* Renamed `normalise.cc' -> `build.cc', `storeexprs.cc' ->

  `derivations.cc', etc.
* Store the SHA-256 content hash of store paths in the database after
  they have been built/added.  This is so that we can check whether
  the store has been messed with (a la `rpm --verify').
* When registering path validity, verify that the closure property
  holds.

1 files changed, 1758 insertions, 0 deletions

diff --git a/src/libstore/build.cc b/src/libstore/build.cc
new file mode 100644
index 0000000000..60e72c9dc1
--- /dev/null
+++ b/src/libstore/build.cc
@@ -0,0 +1,1758 @@
+#include <map>
+#include <boost/shared_ptr.hpp>
+#include <boost/weak_ptr.hpp>
+#include <boost/enable_shared_from_this.hpp>
+
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <errno.h>
+
+#include "build.hh"
+#include "references.hh"
+#include "pathlocks.hh"
+#include "globals.hh"
+
+
+/* !!! TODO storeExprFromPath shouldn't be used here */
+
+
+static string pathNullDevice = "/dev/null";
+
+
+/* Forward definition. */
+class Worker;
+
+
+/* A pointer to a goal. */
+class Goal;
+typedef shared_ptr<Goal> GoalPtr;
+typedef weak_ptr<Goal> WeakGoalPtr;
+
+/* Set of goals. */
+typedef set<GoalPtr> Goals;
+typedef set<WeakGoalPtr> WeakGoals;
+
+/* A map of paths to goals (and the other way around). */
+typedef map<Path, WeakGoalPtr> WeakGoalMap;
+
+
+
+class Goal : public enable_shared_from_this<Goal>
+{
+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;
+
+    /* Whether amDone() has been called. */
+    bool done;
+
+    
+    Goal(Worker & worker) : worker(worker)
+    {
+        done = false;
+        nrFailed = 0;
+    }
+
+    virtual ~Goal()
+    {
+        printMsg(lvlVomit, "goal destroyed");
+    }
+
+public:
+    virtual void work() = 0;
+
+    virtual string name() = 0;
+
+    void addWaitee(GoalPtr waitee);
+
+    virtual void waiteeDone(GoalPtr waitee, bool success);
+
+    virtual void writeLog(int fd, const unsigned char * buf, size_t count)
+    {
+        abort();
+    }
+
+    void trace(const format & f);
+    
+protected:
+    void amDone(bool success = true);
+};
+
+
+/* A mapping used to remember for each child process to what goal it
+   belongs, and a file descriptor for receiving log data. */
+struct Child
+{
+    WeakGoalPtr goal;
+    int fdOutput;
+    bool inBuildSlot;
+};
+
+typedef map<pid_t, Child> Children;
+
+
+/* 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. */
+    Children children;
+
+    /* Number of build slots occupied.  Not all child processes
+       (namely build hooks) count as occupied build slots. */
+    unsigned int nrChildren;
+
+    /* Maps used to prevent multiple instantiations of a goal for the
+       same expression / path. */
+    WeakGoalMap derivationGoals;
+    WeakGoalMap substitutionGoals;
+
+public:
+
+    Worker();
+    ~Worker();
+
+    /* Make a goal (with caching). */
+    GoalPtr makeDerivationGoal(const Path & drvPath);
+    GoalPtr makeSubstitutionGoal(const Path & storePath);
+
+    /* Remove a dead goal. */
+    void removeGoal(GoalPtr goal);
+
+    /* Wake up a goal (i.e., there is something for it to do). */
+    void wakeUp(GoalPtr goal);
+
+    /* Can we start another child process? */
+    bool canBuildMore();
+
+    /* Registers / unregisters a running child process. */
+    void childStarted(GoalPtr goal, pid_t pid, int fdOutput,
+        bool inBuildSlot);
+    void childTerminated(pid_t pid, bool wakeSleepers = true);
+
+    /* Add a goal to the set of goals waiting for a build slot. */
+    void waitForBuildSlot(GoalPtr goal, bool reallyWait = false);
+    
+    /* Loop until the specified top-level goal has finished.  Returns
+       true if it has finished succesfully. */
+    bool run(const Goals & topGoals);
+
+    /* Wait for input to become available. */
+    void waitForInput();
+};
+
+
+class SubstError : public Error
+{
+public:
+    SubstError(const format & f) : Error(f) { };
+};
+
+
+class BuildError : public Error
+{
+public:
+    BuildError(const format & f) : Error(f) { };
+};
+
+
+
+//////////////////////////////////////////////////////////////////////
+
+
+void Goal::addWaitee(GoalPtr waitee)
+{
+    waitees.insert(waitee);
+    waitee->waiters.insert(shared_from_this());
+}
+
+
+void Goal::waiteeDone(GoalPtr waitee, bool success)
+{
+    assert(waitees.find(waitee) != waitees.end());
+    waitees.erase(waitee);
+    
+    if (!success) ++nrFailed;
+    
+    if (waitees.empty() || (!success && !keepGoing)) {
+
+        /* If we failed and keepGoing is not set, we remove all
+           remaining waitees. */
+        for (Goals::iterator i = waitees.begin(); i != waitees.end(); ++i) {
+            GoalPtr goal = *i;
+            WeakGoals waiters2;
+            for (WeakGoals::iterator j = goal->waiters.begin();
+                 j != goal->waiters.end(); ++j)
+                if (j->lock() != shared_from_this())
+                    waiters2.insert(*j);
+            goal->waiters = waiters2;
+        }
+        waitees.clear();
+
+        worker.wakeUp(shared_from_this());
+    }
+}
+
+
+void Goal::amDone(bool success)
+{
+    trace("done");
+    assert(!done);
+    done = true;
+    for (WeakGoals::iterator i = waiters.begin(); i != waiters.end(); ++i) {
+        GoalPtr goal = i->lock();
+        if (goal) goal->waiteeDone(shared_from_this(), success);
+    }
+    waiters.clear();
+    worker.removeGoal(shared_from_this());
+}
+
+
+void Goal::trace(const format & f)
+{
+    debug(format("%1%: %2%") % name() % f);
+}
+
+
+
+//////////////////////////////////////////////////////////////////////
+
+
+/* Common initialisation performed in child processes. */
+void commonChildInit(Pipe & logPipe)
+{
+    /* Put the child in a separate process group so that it doesn't
+       receive terminal signals. */
+    if (setpgid(0, 0) == -1)
+        throw SysError(format("setting process group"));
+
+    /* Dup the write side of the logger pipe into stderr. */
+    if (dup2(logPipe.writeSide, STDERR_FILENO) == -1)
+        throw SysError("cannot pipe standard error into log file");
+    logPipe.readSide.close();
+            
+    /* Dup stderr to stdin. */
+    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");
+}
+
+
+/* Convert a string list to an array of char pointers.  Careful: the
+   string list should outlive the array. */
+const char * * strings2CharPtrs(const Strings & ss)
+{
+    const char * * arr = new const char * [ss.size() + 1];
+    const char * * p = arr;
+    for (Strings::const_iterator i = ss.begin(); i != ss.end(); ++i)
+        *p++ = i->c_str();
+    *p = 0;
+    return arr;
+}
+
+
+
+//////////////////////////////////////////////////////////////////////
+
+
+class DerivationGoal : public Goal
+{
+private:
+    /* The path of the derivation store expression. */
+    Path drvPath;
+
+    /* The derivation store expression stored at drvPath. */
+    Derivation drv;
+    
+    /* 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;
+
+    /* The process ID of the builder. */
+    Pid pid;
+
+    /* The temporary directory. */
+    Path tmpDir;
+
+    /* File descriptor for the log file. */
+    AutoCloseFD fdLogFile;
+
+    /* Pipe for the builder's standard output/error. */
+    Pipe logPipe;
+
+    /* Pipes for talking to the build hook (if any). */
+    Pipe toHook;
+    Pipe fromHook;
+
+    typedef void (DerivationGoal::*GoalState)();
+    GoalState state;
+    
+public:
+    DerivationGoal(const Path & drvPath, Worker & worker);
+    ~DerivationGoal();
+
+    void work();
+
+private:
+    /* The states. */
+    void init();
+    void haveStoreExpr();
+    void inputsRealised();
+    void tryToBuild();
+    void buildDone();
+
+    /* Is the build hook willing to perform the build? */
+    typedef enum {rpAccept, rpDecline, rpPostpone, rpDone} HookReply;
+    HookReply tryBuildHook();
+
+    /* Synchronously wait for a build hook to finish. */
+    void terminateBuildHook();
+
+    /* Acquires locks on the output paths and gathers information
+       about the build (e.g., the input closures).  During this
+       process its possible that we find out that the build is
+       unnecessary, in which case we return false (this is not an
+       error condition!). */
+    bool prepareBuild();
+
+    /* Start building a derivation. */
+    void startBuilder();
+
+    /* Must be called after the output paths have become valid (either
+       due to a successful build or hook, or because they already
+       were). */
+    void computeClosure();
+
+    /* Open a log file and a pipe to it. */
+    void openLogFile();
+
+    /* Common initialisation to be performed in child processes (i.e.,
+       both in builders and in build hooks. */
+    void initChild();
+    
+    /* Delete the temporary directory, if we have one. */
+    void deleteTmpDir(bool force);
+
+    /* Callback used by the worker to write to the log. */
+    void writeLog(int fd, const unsigned char * buf, size_t count);
+
+    /* Return true iff all output paths are valid. */
+    bool allOutputsValid();
+
+    string name();
+};
+
+
+DerivationGoal::DerivationGoal(const Path & drvPath, Worker & worker)
+    : Goal(worker)
+{
+    this->drvPath = drvPath;
+    state = &DerivationGoal::init;
+}
+
+
+DerivationGoal::~DerivationGoal()
+{
+    if (pid != -1) worker.childTerminated(pid);
+    
+    /* Careful: we should never ever throw an exception from a
+       destructor. */
+    try {
+        deleteTmpDir(false);
+    } catch (Error & e) {
+        printMsg(lvlError, format("error (ignored): %1%") % e.msg());
+    }
+}
+
+
+void DerivationGoal::work()
+{
+    (this->*state)();
+}
+
+
+void DerivationGoal::init()
+{
+    trace("init");
+
+    /* The first thing to do is to make sure that the store expression
+       exists.  If it doesn't, it may be created through a
+       substitute. */
+    addWaitee(worker.makeSubstitutionGoal(drvPath));
+
+    state = &DerivationGoal::haveStoreExpr;
+}
+
+
+void DerivationGoal::haveStoreExpr()
+{
+    trace("loading derivation");
+
+    if (nrFailed != 0) {
+        printMsg(lvlError,
+            format("cannot build missing derivation `%1%'")
+            % drvPath);
+        amDone(false);
+        return;
+    }
+
+    assert(isValidPath(drvPath));
+
+    /* Get the derivation. */
+    drv = derivationFromPath(drvPath);
+
+    /* If all the outputs already exist, then we're done. */
+    if (allOutputsValid()) {
+        amDone(true);
+        return;
+    }
+
+    /* Inputs must be built before we can build this goal. */
+    for (PathSet::iterator i = drv.inputDrvs.begin();
+         i != drv.inputDrvs.end(); ++i)
+        addWaitee(worker.makeDerivationGoal(*i));
+
+    for (PathSet::iterator i = drv.inputSrcs.begin();
+         i != drv.inputSrcs.end(); ++i)
+        addWaitee(worker.makeSubstitutionGoal(*i));
+
+    state = &DerivationGoal::inputsRealised;
+}
+
+
+void DerivationGoal::inputsRealised()
+{
+    trace("all inputs realised");
+
+    if (nrFailed != 0) {
+        printMsg(lvlError,
+            format("cannot build derivation `%1%': "
+                "%2% inputs could not be realised")
+            % drvPath % nrFailed);
+        amDone(false);
+        return;
+    }
+
+    /* 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());
+}
+
+
+void DerivationGoal::tryToBuild()
+{
+    trace("trying to build");
+
+    try {
+
+        /* Is the build hook willing to accept this job? */
+        switch (tryBuildHook()) {
+            case rpAccept:
+                /* Yes, it has started doing so.  Wait until we get
+                   EOF from the hook. */
+                state = &DerivationGoal::buildDone;
+                return;
+            case rpPostpone:
+                /* Not now; wait until at least one child finishes. */
+                worker.waitForBuildSlot(shared_from_this(), true);
+                return;
+            case rpDecline:
+                /* We should do it ourselves. */
+                break;
+            case rpDone:
+                /* Somebody else did it. */
+                amDone();
+                return;
+        }
+
+        /* Make sure that we are allowed to start a build. */
+        if (!worker.canBuildMore()) {
+            worker.waitForBuildSlot(shared_from_this());
+            return;
+        }
+
+        /* Acquire locks and such.  If we then see that the build has
+           been done by somebody else, we're done. */
+        if (!prepareBuild()) {
+            amDone();
+            return;
+        }
+
+        /* Okay, we have to build. */
+        startBuilder();
+
+    } catch (BuildError & e) {
+        printMsg(lvlError, e.msg());
+        amDone(false);
+        return;
+    }
+
+    /* This state will be reached when we get EOF on the child's
+       log pipe. */
+    state = &DerivationGoal::buildDone;
+}
+
+
+void DerivationGoal::buildDone()
+{
+    trace("build done");
+
+    /* 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 --- just don't do that
+       :-) */
+    /* !!! this could block! */
+    pid_t savedPid = pid;
+    int status = pid.wait(true);
+
+    /* So the child is gone now. */
+    worker.childTerminated(savedPid);
+
+    /* Close the read side of the logger pipe. */
+    logPipe.readSide.close();
+
+    /* Close the log file. */
+    fdLogFile.close();
+
+    debug(format("builder process for `%1%' finished") % drvPath);
+
+    /* Check the exit status. */
+    if (!statusOk(status)) {
+        deleteTmpDir(false);
+        printMsg(lvlError, format("builder for `%1%' %2%")
+            % drvPath % statusToString(status));
+        amDone(false);
+        return;
+    }
+    
+    deleteTmpDir(true);
+
+    /* Compute the FS closure of the outputs and register them as
+       being valid. */
+    try {
+        computeClosure();
+    } catch (BuildError & e) {
+        printMsg(lvlError, e.msg());
+        amDone(false);
+        return;
+    }
+
+    amDone();
+}
+
+
+static string readLine(int fd)
+{
+    string s;
+    while (1) {
+        char ch;
+        ssize_t rd = read(fd, &ch, 1);
+        if (rd == -1) {
+            if (errno != EINTR)
+                throw SysError("reading a line");
+        } else if (rd == 0)
+            throw Error("unexpected EOF reading a line");
+        else {
+            if (ch == '\n') return s;
+            s += ch;
+        }
+    }
+}
+
+
+static void writeLine(int fd, string s)
+{
+    s += '\n';
+    writeFull(fd, (const unsigned char *) s.c_str(), s.size());
+}
+
+
+/* !!! ugly hack */
+static void drain(int fd)
+{
+    unsigned char buffer[1024];
+    while (1) {
+        ssize_t rd = read(fd, buffer, sizeof buffer);
+        if (rd == -1) {
+            if (errno != EINTR)
+                throw SysError("draining");
+        } else if (rd == 0) break;
+        else writeFull(STDERR_FILENO, buffer, rd);
+    }
+}
+
+
+PathSet outputPaths(const DerivationOutputs & outputs)
+{
+    PathSet paths;
+    for (DerivationOutputs::const_iterator i = outputs.begin();
+         i != outputs.end(); ++i)
+        paths.insert(i->second.path);
+    return paths;
+}
+
+
+string showPaths(const PathSet & paths)
+{
+    string s;
+    for (PathSet::const_iterator i = paths.begin();
+         i != paths.end(); ++i)
+    {
+        if (s.size() != 0) s += ", ";
+        s += "`" + *i + "'";
+    }
+    return s;
+}
+
+
+DerivationGoal::HookReply DerivationGoal::tryBuildHook()
+{
+    Path buildHook = getEnv("NIX_BUILD_HOOK");
+    if (buildHook == "") return rpDecline;
+    buildHook = absPath(buildHook);
+
+    /* Create a directory where we will store files used for
+       communication between us and the build hook. */
+    tmpDir = createTempDir();
+    
+    /* Create the log file and pipe. */
+    openLogFile();
+
+    /* Create the communication pipes. */
+    toHook.create();
+    fromHook.create();
+
+    /* Fork the hook. */
+    pid = fork();
+    switch (pid) {
+        
+    case -1:
+        throw SysError("unable to fork");
+
+    case 0:
+        try { /* child */
+
+            initChild();
+
+            execl(buildHook.c_str(), buildHook.c_str(),
+                (worker.canBuildMore() ? (string) "1" : "0").c_str(),
+                thisSystem.c_str(),
+                drv.platform.c_str(),
+                drvPath.c_str(), 0);
+            
+            throw SysError(format("executing `%1%'") % buildHook);
+            
+        } catch (exception & e) {
+            cerr << format("build error: %1%\n") % e.what();
+        }
+        _exit(1);
+    }
+    
+    /* parent */
+    logPipe.writeSide.close();
+    worker.childStarted(shared_from_this(),
+        pid, logPipe.readSide, false);
+
+    fromHook.writeSide.close();
+    toHook.readSide.close();
+
+    /* Read the first line of input, which should be a word indicating
+       whether the hook wishes to perform the build.  !!! potential
+       for deadlock here: we should also read from the child's logger
+       pipe. */
+    string reply;
+    try {
+        reply = readLine(fromHook.readSide);
+    } catch (Error & e) {
+        drain(logPipe.readSide);
+        throw;
+    }
+
+    debug(format("hook reply is `%1%'") % reply);
+
+    if (reply == "decline" || reply == "postpone") {
+        /* Clean up the child.  !!! hacky / should verify */
+        drain(logPipe.readSide);
+        terminateBuildHook();
+        return reply == "decline" ? rpDecline : rpPostpone;
+    }
+
+    else if (reply == "accept") {
+
+        /* Acquire locks and such.  If we then see that the output
+           paths are now valid, we're done. */
+        if (!prepareBuild()) {
+            /* Tell the hook to exit. */
+            writeLine(toHook.writeSide, "cancel");
+            terminateBuildHook();
+            return rpDone;
+        }
+
+        printMsg(lvlInfo, format("running hook to build path(s) %1%")
+            % showPaths(outputPaths(drv.outputs)));
+        
+        /* Write the information that the hook needs to perform the
+           build, i.e., the set of input paths (including closure
+           expressions), the set of output paths, and [!!!]. */
+        
+        Path inputListFN = tmpDir + "/inputs";
+        Path outputListFN = tmpDir + "/outputs";
+
+        string s;
+        for (PathSet::iterator i = inputPaths.begin();
+             i != inputPaths.end(); ++i)
+            s += *i + "\n";
+        writeStringToFile(inputListFN, s);
+        
+        s = "";
+        for (DerivationOutputs::iterator i = drv.outputs.begin();
+             i != drv.outputs.end(); ++i)
+            s += i->second.path + "\n";
+        writeStringToFile(outputListFN, s);
+        
+        /* Tell the hook to proceed. */ 
+        writeLine(toHook.writeSide, "okay");
+
+        return rpAccept;
+    }
+
+    else throw Error(format("bad hook reply `%1%'") % reply);
+}
+
+
+void DerivationGoal::terminateBuildHook()
+{
+    /* !!! drain stdout of hook */
+    debug("terminating build hook");
+    pid_t savedPid = pid;
+    pid.wait(true);
+    worker.childTerminated(savedPid, false);
+    fromHook.readSide.close();
+    toHook.writeSide.close();
+    fdLogFile.close();
+    logPipe.readSide.close();
+    deleteTmpDir(true); /* get rid of the hook's temporary directory */
+}
+
+
+bool DerivationGoal::prepareBuild()
+{
+    /* Obtain locks on all output paths.  The locks are automatically
+       released when we exit this function or Nix crashes. */
+    /* !!! BUG: this could block, which is not allowed. */
+    outputLocks.lockPaths(outputPaths(drv.outputs));
+
+    /* 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 expression, so no further checks are necessary. */
+    if (allOutputsValid()) {
+        debug(format("skipping build of derivation `%1%', someone beat us to it")
+            % drvPath);
+        outputLocks.setDeletion(true);
+        return false;
+    }
+
+    /* Gather information necessary for computing the closure and/or
+       running the build hook. */
+    
+    /* The outputs are referenceable paths. */
+    for (DerivationOutputs::iterator i = drv.outputs.begin();
+         i != drv.outputs.end(); ++i)
+    {
+        debug(format("building path `%1%'") % i->second.path);
+        allPaths.insert(i->second.path);
+    }
+
+    /* Determine the full set of input paths. */
+
+    /* First, the input derivations. */
+    for (PathSet::iterator i = drv.inputDrvs.begin();
+         i != drv.inputDrvs.end(); ++i)
+    {
+        /* Add all the output closures of the input derivation `*i' as
+           input paths.  !!!  there should be a way to indicate
+           specific outputs. */
+        /* !!! is `*i' present? */
+        assert(isValidPath(*i));
+        Derivation inDrv = derivationFromPath(*i);
+        for (DerivationOutputs::iterator j = inDrv.outputs.begin();
+             j != inDrv.outputs.end(); ++j)
+            computeFSClosure(j->second.path, inputPaths);
+    }
+
+    for (PathSet::iterator i = inputPaths.begin(); i != inputPaths.end(); ++i)
+        debug(format("INPUT %1%") % *i);
+
+    allPaths.insert(inputPaths.begin(), inputPaths.end());
+
+    /* Second, the input sources. */
+    for (PathSet::iterator i = drv.inputSrcs.begin();
+         i != drv.inputSrcs.end(); ++i)
+        computeFSClosure(*i, inputPaths);
+
+    /* We can skip running the builder if all output paths are already
+       valid. */
+    bool fastBuild = true;
+    for (DerivationOutputs::iterator i = drv.outputs.begin();
+         i != drv.outputs.end(); ++i)
+        if (!isValidPath(i->second.path)) { 
+            fastBuild = false;
+            break;
+        }
+
+    if (fastBuild) {
+        printMsg(lvlChatty, format("skipping build; output paths already exist"));
+        computeClosure();
+        return false;
+    }
+
+    return true;
+}
+
+
+void DerivationGoal::startBuilder()
+{
+    startNest(nest, lvlInfo,
+        format("building path(s) %1%") % showPaths(outputPaths(drv.outputs)))
+    
+    /* Right platform? */
+    if (drv.platform != thisSystem)
+        throw BuildError(
+            format("a `%1%' is required to build `%3%', but I am a `%2%'")
+            % drv.platform % thisSystem % drvPath);
+
+    /* If any of the outputs already exist but are not registered,
+       delete them. */
+    for (DerivationOutputs::iterator i = drv.outputs.begin(); 
+         i != drv.outputs.end(); ++i)
+    {
+        Path path = i->second.path;
+        if (isValidPath(path))
+            throw Error(format("obstructed build: path `%1%' exists") % path);
+        if (pathExists(path)) {
+            debug(format("removing unregistered path `%1%'") % path);
+            deletePath(path);
+        }
+    }
+
+    /* Construct the environment passed to the builder. */
+    typedef map<string, string> Environment;
+    Environment env; 
+    
+    /* 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"] = "/homeless-shelter";
+
+    /* 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"] = nixStore;
+
+    /* Add all bindings specified in the derivation expression. */
+    for (StringPairs::iterator i = drv.env.begin();
+         i != drv.env.end(); ++i)
+        env[i->first] = i->second;
+
+    /* Create a temporary directory where the build will take
+       place. */
+    tmpDir = createTempDir();
+
+    /* For convenience, set an environment pointing to the top build
+       directory. */
+    env["NIX_BUILD_TOP"] = tmpDir;
+
+    /* Also set TMPDIR and variants to point to this directory. */
+    env["TMPDIR"] = env["TEMPDIR"] = env["TMP"] = env["TEMP"] = tmpDir;
+
+    /* Run the builder. */
+    printMsg(lvlChatty, format("executing builder `%1%'") %
+        drv.builder);
+
+    /* Create the log file and pipe. */
+    openLogFile();
+    
+    /* Fork a child to build the package.  Note that while we
+       currently use forks to run and wait for the children, it
+       shouldn't be hard to use threads for this on systems where
+       fork() is unavailable or inefficient. */
+    pid = fork();
+    switch (pid) {
+
+    case -1:
+        throw SysError("unable to fork");
+
+    case 0:
+
+        /* Warning: in the child we should absolutely not make any
+           Berkeley DB calls! */
+
+        try { /* child */
+
+            initChild();
+
+            /* Fill in the arguments. */
+            Strings args(drv.args);
+            args.push_front(baseNameOf(drv.builder));
+            const char * * argArr = strings2CharPtrs(args);
+
+            /* Fill in the environment. */
+            Strings envStrs;
+            for (Environment::const_iterator i = env.begin();
+                 i != env.end(); ++i)
+                envStrs.push_back(i->first + "=" + i->second);
+            const char * * envArr = strings2CharPtrs(envStrs);
+
+            /* Execute the program.  This should not return. */
+            execve(drv.builder.c_str(),
+                (char * *) argArr, (char * *) envArr);
+
+            throw SysError(format("executing `%1%'")
+                % drv.builder);
+            
+        } catch (exception & e) {
+            cerr << format("build error: %1%\n") % e.what();
+        }
+        _exit(1);
+    }
+
+    /* parent */
+    pid.setSeparatePG(true);
+    logPipe.writeSide.close();
+    worker.childStarted(shared_from_this(),
+        pid, logPipe.readSide, true);
+}
+
+
+void DerivationGoal::computeClosure()
+{
+    startNest(nest, lvlTalkative,
+        format("determining closure for `%1%'") % drvPath);
+
+    map<Path, PathSet> allReferences;
+    map<Path, Hash> contentHashes;
+    
+    /* 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 (DerivationOutputs::iterator i = drv.outputs.begin(); 
+         i != drv.outputs.end(); ++i)
+    {
+        Path path = i->second.path;
+        if (!pathExists(path)) {
+            throw BuildError(
+                format("builder for `%1%' failed to produce output path `%2%'")
+                % drvPath % path);
+        }
+
+        /* Check that fixed-output derivations produced the right
+           outputs (i.e., the content hash should match the specified
+           hash). */ 
+        if (i->second.hash != "") {
+            HashType ht = parseHashType(i->second.hashAlgo);
+            if (ht == htUnknown)
+                throw Error(format("unknown hash algorithm `%1%'") % i->second.hashAlgo);
+            Hash h = parseHash(ht, i->second.hash);
+            Hash h2 = hashFile(ht, path);
+            if (h != h2)
+                throw Error(
+                    format("output path `%1% should have %2% hash `%3%', instead has `%4%'")
+                    % path % i->second.hashAlgo % printHash(h) % printHash(h2));
+
+            /* Also, the output path should be a regular file withouth
+               execute permission. */
+            struct stat st;
+            if (lstat(path.c_str(), &st))
+                throw SysError(format("getting attributes of path `%1%'") % path);
+            if (!S_ISREG(st.st_mode) || (st.st_mode & S_IXUSR) != 0)
+                throw Error(
+                    format("output path `%1% should be a non-executable regular file")
+                    % path);
+        }
+
+	canonicalisePathMetaData(path);
+
+	/* For this output path, find the references to other paths contained
+	   in it. */
+        PathSet references;
+        if (!pathExists(path + "/nix-support/no-scan")) {
+            startNest(nest2, lvlChatty,
+                format("scanning for store references in `%1%'") % path);
+            Paths references2;
+            references2 = filterReferences(path, 
+                Paths(allPaths.begin(), allPaths.end()));
+            references = PathSet(references2.begin(), references2.end());
+
+            /* For debugging, print out the referenced and
+               unreferenced paths. */
+            for (PathSet::iterator i = inputPaths.begin();
+                 i != inputPaths.end(); ++i)
+            {
+                PathSet::iterator j = references.find(*i);
+                if (j == references.end())
+                    debug(format("unreferenced input: `%1%'") % *i);
+                else
+                    debug(format("referenced input: `%1%'") % *i);
+            }
+            
+            nest2.close();
+        }
+
+        allReferences[path] = references;
+
+        /* Hash the contents of the path.  The hash is stored in the
+           database so that we can verify later on whether nobody has
+           messed with the store.  !!! inefficient: it would be nice
+           if we could combine this with filterReferences(). */
+        contentHashes[path] = hashPath(htSHA256, path);
+    }
+
+    /* Register each output path as valid, and register the sets of
+       paths referenced by each of them.  This is wrapped in one
+       database transaction to ensure that if we crash, either
+       everything is registered or nothing is.  This is for
+       recoverability: unregistered paths in the store can be deleted
+       arbitrarily, while registered paths can only be deleted by
+       running the garbage collector.
+
+       The reason that we do the transaction here and not on the fly
+       while we are scanning (above) is so that we don't hold database
+       locks for too long. */
+    Transaction txn;
+    createStoreTransaction(txn);
+    for (DerivationOutputs::iterator i = drv.outputs.begin(); 
+         i != drv.outputs.end(); ++i)
+    {
+        registerValidPath(txn, i->second.path,
+            contentHashes[i->second.path]);
+        setReferences(txn, i->second.path,
+            allReferences[i->second.path]);
+    }
+    txn.commit();
+
+    /* 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);
+}
+
+
+void DerivationGoal::openLogFile()
+{
+    /* Create a log file. */
+    Path logFileName = nixLogDir + "/" + baseNameOf(drvPath);
+    fdLogFile = open(logFileName.c_str(),
+        O_CREAT | O_WRONLY | O_TRUNC, 0666);
+    if (fdLogFile == -1)
+        throw SysError(format("creating log file `%1%'") % logFileName);
+
+    /* Create a pipe to get the output of the child. */
+    logPipe.create();
+}
+
+
+void DerivationGoal::initChild()
+{
+    commonChildInit(logPipe);
+    
+    if (chdir(tmpDir.c_str()) == -1)
+        throw SysError(format("changing into `%1%'") % tmpDir);
+
+    /* When running a hook, dup the communication pipes. */
+    bool inHook = fromHook.writeSide.isOpen();
+    if (inHook) {
+        fromHook.readSide.close();
+        if (dup2(fromHook.writeSide, 3) == -1)
+            throw SysError("dupping from-hook write side");
+
+        toHook.writeSide.close();
+        if (dup2(toHook.readSide, 4) == -1)
+            throw SysError("dupping to-hook read side");
+    }
+
+    /* Close all other file descriptors. */
+    int maxFD = 0;
+    maxFD = sysconf(_SC_OPEN_MAX);
+    for (int fd = 0; fd < maxFD; ++fd)
+        if (fd != STDIN_FILENO && fd != STDOUT_FILENO && fd != STDERR_FILENO
+            && (!inHook || (fd != 3 && fd != 4)))
+            close(fd); /* ignore result */
+}
+
+
+void DerivationGoal::deleteTmpDir(bool force)
+{
+    if (tmpDir != "") {
+        if (keepFailed && !force)
+	    printMsg(lvlError, 
+		format("builder for `%1%' failed; keeping build directory `%2%'")
+                % drvPath % tmpDir);
+        else
+            deletePath(tmpDir);
+        tmpDir = "";
+    }
+}
+
+
+void DerivationGoal::writeLog(int fd,
+    const unsigned char * buf, size_t count)
+{
+    assert(fd == logPipe.readSide);
+    writeFull(fdLogFile, buf, count);
+}
+
+
+bool DerivationGoal::allOutputsValid()
+{
+    unsigned int nrValid = 0;
+    for (DerivationOutputs::iterator i = drv.outputs.begin();
+         i != drv.outputs.end(); ++i)
+        if (isValidPath(i->second.path)) nrValid++;
+
+    if (nrValid != 0) {
+        if (nrValid == drv.outputs.size()) return true;
+        throw Error(
+            format("derivation `%1%' is blocked by its output paths")
+            % drvPath);
+    }
+
+    return false;
+}
+
+
+string DerivationGoal::name()
+{
+    return (format("building of `%1%'") % drvPath).str();
+}
+
+
+
+//////////////////////////////////////////////////////////////////////
+
+
+class SubstitutionGoal : public Goal
+{
+private:
+    /* The store path that should be realised through a substitute. */
+    Path storePath;
+
+    /* The remaining substitutes for this path. */
+    Substitutes subs;
+
+    /* The current substitute. */
+    Substitute sub;
+
+    /* Pipe for the substitute's standard output/error. */
+    Pipe logPipe;
+
+    /* The process ID of the builder. */
+    Pid pid;
+
+    /* Lock on the store path. */
+    shared_ptr<PathLocks> outputLock;
+    
+    typedef void (SubstitutionGoal::*GoalState)();
+    GoalState state;
+
+public:
+    SubstitutionGoal(const Path & storePath, Worker & worker);
+    ~SubstitutionGoal();
+
+    void work();
+
+    /* The states. */
+    void init();
+    void tryNext();
+    void tryToRun();
+    void finished();
+
+    /* Callback used by the worker to write to the log. */
+    void writeLog(int fd, const unsigned char * buf, size_t count);
+
+    string name();
+};
+
+
+SubstitutionGoal::SubstitutionGoal(const Path & storePath, Worker & worker)
+    : Goal(worker)
+{
+    this->storePath = storePath;
+    state = &SubstitutionGoal::init;
+}
+
+
+SubstitutionGoal::~SubstitutionGoal()
+{
+    if (pid != -1) worker.childTerminated(pid);
+}
+
+
+void SubstitutionGoal::work()
+{
+    (this->*state)();
+}
+
+
+void SubstitutionGoal::init()
+{
+    trace("init");
+
+    /* If the path already exists we're done. */
+    if (isValidPath(storePath)) {
+        amDone();
+        return;
+    }
+
+    /* !!! build the outgoing references of this path first to
+       maintain the closure invariant! */
+
+    /* Otherwise, get the substitutes. */
+    subs = querySubstitutes(storePath);
+
+    /* Try the first one. */
+    tryNext();
+}
+
+
+void SubstitutionGoal::tryNext()
+{
+    trace("trying next substitute");
+
+    if (subs.size() == 0) {
+        /* None left.  Terminate this goal and let someone else deal
+           with it. */
+        printMsg(lvlError,
+            format("path `%1%' is required, but it has no (remaining) substitutes")
+            % storePath);
+        amDone(false);
+        return;
+    }
+    sub = subs.front();
+    subs.pop_front();
+
+    /* Wait until we can run the substitute program. */
+    state = &SubstitutionGoal::tryToRun;
+    worker.waitForBuildSlot(shared_from_this());
+}
+
+
+void SubstitutionGoal::tryToRun()
+{
+    trace("trying to run");
+
+    /* Make sure that we are allowed to start a build. */
+    if (!worker.canBuildMore()) {
+        worker.waitForBuildSlot(shared_from_this());
+        return;
+    }
+
+    /* Acquire a lock on the output path. */
+    PathSet lockPath;
+    lockPath.insert(storePath);
+    outputLock = shared_ptr<PathLocks>(new PathLocks);
+    outputLock->lockPaths(lockPath);
+
+    /* Check again whether the path is invalid. */
+    if (isValidPath(storePath)) {
+        debug(format("store path `%1%' has become valid") % storePath);
+        outputLock->setDeletion(true);
+        amDone();
+        return;
+    }
+
+    printMsg(lvlInfo,
+        format("substituting path `%1%' using substituter `%2%'")
+        % storePath % sub.program);
+    
+    logPipe.create();
+
+    /* Remove the (stale) output path if it exists. */
+    if (pathExists(storePath))
+        deletePath(storePath);
+
+    /* Fork the substitute program. */
+    pid = fork();
+    switch (pid) {
+        
+    case -1:
+        throw SysError("unable to fork");
+
+    case 0:
+        try { /* child */
+
+            logPipe.readSide.close();
+
+            /* !!! close other handles */
+
+            commonChildInit(logPipe);
+
+            /* Fill in the arguments. */
+            Strings args(sub.args);
+            args.push_front(storePath);
+            args.push_front(baseNameOf(sub.program));
+            const char * * argArr = strings2CharPtrs(args);
+
+            execv(sub.program.c_str(), (char * *) argArr);
+            
+            throw SysError(format("executing `%1%'") % sub.program);
+            
+        } catch (exception & e) {
+            cerr << format("substitute error: %1%\n") % e.what();
+        }
+        _exit(1);
+    }
+    
+    /* parent */
+    pid.setSeparatePG(true);
+    logPipe.writeSide.close();
+    worker.childStarted(shared_from_this(),
+        pid, logPipe.readSide, true);
+
+    state = &SubstitutionGoal::finished;
+}
+
+
+void SubstitutionGoal::finished()
+{
+    trace("substitute finished");
+
+    /* Since we got an EOF on the logger pipe, the substitute is
+       presumed to have terminated.  */
+    /* !!! this could block! */
+    pid_t savedPid = pid;
+    int status = pid.wait(true);
+
+    /* So the child is gone now. */
+    worker.childTerminated(savedPid);
+
+    /* Close the read side of the logger pipe. */
+    logPipe.readSide.close();
+
+    debug(format("substitute for `%1%' finished") % storePath);
+
+    /* Check the exit status and the build result. */
+    try {
+        
+        if (!statusOk(status))
+            throw SubstError(format("builder for `%1%' %2%")
+                % storePath % statusToString(status));
+
+        if (!pathExists(storePath))
+            throw SubstError(
+                format("substitute did not produce path `%1%'")
+                % storePath);
+        
+    } catch (SubstError & e) {
+
+        printMsg(lvlInfo,
+            format("substitution of path `%1%' using substituter `%2%' failed: %3%")
+            % storePath % sub.program % e.msg());
+        
+        /* Try the next substitute. */
+        state = &SubstitutionGoal::tryNext;
+        worker.wakeUp(shared_from_this());
+        return;
+    }
+
+    canonicalisePathMetaData(storePath);
+
+    Hash contentHash = hashPath(htSHA256, storePath);
+
+    Transaction txn;
+    createStoreTransaction(txn);
+    registerValidPath(txn, storePath, contentHash);
+    txn.commit();
+
+    outputLock->setDeletion(true);
+    
+    printMsg(lvlChatty,
+        format("substitution of path `%1%' succeeded") % storePath);
+
+    amDone();
+}
+
+
+void SubstitutionGoal::writeLog(int fd,
+    const unsigned char * buf, size_t count)
+{
+    assert(fd == logPipe.readSide);
+    /* Don't write substitution output to a log file for now.  We
+       probably should, though. */
+}
+
+
+string SubstitutionGoal::name()
+{
+    return (format("substitution of `%1%'") % storePath).str();
+}
+
+
+
+//////////////////////////////////////////////////////////////////////
+
+
+/* A fake goal used to receive notification of success or failure of
+   other goals. */
+class PseudoGoal : public Goal
+{
+private:
+    bool success;
+    
+public:
+    PseudoGoal(Worker & worker) : Goal(worker)
+    {
+        success = true;
+    }
+
+    void work() 
+    {
+        abort();
+    }
+
+    void waiteeDone(GoalPtr waitee, bool success)
+    {
+        if (!success) this->success = false;
+    }
+
+    bool isOkay()
+    {
+        return success;
+    }
+
+    string name()
+    {
+        return "pseudo-goal";
+    }
+};
+
+
+
+//////////////////////////////////////////////////////////////////////
+
+
+static bool working = false;
+
+
+Worker::Worker()
+{
+    /* Debugging: prevent recursive workers. */ 
+    if (working) abort();
+    working = true;
+    nrChildren = 0;
+}
+
+
+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();
+}
+
+
+template<class T>
+static GoalPtr addGoal(const Path & path,
+    Worker & worker, WeakGoalMap & goalMap)
+{
+    GoalPtr goal = goalMap[path].lock();
+    if (!goal) {
+        goal = GoalPtr(new T(path, worker));
+        goalMap[path] = goal;
+        worker.wakeUp(goal);
+    }
+    return goal;
+}
+
+
+GoalPtr Worker::makeDerivationGoal(const Path & nePath)
+{
+    return addGoal<DerivationGoal>(nePath, *this, derivationGoals);
+}
+
+
+GoalPtr Worker::makeSubstitutionGoal(const Path & storePath)
+{
+    return addGoal<SubstitutionGoal>(storePath, *this, substitutionGoals);
+}
+
+
+static void removeGoal(GoalPtr goal, WeakGoalMap & goalMap)
+{
+    /* !!! For now we just let dead goals accumulate.  We should
+       probably periodically sweep the goalMap to remove dead
+       goals. */
+}
+
+
+void Worker::removeGoal(GoalPtr goal)
+{
+    topGoals.erase(goal);
+    ::removeGoal(goal, derivationGoals);
+    ::removeGoal(goal, substitutionGoals);
+}
+
+
+void Worker::wakeUp(GoalPtr goal)
+{
+    goal->trace("woken up");
+    awake.insert(goal);
+}
+
+
+bool Worker::canBuildMore()
+{
+    return nrChildren < maxBuildJobs;
+}
+
+
+void Worker::childStarted(GoalPtr goal,
+    pid_t pid, int fdOutput, bool inBuildSlot)
+{
+    Child child;
+    child.goal = goal;
+    child.fdOutput = fdOutput;
+    child.inBuildSlot = inBuildSlot;
+    children[pid] = child;
+    if (inBuildSlot) nrChildren++;
+}
+
+
+void Worker::childTerminated(pid_t pid, bool wakeSleepers)
+{
+    Children::iterator i = children.find(pid);
+    assert(i != children.end());
+
+    if (i->second.inBuildSlot) {
+        assert(nrChildren > 0);
+        nrChildren--;
+    }
+
+    children.erase(pid);
+
+    if (wakeSleepers) {
+        
+        /* Wake up goals waiting for a build slot. */
+        for (WeakGoals::iterator i = wantingToBuild.begin();
+             i != wantingToBuild.end(); ++i)
+        {
+            GoalPtr goal = i->lock();
+            if (goal) wakeUp(goal);
+        }
+
+        wantingToBuild.clear();
+        
+    }
+}
+
+
+void Worker::waitForBuildSlot(GoalPtr goal, bool reallyWait)
+{
+    debug("wait for build slot");
+    if (reallyWait && children.size() == 0)
+        throw Error("waiting for a build slot, yet there are no children - "
+            "maybe the build hook gave an inappropriate `postpone' reply?");
+    if (!reallyWait && canBuildMore())
+        wakeUp(goal); /* we can do it right away */
+    else
+        wantingToBuild.insert(goal);
+}
+
+
+bool Worker::run(const Goals & _topGoals)
+{
+    /* Wrap the specified top-level goal in a pseudo-goal so that we
+       can check whether it succeeded. */
+    shared_ptr<PseudoGoal> pseudo(new PseudoGoal(*this));
+    for (Goals::iterator i = _topGoals.begin();
+         i != _topGoals.end(); ++i)
+    {
+        assert(*i);
+        pseudo->addWaitee(*i);
+        topGoals.insert(*i);
+    }
+    
+    startNest(nest, lvlDebug, format("entered goal loop"));
+
+    while (1) {
+
+        checkInterrupt();
+
+        /* Call every wake goal. */
+        while (!awake.empty()) {
+            WeakGoals awake2(awake);
+            awake.clear();
+            for (WeakGoals::iterator i = awake2.begin(); i != awake2.end(); ++i) {
+                checkInterrupt();
+                GoalPtr goal = i->lock();
+                if (goal) goal->work();
+            }
+        }
+
+        if (topGoals.empty()) break;
+
+        /* !!! not when we're polling */
+        assert(!children.empty());
+        
+        /* Wait for input. */
+        waitForInput();
+    }
+
+    /* 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(!keepGoing || awake.empty());
+    assert(!keepGoing || wantingToBuild.empty());
+    assert(!keepGoing || children.empty());
+
+    return pseudo->isOkay();
+}
+
+
+void Worker::waitForInput()
+{
+    printMsg(lvlVomit, "waiting for children");
+
+    /* Process log output from the children.  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. */
+
+    /* 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 (Children::iterator i = children.begin();
+         i != children.end(); ++i)
+    {
+        int fd = i->second.fdOutput;
+        FD_SET(fd, &fds);
+        if (fd >= fdMax) fdMax = fd + 1;
+    }
+
+    if (select(fdMax, &fds, 0, 0, 0) == -1) {
+        if (errno == EINTR) return;
+        throw SysError("waiting for input");
+    }
+
+    /* Process all available file descriptors. */
+    for (Children::iterator i = children.begin();
+         i != children.end(); ++i)
+    {
+        checkInterrupt();
+        GoalPtr goal = i->second.goal.lock();
+        assert(goal);
+        int fd = i->second.fdOutput;
+        if (FD_ISSET(fd, &fds)) {
+            unsigned char buffer[4096];
+            ssize_t rd = read(fd, buffer, sizeof(buffer));
+            if (rd == -1) {
+                if (errno != EINTR)
+                    throw SysError(format("reading from %1%")
+                        % goal->name());
+            } else if (rd == 0) {
+                debug(format("%1%: got EOF") % goal->name());
+                wakeUp(goal);
+            } else {
+                printMsg(lvlVomit, format("%1%: read %2% bytes")
+                    % goal->name() % rd);
+                goal->writeLog(fd, buffer, (size_t) rd);
+                if (verbosity >= buildVerbosity)
+                    writeFull(STDERR_FILENO, buffer, rd);
+            }
+        }
+    }
+}
+
+
+//////////////////////////////////////////////////////////////////////
+
+
+void buildDerivations(const PathSet & drvPaths)
+{
+    startNest(nest, lvlDebug,
+        format("building %1%") % showPaths(drvPaths));
+
+    Worker worker;
+
+    Goals goals;
+    for (PathSet::const_iterator i = drvPaths.begin();
+         i != drvPaths.end(); ++i)
+        goals.insert(worker.makeDerivationGoal(*i));
+    
+    if (!worker.run(goals))
+        throw Error(format("build failed"));
+}
+
+
+void ensurePath(const Path & path)
+{
+    /* If the path is already valid, we're done. */
+    if (isValidPath(path)) return;
+
+    Worker worker;
+    Goals goals;
+    goals.insert(worker.makeSubstitutionGoal(path));
+    if (!worker.run(goals))
+        throw Error(format("path `%1%' does not exist and cannot be created") % path);
+}