path: root/third_party/nix/src/libstore/build.cc

#include <algorithm>
#include <cerrno>
#include <chrono>
#include <climits>
#include <cstring>
#include <future>
#include <iostream>
#include <map>
#include <memory>
#include <ostream>
#include <queue>
#include <regex>
#include <sstream>
#include <string>
#include <thread>

#include <absl/status/status.h>
#include <absl/strings/ascii.h>
#include <absl/strings/numbers.h>
#include <absl/strings/str_cat.h>
#include <absl/strings/str_format.h>
#include <absl/strings/str_split.h>
#include <fcntl.h>
#include <glog/logging.h>
#include <grp.h>
#include <netdb.h>
#include <pwd.h>
#include <sys/resource.h>
#include <sys/select.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <sys/types.h>
#include <sys/utsname.h>
#include <sys/wait.h>
#include <termios.h>
#include <unistd.h>

#include "libstore/builtins.hh"
#include "libstore/download.hh"
#include "libstore/globals.hh"
#include "libstore/local-store.hh"
#include "libstore/machines.hh"
#include "libstore/nar-info.hh"
#include "libstore/parsed-derivations.hh"
#include "libstore/pathlocks.hh"
#include "libstore/references.hh"
#include "libstore/store-api.hh"
#include "libutil/affinity.hh"
#include "libutil/archive.hh"
#include "libutil/compression.hh"
#include "libutil/finally.hh"
#include "libutil/json.hh"
#include "libutil/util.hh"

/* Includes required for chroot support. */
#if __linux__
#include <net/if.h>
#include <netinet/ip.h>
#include <sched.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <sys/mount.h>
#include <sys/param.h>
#include <sys/personality.h>
#include <sys/socket.h>
#include <sys/syscall.h>
#if HAVE_SECCOMP
#include <seccomp.h>
#endif
#define pivot_root(new_root, put_old) \
  (syscall(SYS_pivot_root, new_root, put_old))
#endif

#if HAVE_STATVFS
#include <sys/statvfs.h>
#endif

#include <nlohmann/json.hpp>
#include <utility>

namespace nix {

static std::string pathNullDevice = "/dev/null";

/* Forward definition. */
class Worker;
struct HookInstance;

/* A pointer to a goal. */
class Goal;
class DerivationGoal;
using GoalPtr = std::shared_ptr<Goal>;
using WeakGoalPtr = std::weak_ptr<Goal>;

struct CompareGoalPtrs {
  bool operator()(const GoalPtr& a, const GoalPtr& b) const;
};

/* Set of goals. */
using Goals = std::set<GoalPtr, CompareGoalPtrs>;
using WeakGoals = std::list<WeakGoalPtr>;

/* A map of paths to goals (and the other way around). */
using WeakGoalMap = std::map<Path, WeakGoalPtr>;

class Goal : public std::enable_shared_from_this<Goal> {
 public:
  using ExitCode = enum {
    ecBusy,
    ecSuccess,
    ecFailed,
    ecNoSubstituters,
    ecIncompleteClosure
  };

 protected:
  /* Backlink to the worker. */
  Worker& worker;

  /* Goals that this goal is waiting for. */
  Goals waitees;

  /* Goals waiting for this one to finish.  Must use weak pointers
     here to prevent cycles. */
  WeakGoals waiters;

  /* Number of goals we are/were waiting for that have failed. */
  unsigned int nrFailed;

  /* Number of substitution goals we are/were waiting for that
     failed because there are no substituters. */
  unsigned int nrNoSubstituters;

  /* Number of substitution goals we are/were waiting for that
     failed because othey had unsubstitutable references. */
  unsigned int nrIncompleteClosure;

  /* Name of this goal for debugging purposes. */
  std::string name;

  /* Whether the goal is finished. */
  ExitCode exitCode;

  // Output stream for build logs.
  // TODO(tazjin): Rename all build_log instances to log_sink.
  std::ostream& log_sink_;

  Goal(Worker& worker, std::ostream& log_sink)
      : worker(worker), log_sink_(log_sink) {
    nrFailed = nrNoSubstituters = nrIncompleteClosure = 0;
    exitCode = ecBusy;
  }

  virtual ~Goal() { trace("goal destroyed"); }

 public:
  virtual void work() = 0;

  void addWaitee(const GoalPtr& waitee);

  virtual void waiteeDone(GoalPtr waitee, ExitCode result);

  virtual void handleChildOutput(int fd, const std::string& data) { abort(); }

  virtual void handleEOF(int fd) { abort(); }

  void trace(const FormatOrString& fs);

  std::string getName() { return name; }

  ExitCode getExitCode() { return exitCode; }

  /* Callback in case of a timeout.  It should wake up its waiters,
     get rid of any running child processes that are being monitored
     by the worker (important!), etc. */
  virtual void timedOut() = 0;

  virtual std::string key() = 0;

 protected:
  virtual void amDone(ExitCode result);
};

bool CompareGoalPtrs::operator()(const GoalPtr& a, const GoalPtr& b) const {
  std::string s1 = a->key();
  std::string s2 = b->key();
  return s1 < s2;
}

using steady_time_point = std::chrono::time_point<std::chrono::steady_clock>;

/* A mapping used to remember for each child process to what goal it
   belongs, and file descriptors for receiving log data and output
   path creation commands. */
struct Child {
  WeakGoalPtr goal;
  Goal* goal2;  // ugly hackery
  std::set<int> fds;
  bool respectTimeouts;
  bool inBuildSlot;
  steady_time_point lastOutput; /* time we last got output on stdout/stderr */
  steady_time_point timeStarted;
};

/* The worker class. */
class Worker {
 private:
  /* Note: the worker should only have strong pointers to the
     top-level goals. */

  /* The top-level goals of the worker. */
  Goals topGoals;

  /* Goals that are ready to do some work. */
  WeakGoals awake;

  /* Goals waiting for a build slot. */
  WeakGoals wantingToBuild;

  /* Child processes currently running. */
  std::list<Child> children;

  /* Number of build slots occupied.  This includes local builds and
     substitutions but not remote builds via the build hook. */
  unsigned int nrLocalBuilds;

  /* Maps used to prevent multiple instantiations of a goal for the
     same derivation / path. */
  WeakGoalMap derivationGoals;
  WeakGoalMap substitutionGoals;

  /* Goals waiting for busy paths to be unlocked. */
  WeakGoals waitingForAnyGoal;

  /* Goals sleeping for a few seconds (polling a lock). */
  WeakGoals waitingForAWhile;

  /* Last time the goals in `waitingForAWhile' where woken up. */
  steady_time_point lastWokenUp;

  /* Cache for pathContentsGood(). */
  std::map<Path, bool> pathContentsGoodCache;

 public:
  /* Set if at least one derivation had a BuildError (i.e. permanent
     failure). */
  bool permanentFailure;

  /* Set if at least one derivation had a timeout. */
  bool timedOut;

  /* Set if at least one derivation fails with a hash mismatch. */
  bool hashMismatch;

  /* Set if at least one derivation is not deterministic in check mode. */
  bool checkMismatch;

  LocalStore& store;

  std::unique_ptr<HookInstance> hook;

  uint64_t expectedBuilds = 0;
  uint64_t doneBuilds = 0;
  uint64_t failedBuilds = 0;
  uint64_t runningBuilds = 0;

  uint64_t expectedSubstitutions = 0;
  uint64_t doneSubstitutions = 0;
  uint64_t failedSubstitutions = 0;
  uint64_t runningSubstitutions = 0;
  uint64_t expectedDownloadSize = 0;
  uint64_t doneDownloadSize = 0;
  uint64_t expectedNarSize = 0;
  uint64_t doneNarSize = 0;

  /* Whether to ask the build hook if it can build a derivation. If
     it answers with "decline-permanently", we don't try again. */
  bool tryBuildHook = true;

  explicit Worker(LocalStore& store);
  ~Worker();

  /* Make a goal (with caching). */
  GoalPtr makeDerivationGoal(std::ostream& log_sink, const Path& drvPath,
                             const StringSet& wantedOutputs,
                             BuildMode buildMode = bmNormal);

  std::shared_ptr<DerivationGoal> makeBasicDerivationGoal(
      std::ostream& log_sink, const Path& drvPath, const BasicDerivation& drv,
      BuildMode buildMode = bmNormal);

  GoalPtr makeSubstitutionGoal(std::ostream& log_sink, const Path& storePath,
                               RepairFlag repair = NoRepair);

  /* Remove a dead goal. */
  void removeGoal(const GoalPtr& goal);

  /* Wake up a goal (i.e., there is something for it to do). */
  void wakeUp(const GoalPtr& goal);

  /* Return the number of local build and substitution processes
     currently running (but not remote builds via the build
     hook). */
  unsigned int getNrLocalBuilds();

  /* Registers a running child process.  `inBuildSlot' means that
     the process counts towards the jobs limit. */
  void childStarted(const GoalPtr& goal, const std::set<int>& fds,
                    bool inBuildSlot, bool respectTimeouts);

  /* Unregisters a running child process.  `wakeSleepers' should be
     false if there is no sense in waking up goals that are sleeping
     because they can't run yet (e.g., there is no free build slot,
     or the hook would still say `postpone'). */
  void childTerminated(Goal* goal, bool wakeSleepers = true);

  /* Put `goal' to sleep until a build slot becomes available (which
     might be right away). */
  void waitForBuildSlot(const GoalPtr& goal);

  /* Wait for any goal to finish.  Pretty indiscriminate way to
     wait for some resource that some other goal is holding. */
  void waitForAnyGoal(GoalPtr goal);

  /* Wait for a few seconds and then retry this goal.  Used when
     waiting for a lock held by another process.  This kind of
     polling is inefficient, but POSIX doesn't really provide a way
     to wait for multiple locks in the main select() loop. */
  void waitForAWhile(GoalPtr goal);

  /* Loop until the specified top-level goals have finished. */
  void run(const Goals& topGoals);

  /* Wait for input to become available. */
  void waitForInput();

  unsigned int exitStatus();

  /* Check whether the given valid path exists and has the right
     contents. */
  bool pathContentsGood(const Path& path);

  void markContentsGood(const Path& path);
};

//////////////////////////////////////////////////////////////////////

void addToWeakGoals(WeakGoals& goals, const GoalPtr& p) {
  // FIXME: necessary?
  // FIXME: O(n)
  for (auto& i : goals) {
    if (i.lock() == p) {
      return;
    }
  }
  goals.push_back(p);
}

void Goal::addWaitee(const GoalPtr& waitee) {
  waitees.insert(waitee);
  addToWeakGoals(waitee->waiters, shared_from_this());
}

void Goal::waiteeDone(GoalPtr waitee, ExitCode result) {
  assert(waitees.find(waitee) != waitees.end());
  waitees.erase(waitee);

  trace(format("waitee '%1%' done; %2% left") % waitee->name % waitees.size());

  if (result == ecFailed || result == ecNoSubstituters ||
      result == ecIncompleteClosure) {
    ++nrFailed;
  }

  if (result == ecNoSubstituters) {
    ++nrNoSubstituters;
  }

  if (result == ecIncompleteClosure) {
    ++nrIncompleteClosure;
  }

  if (waitees.empty() || (result == ecFailed && !settings.keepGoing)) {
    /* If we failed and keepGoing is not set, we remove all
       remaining waitees. */
    for (auto& goal : waitees) {
      WeakGoals waiters2;
      for (auto& j : goal->waiters) {
        if (j.lock() != shared_from_this()) {
          waiters2.push_back(j);
        }
      }
      goal->waiters = waiters2;
    }
    waitees.clear();

    worker.wakeUp(shared_from_this());
  }
}

void Goal::amDone(ExitCode result) {
  trace("done");
  assert(exitCode == ecBusy);
  assert(result == ecSuccess || result == ecFailed ||
         result == ecNoSubstituters || result == ecIncompleteClosure);
  exitCode = result;
  for (auto& i : waiters) {
    GoalPtr goal = i.lock();
    if (goal) {
      goal->waiteeDone(shared_from_this(), result);
    }
  }
  waiters.clear();
  worker.removeGoal(shared_from_this());
}

void Goal::trace(const FormatOrString& fs) {
  DLOG(INFO) << name << ": " << fs.s;
}

//////////////////////////////////////////////////////////////////////

/* Common initialisation performed in child processes. */
static void commonChildInit(Pipe& logPipe) {
  restoreSignals();

  /* Put the child in a separate session (and thus a separate
     process group) so that it has no controlling terminal (meaning
     that e.g. ssh cannot open /dev/tty) and it doesn't receive
     terminal signals. */
  if (setsid() == -1) {
    throw SysError(format("creating a new session"));
  }

  /* Dup the write side of the logger pipe into stderr. */
  if (dup2(logPipe.writeSide.get(), STDERR_FILENO) == -1) {
    throw SysError("cannot pipe standard error into log file");
  }

  /* Dup stderr to stdout. */
  if (dup2(STDERR_FILENO, STDOUT_FILENO) == -1) {
    throw SysError("cannot dup stderr into stdout");
  }

  /* Reroute stdin to /dev/null. */
  int fdDevNull = open(pathNullDevice.c_str(), O_RDWR);
  if (fdDevNull == -1) {
    throw SysError(format("cannot open '%1%'") % pathNullDevice);
  }
  if (dup2(fdDevNull, STDIN_FILENO) == -1) {
    throw SysError("cannot dup null device into stdin");
  }
  close(fdDevNull);
}

void handleDiffHook(uid_t uid, uid_t gid, Path tryA, Path tryB, Path drvPath,
                    Path tmpDir) {
  auto diffHook = settings.diffHook;
  if (diffHook != "" && settings.runDiffHook) {
    try {
      RunOptions diffHookOptions(
          diffHook, {std::move(tryA), std::move(tryB), std::move(drvPath),
                     std::move(tmpDir)});
      diffHookOptions.searchPath = true;
      diffHookOptions.uid = uid;
      diffHookOptions.gid = gid;
      diffHookOptions.chdir = "/";

      auto diffRes = runProgram(diffHookOptions);
      if (!statusOk(diffRes.first)) {
        throw ExecError(diffRes.first,
                        fmt("diff-hook program '%1%' %2%", diffHook,
                            statusToString(diffRes.first)));
      }

      if (!diffRes.second.empty()) {
        LOG(ERROR) << absl::StripTrailingAsciiWhitespace(diffRes.second);
      }
    } catch (Error& error) {
      LOG(ERROR) << "diff hook execution failed: " << error.what();
    }
  }
}

//////////////////////////////////////////////////////////////////////

class UserLock {
 private:
  /* POSIX locks suck.  If we have a lock on a file, and we open and
     close that file again (without closing the original file
     descriptor), we lose the lock.  So we have to be *very* careful
     not to open a lock file on which we are holding a lock. */
  static Sync<PathSet> lockedPaths_;

  Path fnUserLock;
  AutoCloseFD fdUserLock;

  std::string user;
  uid_t uid;
  gid_t gid;
  std::vector<gid_t> supplementaryGIDs;

 public:
  UserLock();
  ~UserLock();

  void kill();

  std::string getUser() { return user; }
  uid_t getUID() {
    assert(uid);
    return uid;
  }
  uid_t getGID() {
    assert(gid);
    return gid;
  }
  std::vector<gid_t> getSupplementaryGIDs() { return supplementaryGIDs; }

  bool enabled() { return uid != 0; }
};

Sync<PathSet> UserLock::lockedPaths_;

UserLock::UserLock() {
  assert(settings.buildUsersGroup != "");

  /* Get the members of the build-users-group. */
  struct group* gr = getgrnam(settings.buildUsersGroup.get().c_str());
  if (gr == nullptr) {
    throw Error(
        format(
            "the group '%1%' specified in 'build-users-group' does not exist") %
        settings.buildUsersGroup);
  }
  gid = gr->gr_gid;

  /* Copy the result of getgrnam. */
  Strings users;
  for (char** p = gr->gr_mem; *p != nullptr; ++p) {
    DLOG(INFO) << "found build user " << *p;
    users.push_back(*p);
  }

  if (users.empty()) {
    throw Error(format("the build users group '%1%' has no members") %
                settings.buildUsersGroup);
  }

  /* Find a user account that isn't currently in use for another
     build. */
  for (auto& i : users) {
    DLOG(INFO) << "trying user " << i;

    struct passwd* pw = getpwnam(i.c_str());
    if (pw == nullptr) {
      throw Error(format("the user '%1%' in the group '%2%' does not exist") %
                  i % settings.buildUsersGroup);
    }

    createDirs(settings.nixStateDir + "/userpool");

    fnUserLock =
        (format("%1%/userpool/%2%") % settings.nixStateDir % pw->pw_uid).str();

    {
      auto lockedPaths(lockedPaths_.lock());
      if (lockedPaths->count(fnUserLock) != 0u) {
        /* We already have a lock on this one. */
        continue;
      }
      lockedPaths->insert(fnUserLock);
    }

    try {
      AutoCloseFD fd(
          open(fnUserLock.c_str(), O_RDWR | O_CREAT | O_CLOEXEC, 0600));
      if (!fd) {
        throw SysError(format("opening user lock '%1%'") % fnUserLock);
      }

      if (lockFile(fd.get(), ltWrite, false)) {
        fdUserLock = std::move(fd);
        user = i;
        uid = pw->pw_uid;

        /* Sanity check... */
        if (uid == getuid() || uid == geteuid()) {
          throw Error(format("the Nix user should not be a member of '%1%'") %
                      settings.buildUsersGroup);
        }

#if __linux__
        /* Get the list of supplementary groups of this build user.  This
           is usually either empty or contains a group such as "kvm".  */
        supplementaryGIDs.resize(10);
        int ngroups = supplementaryGIDs.size();
        int err = getgrouplist(pw->pw_name, pw->pw_gid,
                               supplementaryGIDs.data(), &ngroups);
        if (err == -1) {
          throw Error(
              format("failed to get list of supplementary groups for '%1%'") %
              pw->pw_name);
        }

        supplementaryGIDs.resize(ngroups);
#endif

        return;
      }

    } catch (...) {
      lockedPaths_.lock()->erase(fnUserLock);
    }
  }

  throw Error(format("all build users are currently in use; "
                     "consider creating additional users and adding them to "
                     "the '%1%' group") %
              settings.buildUsersGroup);
}

UserLock::~UserLock() {
  auto lockedPaths(lockedPaths_.lock());
  assert(lockedPaths->count(fnUserLock));
  lockedPaths->erase(fnUserLock);
}

void UserLock::kill() { killUser(uid); }

//////////////////////////////////////////////////////////////////////

struct HookInstance {
  /* Pipes for talking to the build hook. */
  Pipe toHook;

  /* Pipe for the hook's standard output/error. */
  Pipe fromHook;

  /* Pipe for the builder's standard output/error. */
  Pipe builderOut;

  /* The process ID of the hook. */
  Pid pid;

  FdSink sink;

  HookInstance();

  ~HookInstance();
};

HookInstance::HookInstance() {
  DLOG(INFO) << "starting build hook " << settings.buildHook;

  /* Create a pipe to get the output of the child. */
  fromHook.create();

  /* Create the communication pipes. */
  toHook.create();

  /* Create a pipe to get the output of the builder. */
  builderOut.create();

  /* Fork the hook. */
  pid = startProcess([&]() {
    commonChildInit(fromHook);

    if (chdir("/") == -1) {
      throw SysError("changing into /");
    }

    /* Dup the communication pipes. */
    if (dup2(toHook.readSide.get(), STDIN_FILENO) == -1) {
      throw SysError("dupping to-hook read side");
    }

    /* Use fd 4 for the builder's stdout/stderr. */
    if (dup2(builderOut.writeSide.get(), 4) == -1) {
      throw SysError("dupping builder's stdout/stderr");
    }

    /* Hack: pass the read side of that fd to allow build-remote
       to read SSH error messages. */
    if (dup2(builderOut.readSide.get(), 5) == -1) {
      throw SysError("dupping builder's stdout/stderr");
    }

    Strings args = {
        baseNameOf(settings.buildHook),
        // std::to_string(verbosity), // TODO(tazjin): what?
    };

    execv(settings.buildHook.get().c_str(), stringsToCharPtrs(args).data());

    throw SysError("executing '%s'", settings.buildHook);
  });

  pid.setSeparatePG(true);
  fromHook.writeSide = AutoCloseFD(-1);
  toHook.readSide = AutoCloseFD(-1);

  sink = FdSink(toHook.writeSide.get());
  std::map<std::string, Config::SettingInfo> settings;
  globalConfig.getSettings(settings);
  for (auto& setting : settings) {
    sink << 1 << setting.first << setting.second.value;
  }
  sink << 0;
}

HookInstance::~HookInstance() {
  try {
    toHook.writeSide = AutoCloseFD(-1);
    if (pid != Pid(-1)) {
      pid.kill();
    }
  } catch (...) {
    ignoreException();
  }
}

//////////////////////////////////////////////////////////////////////

using StringRewrites = std::map<std::string, std::string>;

std::string rewriteStrings(std::string s, const StringRewrites& rewrites) {
  for (auto& i : rewrites) {
    size_t j = 0;
    while ((j = s.find(i.first, j)) != std::string::npos) {
      s.replace(j, i.first.size(), i.second);
    }
  }
  return s;
}

//////////////////////////////////////////////////////////////////////

using HookReply = enum { rpAccept, rpDecline, rpPostpone };

class SubstitutionGoal;

class DerivationGoal : public Goal {
 private:
  /* Whether to use an on-disk .drv file. */
  bool useDerivation;

  /* The path of the derivation. */
  Path drvPath;

  /* The specific outputs that we need to build.  Empty means all of
     them. */
  StringSet wantedOutputs;

  /* Whether additional wanted outputs have been added. */
  bool needRestart = false;

  /* Whether to retry substituting the outputs after building the
     inputs. */
  bool retrySubstitution;

  /* The derivation stored at drvPath. */
  std::unique_ptr<BasicDerivation> drv;

  std::unique_ptr<ParsedDerivation> parsedDrv;

  /* The remainder is state held during the build. */

  /* Locks on the output paths. */
  PathLocks outputLocks;

  /* All input paths (that is, the union of FS closures of the
     immediate input paths). */
  PathSet inputPaths;

  /* Referenceable paths (i.e., input and output paths). */
  PathSet allPaths;

  /* Outputs that are already valid.  If we're repairing, these are
     the outputs that are valid *and* not corrupt. */
  PathSet validPaths;

  /* Outputs that are corrupt or not valid. */
  PathSet missingPaths;

  /* User selected for running the builder. */
  std::unique_ptr<UserLock> buildUser;

  /* The process ID of the builder. */
  Pid pid;

  /* The temporary directory. */
  Path tmpDir;

  /* The path of the temporary directory in the sandbox. */
  Path tmpDirInSandbox;

  /* File descriptor for the log file. */
  AutoCloseFD fdLogFile;
  std::shared_ptr<BufferedSink> logFileSink, logSink;

  /* Number of bytes received from the builder's stdout/stderr. */
  unsigned long logSize;

  /* The most recent log lines. */
  std::list<std::string> logTail;

  std::string currentLogLine;
  size_t currentLogLinePos = 0;  // to handle carriage return

  std::string currentHookLine;

  /* Pipe for the builder's standard output/error. */
  Pipe builderOut;

  /* Pipe for synchronising updates to the builder user namespace. */
  Pipe userNamespaceSync;

  /* The build hook. */
  std::unique_ptr<HookInstance> hook;

  /* Whether we're currently doing a chroot build. */
  bool useChroot = false;

  Path chrootRootDir;

  /* RAII object to delete the chroot directory. */
  std::shared_ptr<AutoDelete> autoDelChroot;

  /* Whether this is a fixed-output derivation. */
  bool fixedOutput;

  /* Whether to run the build in a private network namespace. */
  bool privateNetwork = false;

  using GoalState = void (DerivationGoal::*)();
  GoalState state;

  /* Stuff we need to pass to initChild(). */
  struct ChrootPath {
    Path source;
    bool optional;
    explicit ChrootPath(Path source = "", bool optional = false)
        : source(std::move(source)), optional(optional) {}
  };
  using DirsInChroot =
      std::map<Path, ChrootPath>;  // maps target path to source path
  DirsInChroot dirsInChroot;

  using Environment = std::map<std::string, std::string>;
  Environment env;

  /* Hash rewriting. */
  StringRewrites inputRewrites, outputRewrites;
  using RedirectedOutputs = std::map<Path, Path>;
  RedirectedOutputs redirectedOutputs;

  BuildMode buildMode;

  /* If we're repairing without a chroot, there may be outputs that
     are valid but corrupt.  So we redirect these outputs to
     temporary paths. */
  PathSet redirectedBadOutputs;

  BuildResult result;

  /* The current round, if we're building multiple times. */
  size_t curRound = 1;

  size_t nrRounds;

  /* Path registration info from the previous round, if we're
     building multiple times. Since this contains the hash, it
     allows us to compare whether two rounds produced the same
     result. */
  std::map<Path, ValidPathInfo> prevInfos;

  const uid_t sandboxUid = 1000;
  const gid_t sandboxGid = 100;

  const static Path homeDir;

  std::unique_ptr<MaintainCount<uint64_t>> mcExpectedBuilds, mcRunningBuilds;

  /* The remote machine on which we're building. */
  std::string machineName;

 public:
  DerivationGoal(Worker& worker, std::ostream& log_sink, const Path& drvPath,
                 StringSet wantedOutputs, BuildMode buildMode = bmNormal);

  DerivationGoal(Worker& worker, std::ostream& log_sink, const Path& drvPath,
                 const BasicDerivation& drv, BuildMode buildMode = bmNormal);

  ~DerivationGoal() override;

  /* Whether we need to perform hash rewriting if there are valid output paths.
   */
  bool needsHashRewrite();

  void timedOut() override;

  std::string key() override {
    /* Ensure that derivations get built in order of their name,
       i.e. a derivation named "aardvark" always comes before
       "baboon". And substitution goals always happen before
       derivation goals (due to "b$"). */
    return "b$" + storePathToName(drvPath) + "$" + drvPath;
  }

  void work() override;

  Path getDrvPath() { return drvPath; }

  /* Add wanted outputs to an already existing derivation goal. */
  void addWantedOutputs(const StringSet& outputs);

  BuildResult getResult() { return result; }

 private:
  /* The states. */
  void getDerivation();
  void loadDerivation();
  void haveDerivation();
  void outputsSubstituted();
  void closureRepaired();
  void inputsRealised();
  void tryToBuild();
  void buildDone();

  /* Is the build hook willing to perform the build? */
  HookReply tryBuildHook();

  /* Start building a derivation. */
  void startBuilder();

  /* Fill in the environment for the builder. */
  void initEnv();

  /* Setup tmp dir location. */
  void initTmpDir();

  /* Write a JSON file containing the derivation attributes. */
  void writeStructuredAttrs();

  /* Make a file owned by the builder. */
  void chownToBuilder(const Path& path);

  /* Run the builder's process. */
  void runChild();

  friend int childEntry(void* /*arg*/);

  /* Check that the derivation outputs all exist and register them
     as valid. */
  void registerOutputs();

  /* Check that an output meets the requirements specified by the
     'outputChecks' attribute (or the legacy
     '{allowed,disallowed}{References,Requisites}' attributes). */
  void checkOutputs(const std::map<std::string, ValidPathInfo>& outputs);

  /* Open a log file and a pipe to it. */
  Path openLogFile();

  /* Close the log file. */
  void closeLogFile();

  /* Delete the temporary directory, if we have one. */
  void deleteTmpDir(bool force);

  /* Callback used by the worker to write to the log. */
  void handleChildOutput(int fd, const std::string& data) override;
  void handleEOF(int fd) override;
  void flushLine();

  /* Return the set of (in)valid paths. */
  PathSet checkPathValidity(bool returnValid, bool checkHash);

  /* Abort the goal if `path' failed to build. */
  bool pathFailed(const Path& path);

  /* Forcibly kill the child process, if any. */
  void killChild();

  Path addHashRewrite(const Path& path);

  void repairClosure();

  void amDone(ExitCode result) override { Goal::amDone(result); }

  void done(BuildResult::Status status, const std::string& msg = "");

  PathSet exportReferences(const PathSet& storePaths);
};

const Path DerivationGoal::homeDir = "/homeless-shelter";

DerivationGoal::DerivationGoal(Worker& worker, std::ostream& log_sink,
                               const Path& drvPath, StringSet wantedOutputs,
                               BuildMode buildMode)
    : Goal(worker, log_sink),
      useDerivation(true),
      drvPath(drvPath),
      wantedOutputs(std::move(wantedOutputs)),
      buildMode(buildMode) {
  state = &DerivationGoal::getDerivation;
  name = (format("building of '%1%'") % drvPath).str();
  trace("created");

  mcExpectedBuilds =
      std::make_unique<MaintainCount<uint64_t>>(worker.expectedBuilds);
}

DerivationGoal::DerivationGoal(Worker& worker, std::ostream& log_sink,
                               const Path& drvPath, const BasicDerivation& drv,
                               BuildMode buildMode)
    : Goal(worker, log_sink),
      useDerivation(false),
      drvPath(drvPath),
      buildMode(buildMode) {
  this->drv = std::make_unique<BasicDerivation>(drv);
  state = &DerivationGoal::haveDerivation;
  name = (format("building of %1%") % showPaths(drv.outputPaths())).str();
  trace("created");

  mcExpectedBuilds =
      std::make_unique<MaintainCount<uint64_t>>(worker.expectedBuilds);

  /* Prevent the .chroot directory from being
     garbage-collected. (See isActiveTempFile() in gc.cc.) */
  worker.store.addTempRoot(drvPath);
}

DerivationGoal::~DerivationGoal() {
  /* Careful: we should never ever throw an exception from a
     destructor. */
  try {
    killChild();
  } catch (...) {
    ignoreException();
  }
  try {
    deleteTmpDir(false);
  } catch (...) {
    ignoreException();
  }
  try {
    closeLogFile();
  } catch (...) {
    ignoreException();
  }
}

inline bool DerivationGoal::needsHashRewrite() { return !useChroot; }

void DerivationGoal::killChild() {
  if (pid != Pid(-1)) {
    worker.childTerminated(this);

    if (buildUser) {
      /* If we're using a build user, then there is a tricky
         race condition: if we kill the build user before the
         child has done its setuid() to the build user uid, then
         it won't be killed, and we'll potentially lock up in
         pid.wait().  So also send a conventional kill to the
         child. */
      ::kill(-static_cast<pid_t>(pid), SIGKILL); /* ignore the result */
      buildUser->kill();
      pid.wait();
    } else {
      pid.kill();
    }

    assert(pid == Pid(-1));
  }

  hook.reset();
}

void DerivationGoal::timedOut() {
  killChild();
  done(BuildResult::TimedOut);
}

void DerivationGoal::work() { (this->*state)(); }

void DerivationGoal::addWantedOutputs(const StringSet& outputs) {
  /* If we already want all outputs, there is nothing to do. */
  if (wantedOutputs.empty()) {
    return;
  }

  if (outputs.empty()) {
    wantedOutputs.clear();
    needRestart = true;
  } else {
    for (auto& i : outputs) {
      if (wantedOutputs.find(i) == wantedOutputs.end()) {
        wantedOutputs.insert(i);
        needRestart = true;
      }
    }
  }
}

void DerivationGoal::getDerivation() {
  trace("init");

  /* The first thing to do is to make sure that the derivation
     exists.  If it doesn't, it may be created through a
     substitute. */
  if (buildMode == bmNormal && worker.store.isValidPath(drvPath)) {
    loadDerivation();
    return;
  }

  addWaitee(worker.makeSubstitutionGoal(log_sink_, drvPath));

  state = &DerivationGoal::loadDerivation;
}

void DerivationGoal::loadDerivation() {
  trace("loading derivation");

  if (nrFailed != 0) {
    LOG(ERROR) << "cannot build missing derivation '" << drvPath << "'";
    done(BuildResult::MiscFailure);
    return;
  }

  /* `drvPath' should already be a root, but let's be on the safe
     side: if the user forgot to make it a root, we wouldn't want
     things being garbage collected while we're busy. */
  worker.store.addTempRoot(drvPath);

  assert(worker.store.isValidPath(drvPath));

  /* Get the derivation. */
  drv = std::unique_ptr<BasicDerivation>(
      new Derivation(worker.store.derivationFromPath(drvPath)));

  haveDerivation();
}

void DerivationGoal::haveDerivation() {
  trace("have derivation");

  retrySubstitution = false;

  for (auto& i : drv->outputs) {
    worker.store.addTempRoot(i.second.path);
  }

  /* Check what outputs paths are not already valid. */
  PathSet invalidOutputs = checkPathValidity(false, buildMode == bmRepair);

  /* If they are all valid, then we're done. */
  if (invalidOutputs.empty() && buildMode == bmNormal) {
    done(BuildResult::AlreadyValid);
    return;
  }

  parsedDrv = std::make_unique<ParsedDerivation>(drvPath, *drv);

  /* We are first going to try to create the invalid output paths
     through substitutes.  If that doesn't work, we'll build
     them. */
  if (settings.useSubstitutes && parsedDrv->substitutesAllowed()) {
    for (auto& i : invalidOutputs) {
      addWaitee(worker.makeSubstitutionGoal(
          log_sink_, i, buildMode == bmRepair ? Repair : NoRepair));
    }
  }

  if (waitees.empty()) { /* to prevent hang (no wake-up event) */
    outputsSubstituted();
  } else {
    state = &DerivationGoal::outputsSubstituted;
  }
}

void DerivationGoal::outputsSubstituted() {
  trace("all outputs substituted (maybe)");

  if (nrFailed > 0 && nrFailed > nrNoSubstituters + nrIncompleteClosure &&
      !settings.tryFallback) {
    done(BuildResult::TransientFailure,
         (format("some substitutes for the outputs of derivation '%1%' failed "
                 "(usually happens due to networking issues); try '--fallback' "
                 "to build derivation from source ") %
          drvPath)
             .str());
    return;
  }

  /*  If the substitutes form an incomplete closure, then we should
      build the dependencies of this derivation, but after that, we
      can still use the substitutes for this derivation itself. */
  if (nrIncompleteClosure > 0) {
    retrySubstitution = true;
  }

  nrFailed = nrNoSubstituters = nrIncompleteClosure = 0;

  if (needRestart) {
    needRestart = false;
    haveDerivation();
    return;
  }

  auto nrInvalid = checkPathValidity(false, buildMode == bmRepair).size();
  if (buildMode == bmNormal && nrInvalid == 0) {
    done(BuildResult::Substituted);
    return;
  }
  if (buildMode == bmRepair && nrInvalid == 0) {
    repairClosure();
    return;
  }
  if (buildMode == bmCheck && nrInvalid > 0) {
    throw Error(format("some outputs of '%1%' are not valid, so checking is "
                       "not possible") %
                drvPath);
  }

  /* Otherwise, at least one of the output paths could not be
     produced using a substitute.  So we have to build instead. */

  /* Make sure checkPathValidity() from now on checks all
     outputs. */
  wantedOutputs = PathSet();

  /* The inputs must be built before we can build this goal. */
  if (useDerivation) {
    for (auto& i : dynamic_cast<Derivation*>(drv.get())->inputDrvs) {
      addWaitee(worker.makeDerivationGoal(
          log_sink_, i.first, i.second,
          buildMode == bmRepair ? bmRepair : bmNormal));
    }
  }

  for (auto& i : drv->inputSrcs) {
    if (worker.store.isValidPath(i)) {
      continue;
    }
    if (!settings.useSubstitutes) {
      throw Error(format("dependency '%1%' of '%2%' does not exist, and "
                         "substitution is disabled") %
                  i % drvPath);
    }
    addWaitee(worker.makeSubstitutionGoal(log_sink_, i));
  }

  if (waitees.empty()) { /* to prevent hang (no wake-up event) */
    inputsRealised();
  } else {
    state = &DerivationGoal::inputsRealised;
  }
}

void DerivationGoal::repairClosure() {
  /* If we're repairing, we now know that our own outputs are valid.
     Now check whether the other paths in the outputs closure are
     good.  If not, then start derivation goals for the derivations
     that produced those outputs. */

  /* Get the output closure. */
  PathSet outputClosure;
  for (auto& i : drv->outputs) {
    if (!wantOutput(i.first, wantedOutputs)) {
      continue;
    }
    worker.store.computeFSClosure(i.second.path, outputClosure);
  }

  /* Filter out our own outputs (which we have already checked). */
  for (auto& i : drv->outputs) {
    outputClosure.erase(i.second.path);
  }

  /* Get all dependencies of this derivation so that we know which
     derivation is responsible for which path in the output
     closure. */
  PathSet inputClosure;
  if (useDerivation) {
    worker.store.computeFSClosure(drvPath, inputClosure);
  }
  std::map<Path, Path> outputsToDrv;
  for (auto& i : inputClosure) {
    if (isDerivation(i)) {
      Derivation drv = worker.store.derivationFromPath(i);
      for (auto& j : drv.outputs) {
        outputsToDrv[j.second.path] = i;
      }
    }
  }

  /* Check each path (slow!). */
  PathSet broken;
  for (auto& i : outputClosure) {
    if (worker.pathContentsGood(i)) {
      continue;
    }
    LOG(ERROR) << "found corrupted or missing path '" << i
               << "' in the output closure of '" << drvPath << "'";
    Path drvPath2 = outputsToDrv[i];
    if (drvPath2.empty()) {
      addWaitee(worker.makeSubstitutionGoal(log_sink_, i, Repair));
    } else {
      addWaitee(
          worker.makeDerivationGoal(log_sink_, drvPath2, PathSet(), bmRepair));
    }
  }

  if (waitees.empty()) {
    done(BuildResult::AlreadyValid);
    return;
  }

  state = &DerivationGoal::closureRepaired;
}

void DerivationGoal::closureRepaired() {
  trace("closure repaired");
  if (nrFailed > 0) {
    throw Error(format("some paths in the output closure of derivation '%1%' "
                       "could not be repaired") %
                drvPath);
  }
  done(BuildResult::AlreadyValid);
}

void DerivationGoal::inputsRealised() {
  trace("all inputs realised");

  if (nrFailed != 0) {
    if (!useDerivation) {
      throw Error(format("some dependencies of '%1%' are missing") % drvPath);
    }
    LOG(ERROR) << "cannot build derivation '" << drvPath << "': " << nrFailed
               << " dependencies couldn't be built";
    done(BuildResult::DependencyFailed);
    return;
  }

  if (retrySubstitution) {
    haveDerivation();
    return;
  }

  /* Gather information necessary for computing the closure and/or
     running the build hook. */

  /* The outputs are referenceable paths. */
  for (auto& i : drv->outputs) {
    DLOG(INFO) << "building path " << i.second.path;
    allPaths.insert(i.second.path);
  }

  /* Determine the full set of input paths. */

  /* First, the input derivations. */
  if (useDerivation) {
    for (auto& i : dynamic_cast<Derivation*>(drv.get())->inputDrvs) {
      /* Add the relevant output closures of the input derivation
         `i' as input paths.  Only add the closures of output paths
         that are specified as inputs. */
      assert(worker.store.isValidPath(i.first));
      Derivation inDrv = worker.store.derivationFromPath(i.first);
      for (auto& j : i.second) {
        if (inDrv.outputs.find(j) != inDrv.outputs.end()) {
          worker.store.computeFSClosure(inDrv.outputs[j].path, inputPaths);
        } else {
          throw Error(format("derivation '%1%' requires non-existent output "
                             "'%2%' from input derivation '%3%'") %
                      drvPath % j % i.first);
        }
      }
    }
  }

  /* Second, the input sources. */
  worker.store.computeFSClosure(drv->inputSrcs, inputPaths);

  DLOG(INFO) << "added input paths " << showPaths(inputPaths);

  allPaths.insert(inputPaths.begin(), inputPaths.end());

  /* Is this a fixed-output derivation? */
  fixedOutput = drv->isFixedOutput();

  /* Don't repeat fixed-output derivations since they're already
     verified by their output hash.*/
  nrRounds = fixedOutput ? 1 : settings.buildRepeat + 1;

  /* Okay, try to build.  Note that here we don't wait for a build
     slot to become available, since we don't need one if there is a
     build hook. */
  state = &DerivationGoal::tryToBuild;
  worker.wakeUp(shared_from_this());

  result = BuildResult();
}

void DerivationGoal::tryToBuild() {
  trace("trying to build");

  /* Obtain locks on all output paths.  The locks are automatically
     released when we exit this function or Nix crashes.  If we
     can't acquire the lock, then continue; hopefully some other
     goal can start a build, and if not, the main loop will sleep a
     few seconds and then retry this goal. */
  PathSet lockFiles;
  for (auto& outPath : drv->outputPaths()) {
    lockFiles.insert(worker.store.toRealPath(outPath));
  }

  if (!outputLocks.lockPaths(lockFiles, "", false)) {
    worker.waitForAWhile(shared_from_this());
    return;
  }

  /* Now check again whether the outputs are valid.  This is because
     another process may have started building in parallel.  After
     it has finished and released the locks, we can (and should)
     reuse its results.  (Strictly speaking the first check can be
     omitted, but that would be less efficient.)  Note that since we
     now hold the locks on the output paths, no other process can
     build this derivation, so no further checks are necessary. */
  validPaths = checkPathValidity(true, buildMode == bmRepair);
  if (buildMode != bmCheck && validPaths.size() == drv->outputs.size()) {
    DLOG(INFO) << "skipping build of derivation '" << drvPath
               << "', someone beat us to it";
    outputLocks.setDeletion(true);
    done(BuildResult::AlreadyValid);
    return;
  }

  missingPaths = drv->outputPaths();
  if (buildMode != bmCheck) {
    for (auto& i : validPaths) {
      missingPaths.erase(i);
    }
  }

  /* If any of the outputs already exist but are not valid, delete
     them. */
  for (auto& i : drv->outputs) {
    Path path = i.second.path;
    if (worker.store.isValidPath(path)) {
      continue;
    }
    DLOG(INFO) << "removing invalid path " << path;
    deletePath(worker.store.toRealPath(path));
  }

  /* Don't do a remote build if the derivation has the attribute
     `preferLocalBuild' set.  Also, check and repair modes are only
     supported for local builds. */
  bool buildLocally = buildMode != bmNormal || parsedDrv->willBuildLocally();

  auto started = [&]() {
    auto msg = fmt(buildMode == bmRepair
                       ? "repairing outputs of '%s'"
                       : buildMode == bmCheck
                             ? "checking outputs of '%s'"
                             : nrRounds > 1 ? "building '%s' (round %d/%d)"
                                            : "building '%s'",
                   drvPath, curRound, nrRounds);

    if (hook) {
      msg += fmt(" on '%s'", machineName);
    }
    log_sink_ << absl::StrCat(msg, "[", drvPath, "]\n");
    mcRunningBuilds =
        std::make_unique<MaintainCount<uint64_t>>(worker.runningBuilds);
  };

  /* Is the build hook willing to accept this job? */
  if (!buildLocally) {
    switch (tryBuildHook()) {
      case rpAccept:
        /* Yes, it has started doing so.  Wait until we get
           EOF from the hook. */
        result.startTime = time(nullptr);  // inexact
        state = &DerivationGoal::buildDone;
        started();
        return;
      case rpPostpone:
        /* Not now; wait until at least one child finishes or
           the wake-up timeout expires. */
        worker.waitForAWhile(shared_from_this());
        outputLocks.unlock();
        return;
      case rpDecline:
        /* We should do it ourselves. */
        break;
    }
  }

  /* Make sure that we are allowed to start a build.  If this
     derivation prefers to be done locally, do it even if
     maxBuildJobs is 0. */
  unsigned int curBuilds = worker.getNrLocalBuilds();
  if (curBuilds >= settings.maxBuildJobs && !(buildLocally && curBuilds == 0)) {
    worker.waitForBuildSlot(shared_from_this());
    outputLocks.unlock();
    return;
  }

  try {
    /* Okay, we have to build. */
    startBuilder();

  } catch (BuildError& e) {
    LOG(ERROR) << e.msg();
    outputLocks.unlock();
    buildUser.reset();
    worker.permanentFailure = true;
    done(BuildResult::InputRejected, e.msg());
    return;
  }

  /* This state will be reached when we get EOF on the child's
     log pipe. */
  state = &DerivationGoal::buildDone;

  started();
}

void replaceValidPath(const Path& storePath, const Path& tmpPath) {
  /* We can't atomically replace storePath (the original) with
     tmpPath (the replacement), so we have to move it out of the
     way first.  We'd better not be interrupted here, because if
     we're repairing (say) Glibc, we end up with a broken system. */
  Path oldPath =
      (format("%1%.old-%2%-%3%") % storePath % getpid() % random()).str();
  if (pathExists(storePath)) {
    rename(storePath.c_str(), oldPath.c_str());
  }
  if (rename(tmpPath.c_str(), storePath.c_str()) == -1) {
    throw SysError(format("moving '%1%' to '%2%'") % tmpPath % storePath);
  }
  deletePath(oldPath);
}

MakeError(NotDeterministic, BuildError);

void DerivationGoal::buildDone() {
  trace("build done");

  /* Release the build user at the end of this function. We don't do
     it right away because we don't want another build grabbing this
     uid and then messing around with our output. */
  Finally releaseBuildUser([&]() { buildUser.reset(); });

  /* Since we got an EOF on the logger pipe, the builder is presumed
     to have terminated.  In fact, the builder could also have
     simply have closed its end of the pipe, so just to be sure,
     kill it. */
  int status = hook ? hook->pid.kill() : pid.kill();

  DLOG(INFO) << "builder process for '" << drvPath << "' finished";

  result.timesBuilt++;
  result.stopTime = time(nullptr);

  /* So the child is gone now. */
  worker.childTerminated(this);

  /* Close the read side of the logger pipe. */
  if (hook) {
    hook->builderOut.readSide = AutoCloseFD(-1);
    hook->fromHook.readSide = AutoCloseFD(-1);
  } else {
    builderOut.readSide = AutoCloseFD(-1);
  }

  /* Close the log file. */
  closeLogFile();

  /* When running under a build user, make sure that all processes
     running under that uid are gone.  This is to prevent a
     malicious user from leaving behind a process that keeps files
     open and modifies them after they have been chown'ed to
     root. */
  if (buildUser) {
    buildUser->kill();
  }

  bool diskFull = false;

  try {
    /* Check the exit status. */
    if (!statusOk(status)) {
      /* Heuristically check whether the build failure may have
         been caused by a disk full condition.  We have no way
         of knowing whether the build actually got an ENOSPC.
         So instead, check if the disk is (nearly) full now.  If
         so, we don't mark this build as a permanent failure. */
#if HAVE_STATVFS
      unsigned long long required =
          8ULL * 1024 * 1024;  // FIXME: make configurable
      struct statvfs st;
      if (statvfs(worker.store.realStoreDir.c_str(), &st) == 0 &&
          static_cast<unsigned long long>(st.f_bavail) * st.f_bsize <
              required) {
        diskFull = true;
      }
      if (statvfs(tmpDir.c_str(), &st) == 0 &&
          static_cast<unsigned long long>(st.f_bavail) * st.f_bsize <
              required) {
        diskFull = true;
      }
#endif

      deleteTmpDir(false);

      /* Move paths out of the chroot for easier debugging of
         build failures. */
      if (useChroot && buildMode == bmNormal) {
        for (auto& i : missingPaths) {
          if (pathExists(chrootRootDir + i)) {
            rename((chrootRootDir + i).c_str(), i.c_str());
          }
        }
      }

      std::string msg =
          (format("builder for '%1%' %2%") % drvPath % statusToString(status))
              .str();

      if (!settings.verboseBuild && !logTail.empty()) {
        msg += (format("; last %d log lines:") % logTail.size()).str();
        for (auto& line : logTail) {
          msg += "\n  " + line;
        }
      }

      if (diskFull) {
        msg +=
            "\nnote: build failure may have been caused by lack of free disk "
            "space";
      }

      throw BuildError(msg);
    }

    /* Compute the FS closure of the outputs and register them as
       being valid. */
    registerOutputs();

    if (settings.postBuildHook != "") {
      LOG(INFO) << "running post-build-hook '" << settings.postBuildHook
                << "' [" << drvPath << "]";
      auto outputPaths = drv->outputPaths();
      std::map<std::string, std::string> hookEnvironment = getEnv();

      hookEnvironment.emplace("DRV_PATH", drvPath);
      hookEnvironment.emplace("OUT_PATHS",
                              absl::StripTrailingAsciiWhitespace(
                                  concatStringsSep(" ", outputPaths)));

      RunOptions opts(settings.postBuildHook, {});
      opts.environment = hookEnvironment;

      struct LogSink : Sink {
        std::string currentLine;

        void operator()(const unsigned char* data, size_t len) override {
          for (size_t i = 0; i < len; i++) {
            auto c = data[i];

            if (c == '\n') {
              flushLine();
            } else {
              currentLine += c;
            }
          }
        }

        void flushLine() {
          if (settings.verboseBuild) {
            LOG(ERROR) << "post-build-hook: " << currentLine;
          }
          currentLine.clear();
        }

        ~LogSink() override {
          if (!currentLine.empty()) {
            currentLine += '\n';
            flushLine();
          }
        }
      };
      LogSink sink;

      opts.standardOut = &sink;
      opts.mergeStderrToStdout = true;
      runProgram2(opts);
    }

    if (buildMode == bmCheck) {
      done(BuildResult::Built);
      return;
    }

    /* Delete unused redirected outputs (when doing hash rewriting). */
    for (auto& i : redirectedOutputs) {
      deletePath(i.second);
    }

    /* Delete the chroot (if we were using one). */
    autoDelChroot.reset(); /* this runs the destructor */

    deleteTmpDir(true);

    /* Repeat the build if necessary. */
    if (curRound++ < nrRounds) {
      outputLocks.unlock();
      state = &DerivationGoal::tryToBuild;
      worker.wakeUp(shared_from_this());
      return;
    }

    /* It is now safe to delete the lock files, since all future
       lockers will see that the output paths are valid; they will
       not create new lock files with the same names as the old
       (unlinked) lock files. */
    outputLocks.setDeletion(true);
    outputLocks.unlock();

  } catch (BuildError& e) {
    LOG(ERROR) << e.msg();

    outputLocks.unlock();

    BuildResult::Status st = BuildResult::MiscFailure;

    if (hook && WIFEXITED(status) && WEXITSTATUS(status) == 101) {
      st = BuildResult::TimedOut;

    } else if (hook && (!WIFEXITED(status) || WEXITSTATUS(status) != 100)) {
    }

    else {
      st = dynamic_cast<NotDeterministic*>(&e) != nullptr
               ? BuildResult::NotDeterministic
               : statusOk(status)
                     ? BuildResult::OutputRejected
                     : fixedOutput || diskFull ? BuildResult::TransientFailure
                                               : BuildResult::PermanentFailure;
    }

    done(st, e.msg());
    return;
  }

  done(BuildResult::Built);
}

HookReply DerivationGoal::tryBuildHook() {
  if (!worker.tryBuildHook || !useDerivation) {
    return rpDecline;
  }

  if (!worker.hook) {
    worker.hook = std::make_unique<HookInstance>();
  }

  try {
    /* Send the request to the hook. */
    worker.hook->sink << "try"
                      << (worker.getNrLocalBuilds() < settings.maxBuildJobs ? 1
                                                                            : 0)
                      << drv->platform << drvPath
                      << parsedDrv->getRequiredSystemFeatures();
    worker.hook->sink.flush();

    /* Read the first line of input, which should be a word indicating
       whether the hook wishes to perform the build. */
    std::string reply;
    while (true) {
      std::string s = readLine(worker.hook->fromHook.readSide.get());
      if (std::string(s, 0, 2) == "# ") {
        reply = std::string(s, 2);
        break;
      }
      s += "\n";
      std::cerr << s;
    }

    DLOG(INFO) << "hook reply is " << reply;

    if (reply == "decline") {
      return rpDecline;
    }
    if (reply == "decline-permanently") {
      worker.tryBuildHook = false;
      worker.hook = nullptr;
      return rpDecline;
    } else if (reply == "postpone") {
      return rpPostpone;
    } else if (reply != "accept") {
      throw Error(format("bad hook reply '%1%'") % reply);
    }
  } catch (SysError& e) {
    if (e.errNo == EPIPE) {
      LOG(ERROR) << "build hook died unexpectedly: "
                 << absl::StripTrailingAsciiWhitespace(
                        drainFD(worker.hook->fromHook.readSide.get()));
      worker.hook = nullptr;
      return rpDecline;
    }
    throw;
  }

  hook = std::move(worker.hook);

  machineName = readLine(hook->fromHook.readSide.get());

  /* Tell the hook all the inputs that have to be copied to the
     remote system. */
  hook->sink << inputPaths;

  /* Tell the hooks the missing outputs that have to be copied back
     from the remote system. */
  hook->sink << missingPaths;

  hook->sink = FdSink();
  hook->toHook.writeSide = AutoCloseFD(-1);

  /* Create the log file and pipe. */
  Path logFile = openLogFile();

  std::set<int> fds;
  fds.insert(hook->fromHook.readSide.get());
  fds.insert(hook->builderOut.readSide.get());
  worker.childStarted(shared_from_this(), fds, false, false);

  return rpAccept;
}

void chmod_(const Path& path, mode_t mode) {
  if (chmod(path.c_str(), mode) == -1) {
    throw SysError(format("setting permissions on '%1%'") % path);
  }
}

int childEntry(void* arg) {
  (static_cast<DerivationGoal*>(arg))->runChild();
  return 1;
}

PathSet DerivationGoal::exportReferences(const PathSet& storePaths) {
  PathSet paths;

  for (auto storePath : storePaths) {
    /* Check that the store path is valid. */
    if (!worker.store.isInStore(storePath)) {
      throw BuildError(
          format("'exportReferencesGraph' contains a non-store path '%1%'") %
          storePath);
    }

    storePath = worker.store.toStorePath(storePath);

    if (inputPaths.count(storePath) == 0u) {
      throw BuildError(
          "cannot export references of path '%s' because it is not in the "
          "input closure of the derivation",
          storePath);
    }

    worker.store.computeFSClosure(storePath, paths);
  }

  /* If there are derivations in the graph, then include their
     outputs as well.  This is useful if you want to do things
     like passing all build-time dependencies of some path to a
     derivation that builds a NixOS DVD image. */
  PathSet paths2(paths);

  for (auto& j : paths2) {
    if (isDerivation(j)) {
      Derivation drv = worker.store.derivationFromPath(j);
      for (auto& k : drv.outputs) {
        worker.store.computeFSClosure(k.second.path, paths);
      }
    }
  }

  return paths;
}

static std::once_flag dns_resolve_flag;

static void preloadNSS() {
  /* builtin:fetchurl can trigger a DNS lookup, which with glibc can trigger a
     dynamic library load of one of the glibc NSS libraries in a sandboxed
     child, which will fail unless the library's already been loaded in the
     parent. So we force a lookup of an invalid domain to force the NSS
     machinery to
     load its lookup libraries in the parent before any child gets a chance to.
   */
  std::call_once(dns_resolve_flag, []() {
    struct addrinfo* res = nullptr;

    if (getaddrinfo("this.pre-initializes.the.dns.resolvers.invalid.", "http",
                    nullptr, &res) != 0) {
      if (res != nullptr) {
        freeaddrinfo(res);
      }
    }
  });
}

void DerivationGoal::startBuilder() {
  /* Right platform? */
  if (!parsedDrv->canBuildLocally()) {
    throw Error(
        "a '%s' with features {%s} is required to build '%s', but I am a '%s' "
        "with features {%s}",
        drv->platform,
        concatStringsSep(", ", parsedDrv->getRequiredSystemFeatures()), drvPath,
        settings.thisSystem, concatStringsSep(", ", settings.systemFeatures));
  }

  if (drv->isBuiltin()) {
    preloadNSS();
  }

  /* Are we doing a chroot build? */
  {
    auto noChroot = parsedDrv->getBoolAttr("__noChroot");
    if (settings.sandboxMode == smEnabled) {
      if (noChroot) {
        throw Error(format("derivation '%1%' has '__noChroot' set, "
                           "but that's not allowed when 'sandbox' is 'true'") %
                    drvPath);
      }
      useChroot = true;
    } else if (settings.sandboxMode == smDisabled) {
      useChroot = false;
    } else if (settings.sandboxMode == smRelaxed) {
      useChroot = !fixedOutput && !noChroot;
    }
  }

  if (worker.store.storeDir != worker.store.realStoreDir) {
    useChroot = true;
  }

  /* If `build-users-group' is not empty, then we have to build as
     one of the members of that group. */
  if (settings.buildUsersGroup != "" && getuid() == 0) {
    buildUser = std::make_unique<UserLock>();

    /* Make sure that no other processes are executing under this
       uid. */
    buildUser->kill();
  }

  /* Create a temporary directory where the build will take
     place. */
  auto drvName = storePathToName(drvPath);
  tmpDir = createTempDir("", "nix-build-" + drvName, false, false, 0700);

  chownToBuilder(tmpDir);

  /* Substitute output placeholders with the actual output paths. */
  for (auto& output : drv->outputs) {
    inputRewrites[hashPlaceholder(output.first)] = output.second.path;
  }

  /* Construct the environment passed to the builder. */
  initEnv();

  writeStructuredAttrs();

  /* Handle exportReferencesGraph(), if set. */
  if (!parsedDrv->getStructuredAttrs()) {
    /* The `exportReferencesGraph' feature allows the references graph
       to be passed to a builder.  This attribute should be a list of
       pairs [name1 path1 name2 path2 ...].  The references graph of
       each `pathN' will be stored in a text file `nameN' in the
       temporary build directory.  The text files have the format used
       by `nix-store --register-validity'.  However, the deriver
       fields are left empty. */
    std::string s = get(drv->env, "exportReferencesGraph");
    std::vector<std::string> ss =
        absl::StrSplit(s, absl::ByAnyChar(" \t\n\r"), absl::SkipEmpty());
    if (ss.size() % 2 != 0) {
      throw BuildError(absl::StrFormat(
          "odd number of tokens %d in 'exportReferencesGraph': '%s'", ss.size(),
          s));
    }
    for (auto i = ss.begin(); i != ss.end();) {
      std::string fileName = *i++;
      checkStoreName(fileName); /* !!! abuse of this function */
      Path storePath = *i++;

      /* Write closure info to <fileName>. */
      writeFile(tmpDir + "/" + fileName,
                worker.store.makeValidityRegistration(
                    exportReferences({storePath}), false, false));
    }
  }

  if (useChroot) {
    /* Allow a user-configurable set of directories from the
       host file system. */
    PathSet dirs = settings.sandboxPaths;
    PathSet dirs2 = settings.extraSandboxPaths;
    dirs.insert(dirs2.begin(), dirs2.end());

    dirsInChroot.clear();

    for (auto i : dirs) {
      if (i.empty()) {
        continue;
      }
      bool optional = false;
      if (i[i.size() - 1] == '?') {
        optional = true;
        i.pop_back();
      }
      size_t p = i.find('=');
      if (p == std::string::npos) {
        dirsInChroot[i] = ChrootPath(i, optional);
      } else {
        dirsInChroot[std::string(i, 0, p)] =
            ChrootPath(std::string(i, p + 1), optional);
      }
    }
    dirsInChroot[tmpDirInSandbox] = ChrootPath(tmpDir);

    /* Add the closure of store paths to the chroot. */
    PathSet closure;
    for (auto& i : dirsInChroot) {
      try {
        if (worker.store.isInStore(i.second.source)) {
          worker.store.computeFSClosure(
              worker.store.toStorePath(i.second.source), closure);
        }
      } catch (InvalidPath& e) {
      } catch (Error& e) {
        throw Error(format("while processing 'sandbox-paths': %s") % e.what());
      }
    }
    for (auto& i : closure) {
      dirsInChroot[i] = ChrootPath(i);
    }

    PathSet allowedPaths = settings.allowedImpureHostPrefixes;

    /* This works like the above, except on a per-derivation level */
    auto impurePaths =
        parsedDrv->getStringsAttr("__impureHostDeps").value_or(Strings());

    for (auto& i : impurePaths) {
      bool found = false;
      /* Note: we're not resolving symlinks here to prevent
         giving a non-root user info about inaccessible
         files. */
      Path canonI = canonPath(i);
      /* If only we had a trie to do this more efficiently :) luckily, these are
       * generally going to be pretty small */
      for (auto& a : allowedPaths) {
        Path canonA = canonPath(a);
        if (canonI == canonA || isInDir(canonI, canonA)) {
          found = true;
          break;
        }
      }
      if (!found) {
        throw Error(format("derivation '%1%' requested impure path '%2%', but "
                           "it was not in allowed-impure-host-deps") %
                    drvPath % i);
      }

      dirsInChroot[i] = ChrootPath(i);
    }

    /* Create a temporary directory in which we set up the chroot
       environment using bind-mounts.  We put it in the Nix store
       to ensure that we can create hard-links to non-directory
       inputs in the fake Nix store in the chroot (see below). */
    chrootRootDir = worker.store.toRealPath(drvPath) + ".chroot";
    deletePath(chrootRootDir);

    /* Clean up the chroot directory automatically. */
    autoDelChroot = std::make_shared<AutoDelete>(chrootRootDir);

    DLOG(INFO) << "setting up chroot environment in '" << chrootRootDir << "'";

    if (mkdir(chrootRootDir.c_str(), 0750) == -1) {
      throw SysError(format("cannot create '%1%'") % chrootRootDir);
    }

    if (buildUser &&
        chown(chrootRootDir.c_str(), 0, buildUser->getGID()) == -1) {
      throw SysError(format("cannot change ownership of '%1%'") %
                     chrootRootDir);
    }

    /* Create a writable /tmp in the chroot.  Many builders need
       this.  (Of course they should really respect $TMPDIR
       instead.) */
    Path chrootTmpDir = chrootRootDir + "/tmp";
    createDirs(chrootTmpDir);
    chmod_(chrootTmpDir, 01777);

    /* Create a /etc/passwd with entries for the build user and the
       nobody account.  The latter is kind of a hack to support
       Samba-in-QEMU. */
    createDirs(chrootRootDir + "/etc");

    writeFile(chrootRootDir + "/etc/passwd",
              fmt("root:x:0:0:Nix build user:%3%:/noshell\n"
                  "nixbld:x:%1%:%2%:Nix build user:%3%:/noshell\n"
                  "nobody:x:65534:65534:Nobody:/:/noshell\n",
                  sandboxUid, sandboxGid, settings.sandboxBuildDir));

    /* Declare the build user's group so that programs get a consistent
       view of the system (e.g., "id -gn"). */
    writeFile(chrootRootDir + "/etc/group", (format("root:x:0:\n"
                                                    "nixbld:!:%1%:\n"
                                                    "nogroup:x:65534:\n") %
                                             sandboxGid)
                                                .str());

    /* Create /etc/hosts with localhost entry. */
    if (!fixedOutput) {
      writeFile(chrootRootDir + "/etc/hosts",
                "127.0.0.1 localhost\n::1 localhost\n");
    }

    /* Make the closure of the inputs available in the chroot,
       rather than the whole Nix store.  This prevents any access
       to undeclared dependencies.  Directories are bind-mounted,
       while other inputs are hard-linked (since only directories
       can be bind-mounted).  !!! As an extra security
       precaution, make the fake Nix store only writable by the
       build user. */
    Path chrootStoreDir = chrootRootDir + worker.store.storeDir;
    createDirs(chrootStoreDir);
    chmod_(chrootStoreDir, 01775);

    if (buildUser &&
        chown(chrootStoreDir.c_str(), 0, buildUser->getGID()) == -1) {
      throw SysError(format("cannot change ownership of '%1%'") %
                     chrootStoreDir);
    }

    for (auto& i : inputPaths) {
      Path r = worker.store.toRealPath(i);
      struct stat st;
      if (lstat(r.c_str(), &st) != 0) {
        throw SysError(format("getting attributes of path '%1%'") % i);
      }
      if (S_ISDIR(st.st_mode)) {
        dirsInChroot[i] = ChrootPath(r);
      } else {
        Path p = chrootRootDir + i;
        DLOG(INFO) << "linking '" << p << "' to '" << r << "'";
        if (link(r.c_str(), p.c_str()) == -1) {
          /* Hard-linking fails if we exceed the maximum
             link count on a file (e.g. 32000 of ext3),
             which is quite possible after a `nix-store
             --optimise'. */
          if (errno != EMLINK) {
            throw SysError(format("linking '%1%' to '%2%'") % p % i);
          }
          StringSink sink;
          dumpPath(r, sink);
          StringSource source(*sink.s);
          restorePath(p, source);
        }
      }
    }

    /* If we're repairing, checking or rebuilding part of a
       multiple-outputs derivation, it's possible that we're
       rebuilding a path that is in settings.dirsInChroot
       (typically the dependencies of /bin/sh).  Throw them
       out. */
    for (auto& i : drv->outputs) {
      dirsInChroot.erase(i.second.path);
    }
  }

  if (needsHashRewrite()) {
    if (pathExists(homeDir)) {
      throw Error(format("directory '%1%' exists; please remove it") % homeDir);
    }

    /* We're not doing a chroot build, but we have some valid
       output paths.  Since we can't just overwrite or delete
       them, we have to do hash rewriting: i.e. in the
       environment/arguments passed to the build, we replace the
       hashes of the valid outputs with unique dummy strings;
       after the build, we discard the redirected outputs
       corresponding to the valid outputs, and rewrite the
       contents of the new outputs to replace the dummy strings
       with the actual hashes. */
    if (!validPaths.empty()) {
      for (auto& i : validPaths) {
        addHashRewrite(i);
      }
    }

    /* If we're repairing, then we don't want to delete the
       corrupt outputs in advance.  So rewrite them as well. */
    if (buildMode == bmRepair) {
      for (auto& i : missingPaths) {
        if (worker.store.isValidPath(i) && pathExists(i)) {
          addHashRewrite(i);
          redirectedBadOutputs.insert(i);
        }
      }
    }
  }

  if (useChroot && settings.preBuildHook != "" &&
      (dynamic_cast<Derivation*>(drv.get()) != nullptr)) {
    DLOG(INFO) << "executing pre-build hook '" << settings.preBuildHook << "'";
    auto args =
        useChroot ? Strings({drvPath, chrootRootDir}) : Strings({drvPath});
    enum BuildHookState { stBegin, stExtraChrootDirs };
    auto state = stBegin;
    auto lines = runProgram(settings.preBuildHook, false, args);
    auto lastPos = std::string::size_type{0};
    for (auto nlPos = lines.find('\n'); nlPos != std::string::npos;
         nlPos = lines.find('\n', lastPos)) {
      auto line = std::string{lines, lastPos, nlPos - lastPos};
      lastPos = nlPos + 1;
      if (state == stBegin) {
        if (line == "extra-sandbox-paths" || line == "extra-chroot-dirs") {
          state = stExtraChrootDirs;
        } else {
          throw Error(format("unknown pre-build hook command '%1%'") % line);
        }
      } else if (state == stExtraChrootDirs) {
        if (line.empty()) {
          state = stBegin;
        } else {
          auto p = line.find('=');
          if (p == std::string::npos) {
            dirsInChroot[line] = ChrootPath(line);
          } else {
            dirsInChroot[std::string(line, 0, p)] =
                ChrootPath(std::string(line, p + 1));
          }
        }
      }
    }
  }

  /* Run the builder. */
  DLOG(INFO) << "executing builder '" << drv->builder << "'";

  /* Create the log file. */
  Path logFile = openLogFile();

  /* Create a pipe to get the output of the builder. */
  // builderOut.create();

  builderOut.readSide = AutoCloseFD(posix_openpt(O_RDWR | O_NOCTTY));
  if (!builderOut.readSide) {
    throw SysError("opening pseudoterminal master");
  }

  std::string slaveName(ptsname(builderOut.readSide.get()));

  if (buildUser) {
    if (chmod(slaveName.c_str(), 0600) != 0) {
      throw SysError("changing mode of pseudoterminal slave");
    }

    if (chown(slaveName.c_str(), buildUser->getUID(), 0) != 0) {
      throw SysError("changing owner of pseudoterminal slave");
    }
  } else {
    if (grantpt(builderOut.readSide.get()) != 0) {
      throw SysError("granting access to pseudoterminal slave");
    }
  }

#if 0
    // Mount the pt in the sandbox so that the "tty" command works.
    // FIXME: this doesn't work with the new devpts in the sandbox.
    if (useChroot)
        dirsInChroot[slaveName] = {slaveName, false};
#endif

  if (unlockpt(builderOut.readSide.get()) != 0) {
    throw SysError("unlocking pseudoterminal");
  }

  builderOut.writeSide =
      AutoCloseFD(open(slaveName.c_str(), O_RDWR | O_NOCTTY));
  if (!builderOut.writeSide) {
    throw SysError("opening pseudoterminal slave");
  }

  // Put the pt into raw mode to prevent \n -> \r\n translation.
  struct termios term;
  if (tcgetattr(builderOut.writeSide.get(), &term) != 0) {
    throw SysError("getting pseudoterminal attributes");
  }

  cfmakeraw(&term);

  if (tcsetattr(builderOut.writeSide.get(), TCSANOW, &term) != 0) {
    throw SysError("putting pseudoterminal into raw mode");
  }

  result.startTime = time(nullptr);

  /* Fork a child to build the package. */
  ProcessOptions options;

#if __linux__
  if (useChroot) {
    /* Set up private namespaces for the build:

       - The PID namespace causes the build to start as PID 1.
         Processes outside of the chroot are not visible to those
         on the inside, but processes inside the chroot are
         visible from the outside (though with different PIDs).

       - The private mount namespace ensures that all the bind
         mounts we do will only show up in this process and its
         children, and will disappear automatically when we're
         done.

       - The private network namespace ensures that the builder
         cannot talk to the outside world (or vice versa).  It
         only has a private loopback interface. (Fixed-output
         derivations are not run in a private network namespace
         to allow functions like fetchurl to work.)

       - The IPC namespace prevents the builder from communicating
         with outside processes using SysV IPC mechanisms (shared
         memory, message queues, semaphores).  It also ensures
         that all IPC objects are destroyed when the builder
         exits.

       - The UTS namespace ensures that builders see a hostname of
         localhost rather than the actual hostname.

       We use a helper process to do the clone() to work around
       clone() being broken in multi-threaded programs due to
       at-fork handlers not being run. Note that we use
       CLONE_PARENT to ensure that the real builder is parented to
       us.
    */

    if (!fixedOutput) {
      privateNetwork = true;
    }

    userNamespaceSync.create();

    Pid helper(startProcess(
        [&]() {
          /* Drop additional groups here because we can't do it
             after we've created the new user namespace.  FIXME:
             this means that if we're not root in the parent
             namespace, we can't drop additional groups; they will
             be mapped to nogroup in the child namespace. There does
             not seem to be a workaround for this. (But who can tell
             from reading user_namespaces(7)?)
             See also https://lwn.net/Articles/621612/. */
          if (getuid() == 0 && setgroups(0, nullptr) == -1) {
            throw SysError("setgroups failed");
          }

          size_t stackSize = 1 * 1024 * 1024;
          char* stack = static_cast<char*>(
              mmap(nullptr, stackSize, PROT_WRITE | PROT_READ,
                   MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0));
          if (stack == MAP_FAILED) {
            throw SysError("allocating stack");
          }

          int flags = CLONE_NEWUSER | CLONE_NEWPID | CLONE_NEWNS |
                      CLONE_NEWIPC | CLONE_NEWUTS | CLONE_PARENT | SIGCHLD;
          if (privateNetwork) {
            flags |= CLONE_NEWNET;
          }

          pid_t child = clone(childEntry, stack + stackSize, flags, this);
          if (child == -1 && errno == EINVAL) {
            /* Fallback for Linux < 2.13 where CLONE_NEWPID and
               CLONE_PARENT are not allowed together. */
            flags &= ~CLONE_NEWPID;
            child = clone(childEntry, stack + stackSize, flags, this);
          }
          if (child == -1 && (errno == EPERM || errno == EINVAL)) {
            /* Some distros patch Linux to not allow unpriveleged
             * user namespaces. If we get EPERM or EINVAL, try
             * without CLONE_NEWUSER and see if that works.
             */
            flags &= ~CLONE_NEWUSER;
            child = clone(childEntry, stack + stackSize, flags, this);
          }
          /* Otherwise exit with EPERM so we can handle this in the
             parent. This is only done when sandbox-fallback is set
             to true (the default). */
          if (child == -1 && (errno == EPERM || errno == EINVAL) &&
              settings.sandboxFallback) {
            _exit(1);
          }
          if (child == -1) {
            throw SysError("cloning builder process");
          }

          writeFull(builderOut.writeSide.get(), std::to_string(child) + "\n");
          _exit(0);
        },
        options));

    int res = helper.wait();
    if (res != 0 && settings.sandboxFallback) {
      useChroot = false;
      initTmpDir();
      goto fallback;
    } else if (res != 0) {
      throw Error("unable to start build process");
    }

    userNamespaceSync.readSide = AutoCloseFD(-1);

    pid_t tmp;
    if (!absl::SimpleAtoi(readLine(builderOut.readSide.get()), &tmp)) {
      abort();
    }
    pid = tmp;

    /* Set the UID/GID mapping of the builder's user namespace
       such that the sandbox user maps to the build user, or to
       the calling user (if build users are disabled). */
    uid_t hostUid = buildUser ? buildUser->getUID() : getuid();
    uid_t hostGid = buildUser ? buildUser->getGID() : getgid();

    writeFile("/proc/" + std::to_string(static_cast<pid_t>(pid)) + "/uid_map",
              (format("%d %d 1") % sandboxUid % hostUid).str());

    writeFile("/proc/" + std::to_string(static_cast<pid_t>(pid)) + "/setgroups",
              "deny");

    writeFile("/proc/" + std::to_string(static_cast<pid_t>(pid)) + "/gid_map",
              (format("%d %d 1") % sandboxGid % hostGid).str());

    /* Signal the builder that we've updated its user
       namespace. */
    writeFull(userNamespaceSync.writeSide.get(), "1");
    userNamespaceSync.writeSide = AutoCloseFD(-1);

  } else
#endif
  {
  fallback:
    pid = startProcess([&]() { runChild(); }, options);
  }

  /* parent */
  pid.setSeparatePG(true);
  builderOut.writeSide = AutoCloseFD(-1);
  worker.childStarted(shared_from_this(), {builderOut.readSide.get()}, true,
                      true);

  /* Check if setting up the build environment failed. */
  while (true) {
    std::string msg = readLine(builderOut.readSide.get());
    if (std::string(msg, 0, 1) == "\1") {
      if (msg.size() == 1) {
        break;
      }
      throw Error(std::string(msg, 1));
    }
    DLOG(INFO) << msg;
  }
}

void DerivationGoal::initTmpDir() {
  /* In a sandbox, for determinism, always use the same temporary
     directory. */
#if __linux__
  tmpDirInSandbox = useChroot ? settings.sandboxBuildDir : tmpDir;
#else
  tmpDirInSandbox = tmpDir;
#endif

  /* In non-structured mode, add all bindings specified in the
     derivation via the environment, except those listed in the
     passAsFile attribute. Those are passed as file names pointing
     to temporary files containing the contents. Note that
     passAsFile is ignored in structure mode because it's not
     needed (attributes are not passed through the environment, so
     there is no size constraint). */
  if (!parsedDrv->getStructuredAttrs()) {
    std::set<std::string> passAsFile =
        absl::StrSplit(get(drv->env, "passAsFile"), absl::ByAnyChar(" \t\n\r"),
                       absl::SkipEmpty());
    for (auto& i : drv->env) {
      if (passAsFile.find(i.first) == passAsFile.end()) {
        env[i.first] = i.second;
      } else {
        auto hash = hashString(htSHA256, i.first);
        std::string fn = ".attr-" + hash.to_string(Base32, false);
        Path p = tmpDir + "/" + fn;
        writeFile(p, rewriteStrings(i.second, inputRewrites));
        chownToBuilder(p);
        env[i.first + "Path"] = tmpDirInSandbox + "/" + fn;
      }
    }
  }

  /* For convenience, set an environment pointing to the top build
     directory. */
  env["NIX_BUILD_TOP"] = tmpDirInSandbox;

  /* Also set TMPDIR and variants to point to this directory. */
  env["TMPDIR"] = env["TEMPDIR"] = env["TMP"] = env["TEMP"] = tmpDirInSandbox;

  /* Explicitly set PWD to prevent problems with chroot builds.  In
     particular, dietlibc cannot figure out the cwd because the
     inode of the current directory doesn't appear in .. (because
     getdents returns the inode of the mount point). */
  env["PWD"] = tmpDirInSandbox;
}

void DerivationGoal::initEnv() {
  env.clear();

  /* Most shells initialise PATH to some default (/bin:/usr/bin:...) when
     PATH is not set.  We don't want this, so we fill it in with some dummy
     value. */
  env["PATH"] = "/path-not-set";

  /* Set HOME to a non-existing path to prevent certain programs from using
     /etc/passwd (or NIS, or whatever) to locate the home directory (for
     example, wget looks for ~/.wgetrc).  I.e., these tools use /etc/passwd
     if HOME is not set, but they will just assume that the settings file
     they are looking for does not exist if HOME is set but points to some
     non-existing path. */
  env["HOME"] = homeDir;

  /* Tell the builder where the Nix store is.  Usually they
     shouldn't care, but this is useful for purity checking (e.g.,
     the compiler or linker might only want to accept paths to files
     in the store or in the build directory). */
  env["NIX_STORE"] = worker.store.storeDir;

  /* The maximum number of cores to utilize for parallel building. */
  env["NIX_BUILD_CORES"] = (format("%d") % settings.buildCores).str();

  initTmpDir();

  /* Compatibility hack with Nix <= 0.7: if this is a fixed-output
     derivation, tell the builder, so that for instance `fetchurl'
     can skip checking the output.  On older Nixes, this environment
     variable won't be set, so `fetchurl' will do the check. */
  if (fixedOutput) {
    env["NIX_OUTPUT_CHECKED"] = "1";
  }

  /* *Only* if this is a fixed-output derivation, propagate the
     values of the environment variables specified in the
     `impureEnvVars' attribute to the builder.  This allows for
     instance environment variables for proxy configuration such as
     `http_proxy' to be easily passed to downloaders like
     `fetchurl'.  Passing such environment variables from the caller
     to the builder is generally impure, but the output of
     fixed-output derivations is by definition pure (since we
     already know the cryptographic hash of the output). */
  if (fixedOutput) {
    for (auto& i :
         parsedDrv->getStringsAttr("impureEnvVars").value_or(Strings())) {
      env[i] = getEnv(i);
    }
  }

  /* Currently structured log messages piggyback on stderr, but we
     may change that in the future. So tell the builder which file
     descriptor to use for that. */
  env["NIX_LOG_FD"] = "2";

  /* Trigger colored output in various tools. */
  env["TERM"] = "xterm-256color";
}

static std::regex shVarName("[A-Za-z_][A-Za-z0-9_]*");

void DerivationGoal::writeStructuredAttrs() {
  auto& structuredAttrs = parsedDrv->getStructuredAttrs();
  if (!structuredAttrs) {
    return;
  }

  auto json = *structuredAttrs;

  /* Add an "outputs" object containing the output paths. */
  nlohmann::json outputs;
  for (auto& i : drv->outputs) {
    outputs[i.first] = rewriteStrings(i.second.path, inputRewrites);
  }
  json["outputs"] = outputs;

  /* Handle exportReferencesGraph. */
  auto e = json.find("exportReferencesGraph");
  if (e != json.end() && e->is_object()) {
    for (auto i = e->begin(); i != e->end(); ++i) {
      std::ostringstream str;
      {
        JSONPlaceholder jsonRoot(str, true);
        PathSet storePaths;
        for (auto& p : *i) {
          storePaths.insert(p.get<std::string>());
        }
        worker.store.pathInfoToJSON(jsonRoot, exportReferences(storePaths),
                                    false, true);
      }
      json[i.key()] = nlohmann::json::parse(str.str());  // urgh
    }
  }

  writeFile(tmpDir + "/.attrs.json",
            rewriteStrings(json.dump(), inputRewrites));
  chownToBuilder(tmpDir + "/.attrs.json");

  /* As a convenience to bash scripts, write a shell file that
     maps all attributes that are representable in bash -
     namely, strings, integers, nulls, Booleans, and arrays and
     objects consisting entirely of those values. (So nested
     arrays or objects are not supported.) */

  auto handleSimpleType =
      [](const nlohmann::json& value) -> std::optional<std::string> {
    if (value.is_string()) {
      return shellEscape(value);
    }

    if (value.is_number()) {
      auto f = value.get<float>();
      if (std::ceil(f) == f) {
        return std::to_string(value.get<int>());
      }
    }

    if (value.is_null()) {
      return std::string("''");
    }

    if (value.is_boolean()) {
      return value.get<bool>() ? std::string("1") : std::string("");
    }

    return {};
  };

  std::string jsonSh;

  for (auto i = json.begin(); i != json.end(); ++i) {
    if (!std::regex_match(i.key(), shVarName)) {
      continue;
    }

    auto& value = i.value();

    auto s = handleSimpleType(value);
    if (s) {
      jsonSh += fmt("declare %s=%s\n", i.key(), *s);

    } else if (value.is_array()) {
      std::string s2;
      bool good = true;

      for (auto i = value.begin(); i != value.end(); ++i) {
        auto s3 = handleSimpleType(i.value());
        if (!s3) {
          good = false;
          break;
        }
        s2 += *s3;
        s2 += ' ';
      }

      if (good) {
        jsonSh += fmt("declare -a %s=(%s)\n", i.key(), s2);
      }
    }

    else if (value.is_object()) {
      std::string s2;
      bool good = true;

      for (auto i = value.begin(); i != value.end(); ++i) {
        auto s3 = handleSimpleType(i.value());
        if (!s3) {
          good = false;
          break;
        }
        s2 += fmt("[%s]=%s ", shellEscape(i.key()), *s3);
      }

      if (good) {
        jsonSh += fmt("declare -A %s=(%s)\n", i.key(), s2);
      }
    }
  }

  writeFile(tmpDir + "/.attrs.sh", rewriteStrings(jsonSh, inputRewrites));
  chownToBuilder(tmpDir + "/.attrs.sh");
}

void DerivationGoal::chownToBuilder(const Path& path) {
  if (!buildUser) {
    return;
  }
  if (chown(path.c_str(), buildUser->getUID(), buildUser->getGID()) == -1) {
    throw SysError(format("cannot change ownership of '%1%'") % path);
  }
}

void setupSeccomp() {
#if __linux__
  if (!settings.filterSyscalls) {
    return;
  }
#if HAVE_SECCOMP
  scmp_filter_ctx ctx;

  if ((ctx = seccomp_init(SCMP_ACT_ALLOW)) == nullptr) {
    throw SysError("unable to initialize seccomp mode 2");
  }

  Finally cleanup([&]() { seccomp_release(ctx); });

  if (nativeSystem == "x86_64-linux" &&
      seccomp_arch_add(ctx, SCMP_ARCH_X86) != 0) {
    throw SysError("unable to add 32-bit seccomp architecture");
  }

  if (nativeSystem == "x86_64-linux" &&
      seccomp_arch_add(ctx, SCMP_ARCH_X32) != 0) {
    throw SysError("unable to add X32 seccomp architecture");
  }

  if (nativeSystem == "aarch64-linux" &&
      seccomp_arch_add(ctx, SCMP_ARCH_ARM) != 0) {
    LOG(ERROR) << "unable to add ARM seccomp architecture; this may result in "
               << "spurious build failures if running 32-bit ARM processes";
  }

  /* Prevent builders from creating setuid/setgid binaries. */
  for (int perm : {S_ISUID, S_ISGID}) {
    if (seccomp_rule_add(ctx, SCMP_ACT_ERRNO(EPERM), SCMP_SYS(chmod), 1,
                         SCMP_A1(SCMP_CMP_MASKED_EQ, (scmp_datum_t)perm,
                                 (scmp_datum_t)perm)) != 0) {
      throw SysError("unable to add seccomp rule");
    }

    if (seccomp_rule_add(ctx, SCMP_ACT_ERRNO(EPERM), SCMP_SYS(fchmod), 1,
                         SCMP_A1(SCMP_CMP_MASKED_EQ, (scmp_datum_t)perm,
                                 (scmp_datum_t)perm)) != 0) {
      throw SysError("unable to add seccomp rule");
    }

    if (seccomp_rule_add(ctx, SCMP_ACT_ERRNO(EPERM), SCMP_SYS(fchmodat), 1,
                         SCMP_A2(SCMP_CMP_MASKED_EQ, (scmp_datum_t)perm,
                                 (scmp_datum_t)perm)) != 0) {
      throw SysError("unable to add seccomp rule");
    }
  }

  /* Prevent builders from creating EAs or ACLs. Not all filesystems
     support these, and they're not allowed in the Nix store because
     they're not representable in the NAR serialisation. */
  if (seccomp_rule_add(ctx, SCMP_ACT_ERRNO(ENOTSUP), SCMP_SYS(setxattr), 0) !=
          0 ||
      seccomp_rule_add(ctx, SCMP_ACT_ERRNO(ENOTSUP), SCMP_SYS(lsetxattr), 0) !=
          0 ||
      seccomp_rule_add(ctx, SCMP_ACT_ERRNO(ENOTSUP), SCMP_SYS(fsetxattr), 0) !=
          0) {
    throw SysError("unable to add seccomp rule");
  }

  if (seccomp_attr_set(ctx, SCMP_FLTATR_CTL_NNP,
                       settings.allowNewPrivileges ? 0 : 1) != 0) {
    throw SysError("unable to set 'no new privileges' seccomp attribute");
  }

  if (seccomp_load(ctx) != 0) {
    throw SysError("unable to load seccomp BPF program");
  }
#else
  throw Error(
      "seccomp is not supported on this platform; "
      "you can bypass this error by setting the option 'filter-syscalls' to "
      "false, but note that untrusted builds can then create setuid binaries!");
#endif
#endif
}

void DerivationGoal::runChild() {
  /* Warning: in the child we should absolutely not make any SQLite
     calls! */

  try { /* child */

    commonChildInit(builderOut);

    try {
      setupSeccomp();
    } catch (...) {
      if (buildUser) {
        throw;
      }
    }

    bool setUser = true;

    /* Make the contents of netrc available to builtin:fetchurl
       (which may run under a different uid and/or in a sandbox). */
    std::string netrcData;
    try {
      if (drv->isBuiltin() && drv->builder == "builtin:fetchurl") {
        const std::string& netrc_file = settings.netrcFile;
        netrcData = readFile(netrc_file);
      }
    } catch (SysError&) {
    }

#if __linux__
    if (useChroot) {
      userNamespaceSync.writeSide = AutoCloseFD(-1);

      if (drainFD(userNamespaceSync.readSide.get()) != "1") {
        throw Error("user namespace initialisation failed");
      }

      userNamespaceSync.readSide = AutoCloseFD(-1);

      if (privateNetwork) {
        /* Initialise the loopback interface. */
        AutoCloseFD fd(socket(PF_INET, SOCK_DGRAM, IPPROTO_IP));
        if (!fd) {
          throw SysError("cannot open IP socket");
        }

        struct ifreq ifr;
        strncpy(ifr.ifr_name, "lo", sizeof("lo"));
        ifr.ifr_flags = IFF_UP | IFF_LOOPBACK | IFF_RUNNING;
        if (ioctl(fd.get(), SIOCSIFFLAGS, &ifr) == -1) {
          throw SysError("cannot set loopback interface flags");
        }
      }

      /* Set the hostname etc. to fixed values. */
      char hostname[] = "localhost";
      if (sethostname(hostname, sizeof(hostname)) == -1) {
        throw SysError("cannot set host name");
      }
      char domainname[] = "(none)";  // kernel default
      if (setdomainname(domainname, sizeof(domainname)) == -1) {
        throw SysError("cannot set domain name");
      }

      /* Make all filesystems private.  This is necessary
         because subtrees may have been mounted as "shared"
         (MS_SHARED).  (Systemd does this, for instance.)  Even
         though we have a private mount namespace, mounting
         filesystems on top of a shared subtree still propagates
         outside of the namespace.  Making a subtree private is
         local to the namespace, though, so setting MS_PRIVATE
         does not affect the outside world. */
      if (mount(nullptr, "/", nullptr, MS_REC | MS_PRIVATE, nullptr) == -1) {
        throw SysError("unable to make '/' private mount");
      }

      /* Bind-mount chroot directory to itself, to treat it as a
         different filesystem from /, as needed for pivot_root. */
      if (mount(chrootRootDir.c_str(), chrootRootDir.c_str(), nullptr, MS_BIND,
                nullptr) == -1) {
        throw SysError(format("unable to bind mount '%1%'") % chrootRootDir);
      }

      /* Set up a nearly empty /dev, unless the user asked to
         bind-mount the host /dev. */
      Strings ss;
      if (dirsInChroot.find("/dev") == dirsInChroot.end()) {
        createDirs(chrootRootDir + "/dev/shm");
        createDirs(chrootRootDir + "/dev/pts");
        ss.push_back("/dev/full");
        if ((settings.systemFeatures.get().count("kvm") != 0u) &&
            pathExists("/dev/kvm")) {
          ss.push_back("/dev/kvm");
        }
        ss.push_back("/dev/null");
        ss.push_back("/dev/random");
        ss.push_back("/dev/tty");
        ss.push_back("/dev/urandom");
        ss.push_back("/dev/zero");
        createSymlink("/proc/self/fd", chrootRootDir + "/dev/fd");
        createSymlink("/proc/self/fd/0", chrootRootDir + "/dev/stdin");
        createSymlink("/proc/self/fd/1", chrootRootDir + "/dev/stdout");
        createSymlink("/proc/self/fd/2", chrootRootDir + "/dev/stderr");
      }

      /* Fixed-output derivations typically need to access the
         network, so give them access to /etc/resolv.conf and so
         on. */
      if (fixedOutput) {
        ss.push_back("/etc/resolv.conf");

        // Only use nss functions to resolve hosts and
        // services. Don’t use it for anything else that may
        // be configured for this system. This limits the
        // potential impurities introduced in fixed outputs.
        writeFile(chrootRootDir + "/etc/nsswitch.conf",
                  "hosts: files dns\nservices: files\n");

        ss.push_back("/etc/services");
        ss.push_back("/etc/hosts");
        if (pathExists("/var/run/nscd/socket")) {
          ss.push_back("/var/run/nscd/socket");
        }
      }

      for (auto& i : ss) {
        dirsInChroot.emplace(i, i);
      }

      /* Bind-mount all the directories from the "host"
         filesystem that we want in the chroot
         environment. */
      auto doBind = [&](const Path& source, const Path& target,
                        bool optional = false) {
        DLOG(INFO) << "bind mounting '" << source << "' to '" << target << "'";
        struct stat st;
        if (stat(source.c_str(), &st) == -1) {
          if (optional && errno == ENOENT) {
            return;
          }
          throw SysError("getting attributes of path '%1%'", source);
        }
        if (S_ISDIR(st.st_mode)) {
          createDirs(target);
        } else {
          createDirs(dirOf(target));
          writeFile(target, "");
        }
        if (mount(source.c_str(), target.c_str(), "", MS_BIND | MS_REC,
                  nullptr) == -1) {
          throw SysError("bind mount from '%1%' to '%2%' failed", source,
                         target);
        }
      };

      for (auto& i : dirsInChroot) {
        if (i.second.source == "/proc") {
          continue;
        }  // backwards compatibility
        doBind(i.second.source, chrootRootDir + i.first, i.second.optional);
      }

      /* Bind a new instance of procfs on /proc. */
      createDirs(chrootRootDir + "/proc");
      if (mount("none", (chrootRootDir + "/proc").c_str(), "proc", 0,
                nullptr) == -1) {
        throw SysError("mounting /proc");
      }

      /* Mount a new tmpfs on /dev/shm to ensure that whatever
         the builder puts in /dev/shm is cleaned up automatically. */
      if (pathExists("/dev/shm") &&
          mount("none", (chrootRootDir + "/dev/shm").c_str(), "tmpfs", 0,
                fmt("size=%s", settings.sandboxShmSize).c_str()) == -1) {
        throw SysError("mounting /dev/shm");
      }

      /* Mount a new devpts on /dev/pts.  Note that this
         requires the kernel to be compiled with
         CONFIG_DEVPTS_MULTIPLE_INSTANCES=y (which is the case
         if /dev/ptx/ptmx exists). */
      if (pathExists("/dev/pts/ptmx") &&
          !pathExists(chrootRootDir + "/dev/ptmx") &&
          (dirsInChroot.count("/dev/pts") == 0u)) {
        if (mount("none", (chrootRootDir + "/dev/pts").c_str(), "devpts", 0,
                  "newinstance,mode=0620") == 0) {
          createSymlink("/dev/pts/ptmx", chrootRootDir + "/dev/ptmx");

          /* Make sure /dev/pts/ptmx is world-writable.  With some
             Linux versions, it is created with permissions 0.  */
          chmod_(chrootRootDir + "/dev/pts/ptmx", 0666);
        } else {
          if (errno != EINVAL) {
            throw SysError("mounting /dev/pts");
          }
          doBind("/dev/pts", chrootRootDir + "/dev/pts");
          doBind("/dev/ptmx", chrootRootDir + "/dev/ptmx");
        }
      }

      /* Do the chroot(). */
      if (chdir(chrootRootDir.c_str()) == -1) {
        throw SysError(format("cannot change directory to '%1%'") %
                       chrootRootDir);
      }

      if (mkdir("real-root", 0) == -1) {
        throw SysError("cannot create real-root directory");
      }

      if (pivot_root(".", "real-root") == -1) {
        throw SysError(format("cannot pivot old root directory onto '%1%'") %
                       (chrootRootDir + "/real-root"));
      }

      if (chroot(".") == -1) {
        throw SysError(format("cannot change root directory to '%1%'") %
                       chrootRootDir);
      }

      if (umount2("real-root", MNT_DETACH) == -1) {
        throw SysError("cannot unmount real root filesystem");
      }

      if (rmdir("real-root") == -1) {
        throw SysError("cannot remove real-root directory");
      }

      /* Switch to the sandbox uid/gid in the user namespace,
         which corresponds to the build user or calling user in
         the parent namespace. */
      if (setgid(sandboxGid) == -1) {
        throw SysError("setgid failed");
      }
      if (setuid(sandboxUid) == -1) {
        throw SysError("setuid failed");
      }

      setUser = false;
    }
#endif

    if (chdir(tmpDirInSandbox.c_str()) == -1) {
      throw SysError(format("changing into '%1%'") % tmpDir);
    }

    /* Close all other file descriptors. */
    closeMostFDs({STDIN_FILENO, STDOUT_FILENO, STDERR_FILENO});

#if __linux__
    /* Change the personality to 32-bit if we're doing an
       i686-linux build on an x86_64-linux machine. */
    struct utsname utsbuf;
    uname(&utsbuf);
    if (drv->platform == "i686-linux" &&
        (settings.thisSystem == "x86_64-linux" ||
         ((strcmp(utsbuf.sysname, "Linux") == 0) &&
          (strcmp(utsbuf.machine, "x86_64") == 0)))) {
      if (personality(PER_LINUX32) == -1) {
        throw SysError("cannot set i686-linux personality");
      }
    }

    /* Impersonate a Linux 2.6 machine to get some determinism in
       builds that depend on the kernel version. */
    if ((drv->platform == "i686-linux" || drv->platform == "x86_64-linux") &&
        settings.impersonateLinux26) {
      int cur = personality(0xffffffff);
      if (cur != -1) {
        personality(cur | 0x0020000 /* == UNAME26 */);
      }
    }

    /* Disable address space randomization for improved
       determinism. */
    int cur = personality(0xffffffff);
    if (cur != -1) {
      personality(cur | ADDR_NO_RANDOMIZE);
    }
#endif

    /* Disable core dumps by default. */
    struct rlimit limit = {0, RLIM_INFINITY};
    setrlimit(RLIMIT_CORE, &limit);

    // FIXME: set other limits to deterministic values?

    /* Fill in the environment. */
    Strings envStrs;
    for (auto& i : env) {
      envStrs.push_back(
          rewriteStrings(i.first + "=" + i.second, inputRewrites));
    }

    /* If we are running in `build-users' mode, then switch to the
       user we allocated above.  Make sure that we drop all root
       privileges.  Note that above we have closed all file
       descriptors except std*, so that's safe.  Also note that
       setuid() when run as root sets the real, effective and
       saved UIDs. */
    if (setUser && buildUser) {
      /* Preserve supplementary groups of the build user, to allow
         admins to specify groups such as "kvm".  */
      if (!buildUser->getSupplementaryGIDs().empty() &&
          setgroups(buildUser->getSupplementaryGIDs().size(),
                    buildUser->getSupplementaryGIDs().data()) == -1) {
        throw SysError("cannot set supplementary groups of build user");
      }

      if (setgid(buildUser->getGID()) == -1 ||
          getgid() != buildUser->getGID() || getegid() != buildUser->getGID()) {
        throw SysError("setgid failed");
      }

      if (setuid(buildUser->getUID()) == -1 ||
          getuid() != buildUser->getUID() || geteuid() != buildUser->getUID()) {
        throw SysError("setuid failed");
      }
    }

    /* Fill in the arguments. */
    Strings args;

    const char* builder = "invalid";

    if (!drv->isBuiltin()) {
      builder = drv->builder.c_str();
      std::string builderBasename = baseNameOf(drv->builder);
      args.push_back(builderBasename);
    }

    for (auto& i : drv->args) {
      args.push_back(rewriteStrings(i, inputRewrites));
    }

    /* Indicate that we managed to set up the build environment. */
    writeFull(STDERR_FILENO, std::string("\1\n"));

    /* Execute the program.  This should not return. */
    if (drv->isBuiltin()) {
      try {
        BasicDerivation drv2(*drv);
        for (auto& e : drv2.env) {
          e.second = rewriteStrings(e.second, inputRewrites);
        }

        if (drv->builder == "builtin:fetchurl") {
          builtinFetchurl(drv2, netrcData);
        } else if (drv->builder == "builtin:buildenv") {
          builtinBuildenv(drv2);
        } else {
          throw Error(format("unsupported builtin function '%1%'") %
                      std::string(drv->builder, 8));
        }
        _exit(0);
      } catch (std::exception& e) {
        writeFull(STDERR_FILENO, "error: " + std::string(e.what()) + "\n");
        _exit(1);
      }
    }

    execve(builder, stringsToCharPtrs(args).data(),
           stringsToCharPtrs(envStrs).data());

    throw SysError(format("executing '%1%'") % drv->builder);

  } catch (std::exception& e) {
    writeFull(STDERR_FILENO, "\1while setting up the build environment: " +
                                 std::string(e.what()) + "\n");
    _exit(1);
  }
}

/* Parse a list of reference specifiers.  Each element must either be
   a store path, or the symbolic name of the output of the derivation
   (such as `out'). */
PathSet parseReferenceSpecifiers(Store& store, const BasicDerivation& drv,
                                 const Strings& paths) {
  PathSet result;
  for (auto& i : paths) {
    if (store.isStorePath(i)) {
      result.insert(i);
    } else if (drv.outputs.find(i) != drv.outputs.end()) {
      result.insert(drv.outputs.find(i)->second.path);
    } else {
      throw BuildError(
          format("derivation contains an illegal reference specifier '%1%'") %
          i);
    }
  }
  return result;
}

void DerivationGoal::registerOutputs() {
  /* When using a build hook, the build hook can register the output
     as valid (by doing `nix-store --import').  If so we don't have
     to do anything here. */
  if (hook) {
    bool allValid = true;
    for (auto& i : drv->outputs) {
      if (!worker.store.isValidPath(i.second.path)) {
        allValid = false;
      }
    }
    if (allValid) {
      return;
    }
  }

  std::map<std::string, ValidPathInfo> infos;

  /* Set of inodes seen during calls to canonicalisePathMetaData()
     for this build's outputs.  This needs to be shared between
     outputs to allow hard links between outputs. */
  InodesSeen inodesSeen;

  Path checkSuffix = ".check";
  bool keepPreviousRound = settings.keepFailed || settings.runDiffHook;

  std::exception_ptr delayedException;

  /* Check whether the output paths were created, and grep each
     output path to determine what other paths it references.  Also make all
     output paths read-only. */
  for (auto& i : drv->outputs) {
    Path path = i.second.path;
    if (missingPaths.find(path) == missingPaths.end()) {
      continue;
    }

    ValidPathInfo info;

    Path actualPath = path;
    if (useChroot) {
      actualPath = chrootRootDir + path;
      if (pathExists(actualPath)) {
        /* Move output paths from the chroot to the Nix store. */
        if (buildMode == bmRepair) {
          replaceValidPath(path, actualPath);
        } else if (buildMode != bmCheck &&
                   rename(actualPath.c_str(),
                          worker.store.toRealPath(path).c_str()) == -1) {
          throw SysError(format("moving build output '%1%' from the sandbox to "
                                "the Nix store") %
                         path);
        }
      }
      if (buildMode != bmCheck) {
        actualPath = worker.store.toRealPath(path);
      }
    }

    if (needsHashRewrite()) {
      Path redirected = redirectedOutputs[path];
      if (buildMode == bmRepair &&
          redirectedBadOutputs.find(path) != redirectedBadOutputs.end() &&
          pathExists(redirected)) {
        replaceValidPath(path, redirected);
      }
      if (buildMode == bmCheck && !redirected.empty()) {
        actualPath = redirected;
      }
    }

    struct stat st;
    if (lstat(actualPath.c_str(), &st) == -1) {
      if (errno == ENOENT) {
        throw BuildError(
            format("builder for '%1%' failed to produce output path '%2%'") %
            drvPath % path);
      }
      throw SysError(format("getting attributes of path '%1%'") % actualPath);
    }

#ifndef __CYGWIN__
    /* Check that the output is not group or world writable, as
       that means that someone else can have interfered with the
       build.  Also, the output should be owned by the build
       user. */
    if ((!S_ISLNK(st.st_mode) && ((st.st_mode & (S_IWGRP | S_IWOTH)) != 0u)) ||
        (buildUser && st.st_uid != buildUser->getUID())) {
      throw BuildError(format("suspicious ownership or permission on '%1%'; "
                              "rejecting this build output") %
                       path);
    }
#endif

    /* Apply hash rewriting if necessary. */
    bool rewritten = false;
    if (!outputRewrites.empty()) {
      LOG(WARNING) << "rewriting hashes in '" << path << "'; cross fingers";

      /* Canonicalise first.  This ensures that the path we're
         rewriting doesn't contain a hard link to /etc/shadow or
         something like that. */
      canonicalisePathMetaData(actualPath, buildUser ? buildUser->getUID() : -1,
                               inodesSeen);

      /* FIXME: this is in-memory. */
      StringSink sink;
      dumpPath(actualPath, sink);
      deletePath(actualPath);
      sink.s = make_ref<std::string>(rewriteStrings(*sink.s, outputRewrites));
      StringSource source(*sink.s);
      restorePath(actualPath, source);

      rewritten = true;
    }

    /* Check that fixed-output derivations produced the right
       outputs (i.e., the content hash should match the specified
       hash). */
    if (fixedOutput) {
      bool recursive;
      Hash h;
      i.second.parseHashInfo(recursive, h);

      if (!recursive) {
        /* The output path should be a regular file without
           execute permission. */
        if (!S_ISREG(st.st_mode) || (st.st_mode & S_IXUSR) != 0) {
          throw BuildError(
              format(
                  "output path '%1%' should be a non-executable regular file") %
              path);
        }
      }

      /* Check the hash. In hash mode, move the path produced by
         the derivation to its content-addressed location. */
      Hash h2 = recursive ? hashPath(h.type, actualPath).first
                          : hashFile(h.type, actualPath);

      Path dest = worker.store.makeFixedOutputPath(recursive, h2,
                                                   storePathToName(path));

      if (h != h2) {
        /* Throw an error after registering the path as
           valid. */
        worker.hashMismatch = true;
        delayedException = std::make_exception_ptr(
            BuildError("hash mismatch in fixed-output derivation '%s':\n  "
                       "wanted: %s\n  got:    %s",
                       dest, h.to_string(), h2.to_string()));

        Path actualDest = worker.store.toRealPath(dest);

        if (worker.store.isValidPath(dest)) {
          std::rethrow_exception(delayedException);
        }

        if (actualPath != actualDest) {
          PathLocks outputLocks({actualDest});
          deletePath(actualDest);
          if (rename(actualPath.c_str(), actualDest.c_str()) == -1) {
            throw SysError(format("moving '%1%' to '%2%'") % actualPath % dest);
          }
        }

        path = dest;
        actualPath = actualDest;
      } else {
        assert(path == dest);
      }

      info.ca = makeFixedOutputCA(recursive, h2);
    }

    /* Get rid of all weird permissions.  This also checks that
       all files are owned by the build user, if applicable. */
    canonicalisePathMetaData(actualPath,
                             buildUser && !rewritten ? buildUser->getUID() : -1,
                             inodesSeen);

    /* For this output path, find the references to other paths
       contained in it.  Compute the SHA-256 NAR hash at the same
       time.  The hash is stored in the database so that we can
       verify later on whether nobody has messed with the store. */
    DLOG(INFO) << "scanning for references inside '" << path << "'";
    HashResult hash;
    PathSet references = scanForReferences(actualPath, allPaths, hash);

    if (buildMode == bmCheck) {
      if (!worker.store.isValidPath(path)) {
        continue;
      }
      auto info = *worker.store.queryPathInfo(path);
      if (hash.first != info.narHash) {
        worker.checkMismatch = true;
        if (settings.runDiffHook || settings.keepFailed) {
          Path dst = worker.store.toRealPath(path + checkSuffix);
          deletePath(dst);
          if (rename(actualPath.c_str(), dst.c_str()) != 0) {
            throw SysError(format("renaming '%1%' to '%2%'") % actualPath %
                           dst);
          }

          handleDiffHook(buildUser ? buildUser->getUID() : getuid(),
                         buildUser ? buildUser->getGID() : getgid(), path, dst,
                         drvPath, tmpDir);

          throw NotDeterministic(
              format("derivation '%1%' may not be deterministic: output '%2%' "
                     "differs from '%3%'") %
              drvPath % path % dst);
        }
        throw NotDeterministic(format("derivation '%1%' may not be "
                                      "deterministic: output '%2%' differs") %
                               drvPath % path);
      }

      /* Since we verified the build, it's now ultimately
         trusted. */
      if (!info.ultimate) {
        info.ultimate = true;
        worker.store.signPathInfo(info);
        worker.store.registerValidPaths({info});
      }

      continue;
    }

    /* For debugging, print out the referenced and unreferenced
       paths. */
    for (auto& i : inputPaths) {
      auto j = references.find(i);
      if (j == references.end()) {
        DLOG(INFO) << "unreferenced input: '" << i << "'";
      } else {
        DLOG(INFO) << "referenced input: '" << i << "'";
      }
    }

    if (curRound == nrRounds) {
      worker.store.optimisePath(
          actualPath);  // FIXME: combine with scanForReferences()
      worker.markContentsGood(path);
    }

    info.path = path;
    info.narHash = hash.first;
    info.narSize = hash.second;
    info.references = references;
    info.deriver = drvPath;
    info.ultimate = true;
    worker.store.signPathInfo(info);

    if (!info.references.empty()) {
      info.ca.clear();
    }

    infos[i.first] = info;
  }

  if (buildMode == bmCheck) {
    return;
  }

  /* Apply output checks. */
  checkOutputs(infos);

  /* Compare the result with the previous round, and report which
     path is different, if any.*/
  if (curRound > 1 && prevInfos != infos) {
    assert(prevInfos.size() == infos.size());
    for (auto i = prevInfos.begin(), j = infos.begin(); i != prevInfos.end();
         ++i, ++j) {
      if (!(*i == *j)) {
        result.isNonDeterministic = true;
        Path prev = i->second.path + checkSuffix;
        bool prevExists = keepPreviousRound && pathExists(prev);
        auto msg =
            prevExists
                ? fmt("output '%1%' of '%2%' differs from '%3%' from previous "
                      "round",
                      i->second.path, drvPath, prev)
                : fmt("output '%1%' of '%2%' differs from previous round",
                      i->second.path, drvPath);

        handleDiffHook(buildUser ? buildUser->getUID() : getuid(),
                       buildUser ? buildUser->getGID() : getgid(), prev,
                       i->second.path, drvPath, tmpDir);

        if (settings.enforceDeterminism) {
          throw NotDeterministic(msg);
        }

        LOG(ERROR) << msg;
        curRound = nrRounds;  // we know enough, bail out early
      }
    }
  }

  /* If this is the first round of several, then move the output out
     of the way. */
  if (nrRounds > 1 && curRound == 1 && curRound < nrRounds &&
      keepPreviousRound) {
    for (auto& i : drv->outputs) {
      Path prev = i.second.path + checkSuffix;
      deletePath(prev);
      Path dst = i.second.path + checkSuffix;
      if (rename(i.second.path.c_str(), dst.c_str()) != 0) {
        throw SysError(format("renaming '%1%' to '%2%'") % i.second.path % dst);
      }
    }
  }

  if (curRound < nrRounds) {
    prevInfos = infos;
    return;
  }

  /* Remove the .check directories if we're done. FIXME: keep them
     if the result was not determistic? */
  if (curRound == nrRounds) {
    for (auto& i : drv->outputs) {
      Path prev = i.second.path + checkSuffix;
      deletePath(prev);
    }
  }

  /* Register each output path as valid, and register the sets of
     paths referenced by each of them.  If there are cycles in the
     outputs, this will fail. */
  {
    ValidPathInfos infos2;
    for (auto& i : infos) {
      infos2.push_back(i.second);
    }
    worker.store.registerValidPaths(infos2);
  }

  /* In case of a fixed-output derivation hash mismatch, throw an
     exception now that we have registered the output as valid. */
  if (delayedException) {
    std::rethrow_exception(delayedException);
  }
}

void DerivationGoal::checkOutputs(
    const std::map<Path, ValidPathInfo>& outputs) {
  std::map<Path, const ValidPathInfo&> outputsByPath;
  for (auto& output : outputs) {
    outputsByPath.emplace(output.second.path, output.second);
  }

  for (auto& output : outputs) {
    auto& outputName = output.first;
    auto& info = output.second;

    struct Checks {
      bool ignoreSelfRefs = false;
      std::optional<uint64_t> maxSize, maxClosureSize;
      std::optional<Strings> allowedReferences, allowedRequisites,
          disallowedReferences, disallowedRequisites;
    };

    /* Compute the closure and closure size of some output. This
       is slightly tricky because some of its references (namely
       other outputs) may not be valid yet. */
    auto getClosure = [&](const Path& path) {
      uint64_t closureSize = 0;
      PathSet pathsDone;
      std::queue<Path> pathsLeft;
      pathsLeft.push(path);

      while (!pathsLeft.empty()) {
        auto path = pathsLeft.front();
        pathsLeft.pop();
        if (!pathsDone.insert(path).second) {
          continue;
        }

        auto i = outputsByPath.find(path);
        if (i != outputsByPath.end()) {
          closureSize += i->second.narSize;
          for (auto& ref : i->second.references) {
            pathsLeft.push(ref);
          }
        } else {
          auto info = worker.store.queryPathInfo(path);
          closureSize += info->narSize;
          for (auto& ref : info->references) {
            pathsLeft.push(ref);
          }
        }
      }

      return std::make_pair(pathsDone, closureSize);
    };

    auto applyChecks = [&](const Checks& checks) {
      if (checks.maxSize && info.narSize > *checks.maxSize) {
        throw BuildError(
            "path '%s' is too large at %d bytes; limit is %d bytes", info.path,
            info.narSize, *checks.maxSize);
      }

      if (checks.maxClosureSize) {
        uint64_t closureSize = getClosure(info.path).second;
        if (closureSize > *checks.maxClosureSize) {
          throw BuildError(
              "closure of path '%s' is too large at %d bytes; limit is %d "
              "bytes",
              info.path, closureSize, *checks.maxClosureSize);
        }
      }

      auto checkRefs = [&](const std::optional<Strings>& value, bool allowed,
                           bool recursive) {
        if (!value) {
          return;
        }

        PathSet spec = parseReferenceSpecifiers(worker.store, *drv, *value);

        PathSet used =
            recursive ? getClosure(info.path).first : info.references;

        if (recursive && checks.ignoreSelfRefs) {
          used.erase(info.path);
        }

        PathSet badPaths;

        for (auto& i : used) {
          if (allowed) {
            if (spec.count(i) == 0u) {
              badPaths.insert(i);
            }
          } else {
            if (spec.count(i) != 0u) {
              badPaths.insert(i);
            }
          }
        }

        if (!badPaths.empty()) {
          std::string badPathsStr;
          for (auto& i : badPaths) {
            badPathsStr += "\n  ";
            badPathsStr += i;
          }
          throw BuildError(
              "output '%s' is not allowed to refer to the following paths:%s",
              info.path, badPathsStr);
        }
      };

      checkRefs(checks.allowedReferences, true, false);
      checkRefs(checks.allowedRequisites, true, true);
      checkRefs(checks.disallowedReferences, false, false);
      checkRefs(checks.disallowedRequisites, false, true);
    };

    if (auto structuredAttrs = parsedDrv->getStructuredAttrs()) {
      auto outputChecks = structuredAttrs->find("outputChecks");
      if (outputChecks != structuredAttrs->end()) {
        auto output = outputChecks->find(outputName);

        if (output != outputChecks->end()) {
          Checks checks;

          auto maxSize = output->find("maxSize");
          if (maxSize != output->end()) {
            checks.maxSize = maxSize->get<uint64_t>();
          }

          auto maxClosureSize = output->find("maxClosureSize");
          if (maxClosureSize != output->end()) {
            checks.maxClosureSize = maxClosureSize->get<uint64_t>();
          }

          auto get = [&](const std::string& name) -> std::optional<Strings> {
            auto i = output->find(name);
            if (i != output->end()) {
              Strings res;
              for (auto& j : *i) {
                if (!j.is_string()) {
                  throw Error(
                      "attribute '%s' of derivation '%s' must be a list of "
                      "strings",
                      name, drvPath);
                }
                res.push_back(j.get<std::string>());
              }
              checks.disallowedRequisites = res;
              return res;
            }
            return {};
          };

          checks.allowedReferences = get("allowedReferences");
          checks.allowedRequisites = get("allowedRequisites");
          checks.disallowedReferences = get("disallowedReferences");
          checks.disallowedRequisites = get("disallowedRequisites");

          applyChecks(checks);
        }
      }
    } else {
      // legacy non-structured-attributes case
      Checks checks;
      checks.ignoreSelfRefs = true;
      checks.allowedReferences = parsedDrv->getStringsAttr("allowedReferences");
      checks.allowedRequisites = parsedDrv->getStringsAttr("allowedRequisites");
      checks.disallowedReferences =
          parsedDrv->getStringsAttr("disallowedReferences");
      checks.disallowedRequisites =
          parsedDrv->getStringsAttr("disallowedRequisites");
      applyChecks(checks);
    }
  }
}

Path DerivationGoal::openLogFile() {
  logSize = 0;

  if (!settings.keepLog) {
    return "";
  }

  std::string baseName = baseNameOf(drvPath);

  /* Create a log file. */
  Path dir = fmt("%s/%s/%s/", worker.store.logDir, nix::LocalStore::drvsLogDir,
                 std::string(baseName, 0, 2));
  createDirs(dir);

  Path logFileName = fmt("%s/%s%s", dir, std::string(baseName, 2),
                         settings.compressLog ? ".bz2" : "");

  fdLogFile = AutoCloseFD(open(logFileName.c_str(),
                               O_CREAT | O_WRONLY | O_TRUNC | O_CLOEXEC, 0666));
  if (!fdLogFile) {
    throw SysError(format("creating log file '%1%'") % logFileName);
  }

  logFileSink = std::make_shared<FdSink>(fdLogFile.get());

  if (settings.compressLog) {
    logSink = std::shared_ptr<CompressionSink>(
        makeCompressionSink("bzip2", *logFileSink));
  } else {
    logSink = logFileSink;
  }

  return logFileName;
}

void DerivationGoal::closeLogFile() {
  auto logSink2 = std::dynamic_pointer_cast<CompressionSink>(logSink);
  if (logSink2) {
    logSink2->finish();
  }
  if (logFileSink) {
    logFileSink->flush();
  }
  logSink = logFileSink = nullptr;
  fdLogFile = AutoCloseFD(-1);
}

void DerivationGoal::deleteTmpDir(bool force) {
  if (!tmpDir.empty()) {
    /* Don't keep temporary directories for builtins because they
       might have privileged stuff (like a copy of netrc). */
    if (settings.keepFailed && !force && !drv->isBuiltin()) {
      LOG(INFO) << "note: keeping build directory '" << tmpDir << "'";
      chmod(tmpDir.c_str(), 0755);
    } else {
      deletePath(tmpDir);
    }
    tmpDir = "";
  }
}

// TODO(tazjin): What ... what does this function ... do?
void DerivationGoal::handleChildOutput(int fd, const std::string& data) {
  if ((hook && fd == hook->builderOut.readSide.get()) ||
      (!hook && fd == builderOut.readSide.get())) {
    logSize += data.size();
    if (settings.maxLogSize && logSize > settings.maxLogSize) {
      LOG(ERROR) << getName()
                 << " killed after writing more than %2% bytes of log output"
                 << settings.maxLogSize;
      killChild();
      done(BuildResult::LogLimitExceeded);
      return;
    }

    for (auto c : data) {
      if (c == '\r') {
        currentLogLinePos = 0;
      } else if (c == '\n') {
        flushLine();
      } else {
        if (currentLogLinePos >= currentLogLine.size()) {
          currentLogLine.resize(currentLogLinePos + 1);
        }
        currentLogLine[currentLogLinePos++] = c;
      }
    }

    if (logSink) {
      (*logSink)(data);
    }
  }

  if (hook && fd == hook->fromHook.readSide.get()) {
    for (auto c : data) {
      if (c == '\n') {
        currentHookLine.clear();
      } else {
        currentHookLine += c;
      }
    }
  }
}

void DerivationGoal::handleEOF(int /* fd */) {
  if (!currentLogLine.empty()) {
    flushLine();
  }
  worker.wakeUp(shared_from_this());
}

void DerivationGoal::flushLine() {
  if (settings.verboseBuild &&
      (settings.printRepeatedBuilds || curRound == 1)) {
    log_sink_ << absl::StrCat(currentLogLine, "\n");
  } else {
    logTail.push_back(currentLogLine);
    if (logTail.size() > settings.logLines) {
      logTail.pop_front();
    }
  }

  currentLogLine = "";
  currentLogLinePos = 0;
}

PathSet DerivationGoal::checkPathValidity(bool returnValid, bool checkHash) {
  PathSet result;
  for (auto& i : drv->outputs) {
    if (!wantOutput(i.first, wantedOutputs)) {
      continue;
    }
    bool good = worker.store.isValidPath(i.second.path) &&
                (!checkHash || worker.pathContentsGood(i.second.path));
    if (good == returnValid) {
      result.insert(i.second.path);
    }
  }
  return result;
}

Path DerivationGoal::addHashRewrite(const Path& path) {
  std::string h1 = std::string(path, worker.store.storeDir.size() + 1, 32);
  std::string h2 =
      std::string(hashString(htSHA256, "rewrite:" + drvPath + ":" + path)
                      .to_string(Base32, false),
                  0, 32);
  Path p = worker.store.storeDir + "/" + h2 +
           std::string(path, worker.store.storeDir.size() + 33);
  deletePath(p);
  assert(path.size() == p.size());
  inputRewrites[h1] = h2;
  outputRewrites[h2] = h1;
  redirectedOutputs[path] = p;
  return p;
}

void DerivationGoal::done(BuildResult::Status status, const std::string& msg) {
  result.status = status;
  result.errorMsg = msg;
  amDone(result.success() ? ecSuccess : ecFailed);
  if (result.status == BuildResult::TimedOut) {
    worker.timedOut = true;
  }
  if (result.status == BuildResult::PermanentFailure) {
    worker.permanentFailure = true;
  }

  mcExpectedBuilds.reset();
  mcRunningBuilds.reset();

  if (result.success()) {
    if (status == BuildResult::Built) {
      worker.doneBuilds++;
    }
  } else {
    if (status != BuildResult::DependencyFailed) {
      worker.failedBuilds++;
    }
  }
}

//////////////////////////////////////////////////////////////////////

class SubstitutionGoal : public Goal {
  friend class Worker;

 private:
  /* The store path that should be realised through a substitute. */
  Path storePath;

  /* The remaining substituters. */
  std::list<ref<Store>> subs;

  /* The current substituter. */
  std::shared_ptr<Store> sub;

  /* Whether a substituter failed. */
  bool substituterFailed = false;

  /* Path info returned by the substituter's query info operation. */
  std::shared_ptr<const ValidPathInfo> info;

  /* Pipe for the substituter's standard output. */
  Pipe outPipe;

  /* The substituter thread. */
  std::thread thr;

  std::promise<void> promise;

  /* Whether to try to repair a valid path. */
  RepairFlag repair;

  /* Location where we're downloading the substitute.  Differs from
     storePath when doing a repair. */
  Path destPath;

  std::unique_ptr<MaintainCount<uint64_t>> maintainExpectedSubstitutions,
      maintainRunningSubstitutions, maintainExpectedNar,
      maintainExpectedDownload;

  using GoalState = void (SubstitutionGoal::*)();
  GoalState state;

 public:
  SubstitutionGoal(Worker& worker, std::ostream& log_sink,
                   const Path& storePath, RepairFlag repair = NoRepair);

  ~SubstitutionGoal() override;

  void timedOut() override { abort(); };

  std::string key() override {
    /* "a$" ensures substitution goals happen before derivation
       goals. */
    return "a$" + storePathToName(storePath) + "$" + storePath;
  }

  void work() override;

  /* The states. */
  void init();
  void tryNext();
  void gotInfo();
  void referencesValid();
  void tryToRun();
  void finished();

  /* Callback used by the worker to write to the log. */
  void handleChildOutput(int fd, const std::string& data) override;
  void handleEOF(int fd) override;

  Path getStorePath() { return storePath; }

  void amDone(ExitCode result) override { Goal::amDone(result); }
};

SubstitutionGoal::SubstitutionGoal(Worker& worker, std::ostream& log_sink,
                                   const Path& storePath, RepairFlag repair)
    : Goal(worker, log_sink), repair(repair) {
  this->storePath = storePath;
  state = &SubstitutionGoal::init;
  name = absl::StrCat("substitution of ", storePath);
  trace("created");
  maintainExpectedSubstitutions =
      std::make_unique<MaintainCount<uint64_t>>(worker.expectedSubstitutions);
}

SubstitutionGoal::~SubstitutionGoal() {
  try {
    if (thr.joinable()) {
      // FIXME: signal worker thread to quit.
      thr.join();
      worker.childTerminated(this);
    }
  } catch (...) {
    ignoreException();
  }
}

void SubstitutionGoal::work() { (this->*state)(); }

void SubstitutionGoal::init() {
  trace("init");

  worker.store.addTempRoot(storePath);

  /* If the path already exists we're done. */
  if ((repair == 0u) && worker.store.isValidPath(storePath)) {
    amDone(ecSuccess);
    return;
  }

  if (settings.readOnlyMode) {
    throw Error(
        format(
            "cannot substitute path '%1%' - no write access to the Nix store") %
        storePath);
  }

  subs = settings.useSubstitutes ? getDefaultSubstituters()
                                 : std::list<ref<Store>>();

  tryNext();
}

void SubstitutionGoal::tryNext() {
  trace("trying next substituter");

  if (subs.empty()) {
    /* None left.  Terminate this goal and let someone else deal
       with it. */
    DLOG(WARNING)
        << "path '" << storePath
        << "' is required, but there is no substituter that can build it";

    /* Hack: don't indicate failure if there were no substituters.
       In that case the calling derivation should just do a
       build. */
    amDone(substituterFailed ? ecFailed : ecNoSubstituters);

    if (substituterFailed) {
      worker.failedSubstitutions++;
    }

    return;
  }

  sub = subs.front();
  subs.pop_front();

  if (sub->storeDir != worker.store.storeDir) {
    tryNext();
    return;
  }

  try {
    // FIXME: make async
    info = sub->queryPathInfo(storePath);
  } catch (InvalidPath&) {
    tryNext();
    return;
  } catch (SubstituterDisabled&) {
    if (settings.tryFallback) {
      tryNext();
      return;
    }
    throw;
  } catch (Error& e) {
    if (settings.tryFallback) {
      LOG(ERROR) << e.what();
      tryNext();
      return;
    }
    throw;
  }

  /* Update the total expected download size. */
  auto narInfo = std::dynamic_pointer_cast<const NarInfo>(info);

  maintainExpectedNar = std::make_unique<MaintainCount<uint64_t>>(
      worker.expectedNarSize, info->narSize);

  maintainExpectedDownload =
      narInfo && (narInfo->fileSize != 0u)
          ? std::make_unique<MaintainCount<uint64_t>>(
                worker.expectedDownloadSize, narInfo->fileSize)
          : nullptr;

  /* Bail out early if this substituter lacks a valid
     signature. LocalStore::addToStore() also checks for this, but
     only after we've downloaded the path. */
  if (worker.store.requireSigs && !sub->isTrusted &&
      (info->checkSignatures(worker.store, worker.store.getPublicKeys()) ==
       0u)) {
    LOG(WARNING) << "substituter '" << sub->getUri()
                 << "' does not have a valid signature for path '" << storePath
                 << "'";
    tryNext();
    return;
  }

  /* To maintain the closure invariant, we first have to realise the
     paths referenced by this one. */
  for (auto& i : info->references) {
    if (i != storePath) { /* ignore self-references */
      addWaitee(worker.makeSubstitutionGoal(log_sink_, i));
    }
  }

  if (waitees.empty()) { /* to prevent hang (no wake-up event) */
    referencesValid();
  } else {
    state = &SubstitutionGoal::referencesValid;
  }
}

void SubstitutionGoal::referencesValid() {
  trace("all references realised");

  if (nrFailed > 0) {
    DLOG(WARNING) << "some references of path '" << storePath
                  << "' could not be realised";
    amDone(nrNoSubstituters > 0 || nrIncompleteClosure > 0 ? ecIncompleteClosure
                                                           : ecFailed);
    return;
  }

  for (auto& i : info->references) {
    if (i != storePath) { /* ignore self-references */
      assert(worker.store.isValidPath(i));
    }
  }

  state = &SubstitutionGoal::tryToRun;
  worker.wakeUp(shared_from_this());
}

void SubstitutionGoal::tryToRun() {
  trace("trying to run");

  /* Make sure that we are allowed to start a build.  Note that even
     if maxBuildJobs == 0 (no local builds allowed), we still allow
     a substituter to run.  This is because substitutions cannot be
     distributed to another machine via the build hook. */
  if (worker.getNrLocalBuilds() >=
      std::max(1U, (unsigned int)settings.maxBuildJobs)) {
    worker.waitForBuildSlot(shared_from_this());
    return;
  }

  maintainRunningSubstitutions =
      std::make_unique<MaintainCount<uint64_t>>(worker.runningSubstitutions);

  outPipe.create();

  promise = std::promise<void>();

  thr = std::thread([this]() {
    try {
      /* Wake up the worker loop when we're done. */
      Finally updateStats([this]() { outPipe.writeSide = AutoCloseFD(-1); });

      copyStorePath(ref<Store>(sub),
                    ref<Store>(worker.store.shared_from_this()), storePath,
                    repair, sub->isTrusted ? NoCheckSigs : CheckSigs);

      promise.set_value();
    } catch (...) {
      promise.set_exception(std::current_exception());
    }
  });

  worker.childStarted(shared_from_this(), {outPipe.readSide.get()}, true,
                      false);

  state = &SubstitutionGoal::finished;
}

void SubstitutionGoal::finished() {
  trace("substitute finished");

  thr.join();
  worker.childTerminated(this);

  try {
    promise.get_future().get();
  } catch (std::exception& e) {
    LOG(ERROR) << e.what();

    /* Cause the parent build to fail unless --fallback is given,
       or the substitute has disappeared. The latter case behaves
       the same as the substitute never having existed in the
       first place. */
    try {
      throw;
    } catch (SubstituteGone&) {
    } catch (...) {
      substituterFailed = true;
    }

    /* Try the next substitute. */
    state = &SubstitutionGoal::tryNext;
    worker.wakeUp(shared_from_this());
    return;
  }

  worker.markContentsGood(storePath);

  DLOG(INFO) << "substitution of path '" << storePath << "' succeeded";

  maintainRunningSubstitutions.reset();

  maintainExpectedSubstitutions.reset();
  worker.doneSubstitutions++;

  if (maintainExpectedDownload) {
    auto fileSize = maintainExpectedDownload->delta;
    maintainExpectedDownload.reset();
    worker.doneDownloadSize += fileSize;
  }

  worker.doneNarSize += maintainExpectedNar->delta;
  maintainExpectedNar.reset();

  amDone(ecSuccess);
}

void SubstitutionGoal::handleChildOutput(int fd, const std::string& data) {}

void SubstitutionGoal::handleEOF(int fd) {
  if (fd == outPipe.readSide.get()) {
    worker.wakeUp(shared_from_this());
  }
}

//////////////////////////////////////////////////////////////////////

static bool working = false;

Worker::Worker(LocalStore& store) : store(store) {
  /* Debugging: prevent recursive workers. */
  if (working) {
    abort();
  }
  working = true;
  nrLocalBuilds = 0;
  lastWokenUp = steady_time_point::min();
  permanentFailure = false;
  timedOut = false;
  hashMismatch = false;
  checkMismatch = false;
}

Worker::~Worker() {
  working = false;

  /* Explicitly get rid of all strong pointers now.  After this all
     goals that refer to this worker should be gone.  (Otherwise we
     are in trouble, since goals may call childTerminated() etc. in
     their destructors). */
  topGoals.clear();

  assert(expectedSubstitutions == 0);
  assert(expectedDownloadSize == 0);
  assert(expectedNarSize == 0);
}

GoalPtr Worker::makeDerivationGoal(std::ostream& log_sink, const Path& drv_path,
                                   const StringSet& wantedOutputs,
                                   BuildMode buildMode) {
  GoalPtr goal = derivationGoals[drv_path].lock();
  if (!goal) {
    goal = std::make_shared<DerivationGoal>(*this, log_sink, drv_path,
                                            wantedOutputs, buildMode);
    derivationGoals[drv_path] = goal;
    wakeUp(goal);
  } else {
    (dynamic_cast<DerivationGoal*>(goal.get()))
        ->addWantedOutputs(wantedOutputs);
  }
  return goal;
}

std::shared_ptr<DerivationGoal> Worker::makeBasicDerivationGoal(
    std::ostream& log_sink, const Path& drvPath, const BasicDerivation& drv,
    BuildMode buildMode) {
  std::shared_ptr<DerivationGoal> goal = std::make_shared<DerivationGoal>(
      *this, log_sink, drvPath, drv, buildMode);
  wakeUp(goal);
  return goal;
}

GoalPtr Worker::makeSubstitutionGoal(std::ostream& log_sink, const Path& path,
                                     RepairFlag repair) {
  GoalPtr goal = substitutionGoals[path].lock();
  if (!goal) {
    goal = std::make_shared<SubstitutionGoal>(*this, log_sink, path, repair);
    substitutionGoals[path] = goal;
    wakeUp(goal);
  }
  return goal;
}

static void removeGoal(const GoalPtr& goal, WeakGoalMap& goalMap) {
  /* !!! inefficient */
  for (auto i = goalMap.begin(); i != goalMap.end();) {
    if (i->second.lock() == goal) {
      auto j = i;
      ++j;
      goalMap.erase(i);
      i = j;
    } else {
      ++i;
    }
  }
}

void Worker::removeGoal(const GoalPtr& goal) {
  nix::removeGoal(goal, derivationGoals);
  nix::removeGoal(goal, substitutionGoals);
  if (topGoals.find(goal) != topGoals.end()) {
    topGoals.erase(goal);
    /* If a top-level goal failed, then kill all other goals
       (unless keepGoing was set). */
    if (goal->getExitCode() == Goal::ecFailed && !settings.keepGoing) {
      topGoals.clear();
    }
  }

  /* Wake up goals waiting for any goal to finish. */
  for (auto& i : waitingForAnyGoal) {
    GoalPtr goal = i.lock();
    if (goal) {
      wakeUp(goal);
    }
  }

  waitingForAnyGoal.clear();
}

void Worker::wakeUp(const GoalPtr& goal) {
  goal->trace("woken up");
  addToWeakGoals(awake, goal);
}

unsigned Worker::getNrLocalBuilds() { return nrLocalBuilds; }

void Worker::childStarted(const GoalPtr& goal, const std::set<int>& fds,
                          bool inBuildSlot, bool respectTimeouts) {
  Child child;
  child.goal = goal;
  child.goal2 = goal.get();
  child.fds = fds;
  child.timeStarted = child.lastOutput = steady_time_point::clock::now();
  child.inBuildSlot = inBuildSlot;
  child.respectTimeouts = respectTimeouts;
  children.emplace_back(child);
  if (inBuildSlot) {
    nrLocalBuilds++;
  }
}

void Worker::childTerminated(Goal* goal, bool wakeSleepers) {
  auto i =
      std::find_if(children.begin(), children.end(),
                   [&](const Child& child) { return child.goal2 == goal; });
  if (i == children.end()) {
    return;
  }

  if (i->inBuildSlot) {
    assert(nrLocalBuilds > 0);
    nrLocalBuilds--;
  }

  children.erase(i);

  if (wakeSleepers) {
    /* Wake up goals waiting for a build slot. */
    for (auto& j : wantingToBuild) {
      GoalPtr goal = j.lock();
      if (goal) {
        wakeUp(goal);
      }
    }

    wantingToBuild.clear();
  }
}

void Worker::waitForBuildSlot(const GoalPtr& goal) {
  DLOG(INFO) << "wait for build slot";
  if (getNrLocalBuilds() < settings.maxBuildJobs) {
    wakeUp(goal); /* we can do it right away */
  } else {
    addToWeakGoals(wantingToBuild, goal);
  }
}

void Worker::waitForAnyGoal(GoalPtr goal) {
  DLOG(INFO) << "wait for any goal";
  addToWeakGoals(waitingForAnyGoal, std::move(goal));
}

void Worker::waitForAWhile(GoalPtr goal) {
  DLOG(INFO) << "wait for a while";
  addToWeakGoals(waitingForAWhile, std::move(goal));
}

void Worker::run(const Goals& _topGoals) {
  for (auto& i : _topGoals) {
    topGoals.insert(i);
  }

  DLOG(INFO) << "entered goal loop";

  while (true) {
    checkInterrupt();

    store.autoGC(false);

    /* Call every wake goal (in the ordering established by
       CompareGoalPtrs). */
    while (!awake.empty() && !topGoals.empty()) {
      Goals awake2;
      for (auto& i : awake) {
        GoalPtr goal = i.lock();
        if (goal) {
          awake2.insert(goal);
        }
      }
      awake.clear();
      for (auto& goal : awake2) {
        checkInterrupt();
        goal->work();
        if (topGoals.empty()) {
          break;
        }  // stuff may have been cancelled
      }
    }

    if (topGoals.empty()) {
      break;
    }

    /* Wait for input. */
    if (!children.empty() || !waitingForAWhile.empty()) {
      waitForInput();
    } else {
      if (awake.empty() && 0 == settings.maxBuildJobs) {
        throw Error(
            "unable to start any build; either increase '--max-jobs' "
            "or enable remote builds");
      }
      assert(!awake.empty());
    }
  }

  /* If --keep-going is not set, it's possible that the main goal
     exited while some of its subgoals were still active.  But if
     --keep-going *is* set, then they must all be finished now. */
  assert(!settings.keepGoing || awake.empty());
  assert(!settings.keepGoing || wantingToBuild.empty());
  assert(!settings.keepGoing || children.empty());
}

void Worker::waitForInput() {
  DLOG(INFO) << "waiting for children";

  /* Process output from the file descriptors attached to the
     children, namely log output and output path creation commands.
     We also use this to detect child termination: if we get EOF on
     the logger pipe of a build, we assume that the builder has
     terminated. */

  bool useTimeout = false;
  struct timeval timeout;
  timeout.tv_usec = 0;
  auto before = steady_time_point::clock::now();

  /* If we're monitoring for silence on stdout/stderr, or if there
     is a build timeout, then wait for input until the first
     deadline for any child. */
  auto nearest = steady_time_point::max();  // nearest deadline
  if (settings.minFree.get() != 0) {
    // Periodicallty wake up to see if we need to run the garbage collector.
    nearest = before + std::chrono::seconds(10);
  }
  for (auto& i : children) {
    if (!i.respectTimeouts) {
      continue;
    }
    if (0 != settings.maxSilentTime) {
      nearest = std::min(
          nearest, i.lastOutput + std::chrono::seconds(settings.maxSilentTime));
    }
    if (0 != settings.buildTimeout) {
      nearest = std::min(
          nearest, i.timeStarted + std::chrono::seconds(settings.buildTimeout));
    }
  }
  if (nearest != steady_time_point::max()) {
    timeout.tv_sec = std::max(
        1L, static_cast<long>(std::chrono::duration_cast<std::chrono::seconds>(
                                  nearest - before)
                                  .count()));
    useTimeout = true;
  }

  /* If we are polling goals that are waiting for a lock, then wake
     up after a few seconds at most. */
  if (!waitingForAWhile.empty()) {
    useTimeout = true;
    if (lastWokenUp == steady_time_point::min()) {
      DLOG(WARNING) << "waiting for locks or build slots...";
    }
    if (lastWokenUp == steady_time_point::min() || lastWokenUp > before) {
      lastWokenUp = before;
    }
    timeout.tv_sec = std::max(
        1L, static_cast<long>(std::chrono::duration_cast<std::chrono::seconds>(
                                  lastWokenUp +
                                  std::chrono::seconds(settings.pollInterval) -
                                  before)
                                  .count()));
  } else {
    lastWokenUp = steady_time_point::min();
  }

  if (useTimeout) {
    DLOG(INFO) << "sleeping " << timeout.tv_sec << " seconds";
  }

  /* Use select() to wait for the input side of any logger pipe to
     become `available'.  Note that `available' (i.e., non-blocking)
     includes EOF. */
  fd_set fds;
  FD_ZERO(&fds);
  int fdMax = 0;
  for (auto& i : children) {
    for (auto& j : i.fds) {
      if (j >= FD_SETSIZE) {
        throw Error("reached FD_SETSIZE limit");
      }
      FD_SET(j, &fds);
      if (j >= fdMax) {
        fdMax = j + 1;
      }
    }
  }

  if (select(fdMax, &fds, nullptr, nullptr, useTimeout ? &timeout : nullptr) ==
      -1) {
    if (errno == EINTR) {
      return;
    }
    throw SysError("waiting for input");
  }

  auto after = steady_time_point::clock::now();

  /* Process all available file descriptors. FIXME: this is
     O(children * fds). */
  decltype(children)::iterator i;
  for (auto j = children.begin(); j != children.end(); j = i) {
    i = std::next(j);

    checkInterrupt();

    GoalPtr goal = j->goal.lock();
    assert(goal);

    std::set<int> fds2(j->fds);
    std::vector<unsigned char> buffer(4096);
    for (auto& k : fds2) {
      if (FD_ISSET(k, &fds)) {
        ssize_t rd = read(k, buffer.data(), buffer.size());
        // FIXME: is there a cleaner way to handle pt close
        // than EIO? Is this even standard?
        if (rd == 0 || (rd == -1 && errno == EIO)) {
          DLOG(WARNING) << goal->getName() << ": got EOF";
          goal->handleEOF(k);
          j->fds.erase(k);
        } else if (rd == -1) {
          if (errno != EINTR) {
            throw SysError("%s: read failed", goal->getName());
          }
        } else {
          DLOG(INFO) << goal->getName() << ": read " << rd << " bytes";
          std::string data(reinterpret_cast<char*>(buffer.data()), rd);
          j->lastOutput = after;
          goal->handleChildOutput(k, data);
        }
      }
    }

    if (goal->getExitCode() == Goal::ecBusy && 0 != settings.maxSilentTime &&
        j->respectTimeouts &&
        after - j->lastOutput >= std::chrono::seconds(settings.maxSilentTime)) {
      LOG(ERROR) << goal->getName() << " timed out after "
                 << settings.maxSilentTime << " seconds of silence";
      goal->timedOut();
    }

    else if (goal->getExitCode() == Goal::ecBusy &&
             0 != settings.buildTimeout && j->respectTimeouts &&
             after - j->timeStarted >=
                 std::chrono::seconds(settings.buildTimeout)) {
      LOG(ERROR) << goal->getName() << " timed out after "
                 << settings.buildTimeout << " seconds";
      goal->timedOut();
    }
  }

  if (!waitingForAWhile.empty() &&
      lastWokenUp + std::chrono::seconds(settings.pollInterval) <= after) {
    lastWokenUp = after;
    for (auto& i : waitingForAWhile) {
      GoalPtr goal = i.lock();
      if (goal) {
        wakeUp(goal);
      }
    }
    waitingForAWhile.clear();
  }
}

unsigned int Worker::exitStatus() {
  /*
   * 1100100
   *    ^^^^
   *    |||`- timeout
   *    ||`-- output hash mismatch
   *    |`--- build failure
   *    `---- not deterministic
   */
  unsigned int mask = 0;
  bool buildFailure = permanentFailure || timedOut || hashMismatch;
  if (buildFailure) {
    mask |= 0x04;  // 100
  }
  if (timedOut) {
    mask |= 0x01;  // 101
  }
  if (hashMismatch) {
    mask |= 0x02;  // 102
  }
  if (checkMismatch) {
    mask |= 0x08;  // 104
  }

  if (mask != 0u) {
    mask |= 0x60;
  }
  return mask != 0u ? mask : 1;
}

bool Worker::pathContentsGood(const Path& path) {
  auto i = pathContentsGoodCache.find(path);
  if (i != pathContentsGoodCache.end()) {
    return i->second;
  }
  LOG(INFO) << "checking path '" << path << "'...";
  auto info = store.queryPathInfo(path);
  bool res;
  if (!pathExists(path)) {
    res = false;
  } else {
    HashResult current = hashPath(info->narHash.type, path);
    Hash nullHash(htSHA256);
    res = info->narHash == nullHash || info->narHash == current.first;
  }
  pathContentsGoodCache[path] = res;
  if (!res) {
    LOG(ERROR) << "path '" << path << "' is corrupted or missing!";
  }
  return res;
}

void Worker::markContentsGood(const Path& path) {
  pathContentsGoodCache[path] = true;
}

//////////////////////////////////////////////////////////////////////

static void primeCache(Store& store, const PathSet& paths) {
  PathSet willBuild;
  PathSet willSubstitute;
  PathSet unknown;
  unsigned long long downloadSize;
  unsigned long long narSize;
  store.queryMissing(paths, willBuild, willSubstitute, unknown, downloadSize,
                     narSize);

  if (!willBuild.empty() && 0 == settings.maxBuildJobs &&
      getMachines().empty()) {
    throw Error(
        "%d derivations need to be built, but neither local builds "
        "('--max-jobs') "
        "nor remote builds ('--builders') are enabled",
        willBuild.size());
  }
}

absl::Status LocalStore::buildPaths(std::ostream& log_sink,
                                    const PathSet& drvPaths,
                                    BuildMode build_mode) {
  Worker worker(*this);

  primeCache(*this, drvPaths);

  Goals goals;
  for (auto& i : drvPaths) {
    DrvPathWithOutputs i2 = parseDrvPathWithOutputs(i);
    if (isDerivation(i2.first)) {
      goals.insert(
          worker.makeDerivationGoal(log_sink, i2.first, i2.second, build_mode));
    } else {
      goals.insert(worker.makeSubstitutionGoal(
          log_sink, i, build_mode == bmRepair ? Repair : NoRepair));
    }
  }

  worker.run(goals);

  PathSet failed;
  for (auto& i : goals) {
    if (i->getExitCode() != Goal::ecSuccess) {
      auto* i2 = dynamic_cast<DerivationGoal*>(i.get());
      if (i2 != nullptr) {
        failed.insert(i2->getDrvPath());
      } else {
        failed.insert(dynamic_cast<SubstitutionGoal*>(i.get())->getStorePath());
      }
    }
  }

  if (!failed.empty()) {
    return absl::Status(
        absl::StatusCode::kInternal,
        absl::StrFormat("build of %s failed (exit code %d)", showPaths(failed),
                        worker.exitStatus()));
  }
  return absl::OkStatus();
}

BuildResult LocalStore::buildDerivation(const Path& drvPath,
                                        const BasicDerivation& drv,
                                        BuildMode buildMode) {
  Worker worker(*this);
  auto discard_logs = DiscardLogsSink();
  auto goal =
      worker.makeBasicDerivationGoal(discard_logs, drvPath, drv, buildMode);

  BuildResult result;

  try {
    worker.run(Goals{goal});
    result = goal->getResult();
  } catch (Error& e) {
    result.status = BuildResult::MiscFailure;
    result.errorMsg = e.msg();
  }

  return result;
}

void LocalStore::ensurePath(const Path& path) {
  /* If the path is already valid, we're done. */
  if (isValidPath(path)) {
    return;
  }

  primeCache(*this, {path});

  Worker worker(*this);
  auto discard_logs = DiscardLogsSink();
  GoalPtr goal = worker.makeSubstitutionGoal(discard_logs, path);
  Goals goals = {goal};

  worker.run(goals);

  if (goal->getExitCode() != Goal::ecSuccess) {
    throw Error(worker.exitStatus(),
                "path '%s' does not exist and cannot be created", path);
  }
}

void LocalStore::repairPath(const Path& path) {
  Worker worker(*this);
  auto discard_logs = DiscardLogsSink();
  GoalPtr goal = worker.makeSubstitutionGoal(discard_logs, path, Repair);
  Goals goals = {goal};

  worker.run(goals);

  if (goal->getExitCode() != Goal::ecSuccess) {
    /* Since substituting the path didn't work, if we have a valid
       deriver, then rebuild the deriver. */
    auto deriver = queryPathInfo(path)->deriver;
    if (!deriver.empty() && isValidPath(deriver)) {
      goals.clear();
      goals.insert(worker.makeDerivationGoal(discard_logs, deriver, StringSet(),
                                             bmRepair));
      worker.run(goals);
    } else {
      throw Error(worker.exitStatus(), "cannot repair path '%s'", path);
    }
  }
}

}  // namespace nix