path: root/src/libstore/gc.cc

#include "globals.hh"
#include "misc.hh"
#include "pathlocks.hh"
#include "local-store.hh"

#include <boost/shared_ptr.hpp>

#include <functional>
#include <queue>
#include <algorithm>

#include <sys/types.h>
#include <sys/stat.h>
#include <errno.h>
#include <fcntl.h>
#include <unistd.h>


namespace nix {


static string gcLockName = "gc.lock";
static string tempRootsDir = "temproots";
static string gcRootsDir = "gcroots";

static const int defaultGcLevel = 1000;


/* Acquire the global GC lock.  This is used to prevent new Nix
   processes from starting after the temporary root files have been
   read.  To be precise: when they try to create a new temporary root
   file, they will block until the garbage collector has finished /
   yielded the GC lock. */
static int openGCLock(LockType lockType)
{
    Path fnGCLock = (format("%1%/%2%")
        % nixStateDir % gcLockName).str();
         
    debug(format("acquiring global GC lock `%1%'") % fnGCLock);
    
    AutoCloseFD fdGCLock = open(fnGCLock.c_str(), O_RDWR | O_CREAT, 0600);
    if (fdGCLock == -1)
        throw SysError(format("opening global GC lock `%1%'") % fnGCLock);

    if (!lockFile(fdGCLock, lockType, false)) {
        printMsg(lvlError, format("waiting for the big garbage collector lock..."));
        lockFile(fdGCLock, lockType, true);
    }

    /* !!! Restrict read permission on the GC root.  Otherwise any
       process that can open the file for reading can DoS the
       collector. */
    
    return fdGCLock.borrow();
}


void createSymlink(const Path & link, const Path & target, bool careful)
{
    /* Create directories up to `gcRoot'. */
    createDirs(dirOf(link));

    /* !!! shouldn't removing and creating the symlink be atomic? */

    /* Remove the old symlink. */
    if (pathExists(link)) {
        if (careful && (!isLink(link) || !isInStore(readLink(link))))
            throw Error(format("cannot create symlink `%1%'; already exists") % link);
        unlink(link.c_str());
    }

    /* And create the new one. */
    if (symlink(target.c_str(), link.c_str()) == -1)
        throw SysError(format("symlinking `%1%' to `%2%'")
            % link % target);
}


void LocalStore::syncWithGC()
{
    AutoCloseFD fdGCLock = openGCLock(ltRead);
}


void LocalStore::addIndirectRoot(const Path & path)
{
    string hash = printHash32(hashString(htSHA1, path));
    Path realRoot = canonPath((format("%1%/%2%/auto/%3%")
        % nixStateDir % gcRootsDir % hash).str());
    createSymlink(realRoot, path, false);
}


Path addPermRoot(const Path & _storePath, const Path & _gcRoot,
    bool indirect, bool allowOutsideRootsDir)
{
    Path storePath(canonPath(_storePath));
    Path gcRoot(canonPath(_gcRoot));
    assertStorePath(storePath);

    if (isInStore(gcRoot))
        throw Error(format(
                "creating a garbage collector root (%1%) in the Nix store is forbidden "
                "(are you running nix-build inside the store?)") % gcRoot);

    if (indirect) {
        createSymlink(gcRoot, storePath, true);
        store->addIndirectRoot(gcRoot);
    }

    else {
        if (!allowOutsideRootsDir) {
            Path rootsDir = canonPath((format("%1%/%2%") % nixStateDir % gcRootsDir).str());
    
            if (string(gcRoot, 0, rootsDir.size() + 1) != rootsDir + "/")
                throw Error(format(
                    "path `%1%' is not a valid garbage collector root; "
                    "it's not in the directory `%2%'")
                    % gcRoot % rootsDir);
        }
            
        createSymlink(gcRoot, storePath, false);
    }

    /* Check that the root can be found by the garbage collector.
       !!! This can be very slow on machines that have many roots.
       Instead of reading all the roots, it would be more efficient to
       check if the root is in a directory in or linked from the
       gcroots directory. */
    if (queryBoolSetting("gc-check-reachability", true)) {
        Roots roots = store->findRoots();
        if (roots.find(gcRoot) == roots.end())
            printMsg(lvlError, 
                format(
                    "warning: `%1%' is not in a directory where the garbage collector looks for roots; "
                    "therefore, `%2%' might be removed by the garbage collector")
                % gcRoot % storePath);
    }
        
    /* Grab the global GC root, causing us to block while a GC is in
       progress.  This prevents the set of permanent roots from
       increasing while a GC is in progress. */
    store->syncWithGC();
    
    return gcRoot;
}


/* The file to which we write our temporary roots. */
static Path fnTempRoots;
static AutoCloseFD fdTempRoots;


void LocalStore::addTempRoot(const Path & path)
{
    /* Create the temporary roots file for this process. */
    if (fdTempRoots == -1) {

        while (1) {
            Path dir = (format("%1%/%2%") % nixStateDir % tempRootsDir).str();
            createDirs(dir);
            
            fnTempRoots = (format("%1%/%2%")
                % dir % getpid()).str();

            AutoCloseFD fdGCLock = openGCLock(ltRead);
            
            if (pathExists(fnTempRoots))
                /* It *must* be stale, since there can be no two
                   processes with the same pid. */
                deletePath(fnTempRoots);

	    fdTempRoots = openLockFile(fnTempRoots, true);

            fdGCLock.close();
      
            debug(format("acquiring read lock on `%1%'") % fnTempRoots);
            lockFile(fdTempRoots, ltRead, true);

            /* Check whether the garbage collector didn't get in our
               way. */
            struct stat st;
            if (fstat(fdTempRoots, &st) == -1)
                throw SysError(format("statting `%1%'") % fnTempRoots);
            if (st.st_size == 0) break;
            
            /* The garbage collector deleted this file before we could
               get a lock.  (It won't delete the file after we get a
               lock.)  Try again. */
        }

    }

    /* Upgrade the lock to a write lock.  This will cause us to block
       if the garbage collector is holding our lock. */
    debug(format("acquiring write lock on `%1%'") % fnTempRoots);
    lockFile(fdTempRoots, ltWrite, true);

    string s = path + '\0';
    writeFull(fdTempRoots, (const unsigned char *) s.c_str(), s.size());

    /* Downgrade to a read lock. */
    debug(format("downgrading to read lock on `%1%'") % fnTempRoots);
    lockFile(fdTempRoots, ltRead, true);
}


void removeTempRoots()
{
    if (fdTempRoots != -1) {
        fdTempRoots.close();
        unlink(fnTempRoots.c_str());
    }
}


typedef boost::shared_ptr<AutoCloseFD> FDPtr;
typedef list<FDPtr> FDs;


static void readTempRoots(PathSet & tempRoots, FDs & fds)
{
    /* Read the `temproots' directory for per-process temporary root
       files. */
    Strings tempRootFiles = readDirectory(
        (format("%1%/%2%") % nixStateDir % tempRootsDir).str());

    foreach (Strings::iterator, i, tempRootFiles) {
        Path path = (format("%1%/%2%/%3%") % nixStateDir % tempRootsDir % *i).str();

        debug(format("reading temporary root file `%1%'") % path);
        FDPtr fd(new AutoCloseFD(open(path.c_str(), O_RDWR, 0666)));
        if (*fd == -1) {
            /* It's okay if the file has disappeared. */
            if (errno == ENOENT) continue;
            throw SysError(format("opening temporary roots file `%1%'") % path);
        }

        /* This should work, but doesn't, for some reason. */
        //FDPtr fd(new AutoCloseFD(openLockFile(path, false)));
        //if (*fd == -1) continue;

        /* Try to acquire a write lock without blocking.  This can
           only succeed if the owning process has died.  In that case
           we don't care about its temporary roots. */
        if (lockFile(*fd, ltWrite, false)) {
            printMsg(lvlError, format("removing stale temporary roots file `%1%'")
                % path);
            unlink(path.c_str());
            writeFull(*fd, (const unsigned char *) "d", 1);
            continue;
        }

        /* Acquire a read lock.  This will prevent the owning process
           from upgrading to a write lock, therefore it will block in
           addTempRoot(). */
        debug(format("waiting for read lock on `%1%'") % path);
        lockFile(*fd, ltRead, true);

        /* Read the entire file. */
        string contents = readFile(*fd);

        /* Extract the roots. */
        string::size_type pos = 0, end;

        while ((end = contents.find((char) 0, pos)) != string::npos) {
            Path root(contents, pos, end - pos);
            debug(format("got temporary root `%1%'") % root);
            assertStorePath(root);
            tempRoots.insert(root);
            pos = end + 1;
        }

        fds.push_back(fd); /* keep open */
    }
}


static void findRoots(const Path & path, bool recurseSymlinks,
    bool deleteStale, Roots & roots)
{
    try {
        
        struct stat st;
        if (lstat(path.c_str(), &st) == -1)
            throw SysError(format("statting `%1%'") % path);

        printMsg(lvlVomit, format("looking at `%1%'") % path);

        if (S_ISDIR(st.st_mode)) {
            Strings names = readDirectory(path);
            foreach (Strings::iterator, i, names)
                findRoots(path + "/" + *i, recurseSymlinks, deleteStale, roots);
        }

        else if (S_ISLNK(st.st_mode)) {
            Path target = absPath(readLink(path), dirOf(path));

            if (isInStore(target)) {
                debug(format("found root `%1%' in `%2%'")
                    % target % path);
                Path storePath = toStorePath(target);
                if (store->isValidPath(storePath)) 
                    roots[path] = storePath;
                else
                    printMsg(lvlInfo, format("skipping invalid root from `%1%' to `%2%'")
                        % path % storePath);
            }

            else if (recurseSymlinks) {
                if (pathExists(target))
                    findRoots(target, false, deleteStale, roots);
                else if (deleteStale) {
                    printMsg(lvlInfo, format("removing stale link from `%1%' to `%2%'") % path % target);
                    /* Note that we only delete when recursing, i.e.,
                       when we are still in the `gcroots' tree.  We
                       never delete stuff outside that tree. */
                    unlink(path.c_str());
                }
            }
        }

    }

    catch (SysError & e) {
        /* We only ignore permanent failures. */
        if (e.errNo == EACCES || e.errNo == ENOENT || e.errNo == ENOTDIR)
            printMsg(lvlInfo, format("cannot read potential root `%1%'") % path);
        else
            throw;
    }
}


static Roots findRoots(bool deleteStale)
{
    Roots roots;
    Path rootsDir = canonPath((format("%1%/%2%") % nixStateDir % gcRootsDir).str());
    findRoots(rootsDir, true, deleteStale, roots);
    return roots;
}


Roots LocalStore::findRoots()
{
    return nix::findRoots(false);
}


static void addAdditionalRoots(PathSet & roots)
{
    Path rootFinder = getEnv("NIX_ROOT_FINDER",
        nixLibexecDir + "/nix/find-runtime-roots.pl");

    if (rootFinder.empty()) return;
    
    debug(format("executing `%1%' to find additional roots") % rootFinder);

    string result = runProgram(rootFinder);

    Strings paths = tokenizeString(result, "\n");
    
    foreach (Strings::iterator, i, paths) {
        if (isInStore(*i)) {
            Path path = toStorePath(*i);
            if (roots.find(path) == roots.end() && store->isValidPath(path)) {
                debug(format("got additional root `%1%'") % path);
                roots.insert(path);
            }
        }
    }
}


static void dfsVisit(const PathSet & paths, const Path & path,
    PathSet & visited, Paths & sorted)
{
    if (visited.find(path) != visited.end()) return;
    visited.insert(path);
    
    PathSet references;
    if (store->isValidPath(path))
        store->queryReferences(path, references);
    
    foreach (PathSet::iterator, i, references)
        /* Don't traverse into paths that don't exist.  That can
           happen due to substitutes for non-existent paths. */
        if (*i != path && paths.find(*i) != paths.end())
            dfsVisit(paths, *i, visited, sorted);

    sorted.push_front(path);
}


Paths topoSortPaths(const PathSet & paths)
{
    Paths sorted;
    PathSet visited;
    foreach (PathSet::const_iterator, i, paths)
        dfsVisit(paths, *i, visited, sorted);
    return sorted;
}


struct GCLimitReached { };


struct LocalStore::GCState
{
    GCOptions options;
    GCResults & results;
    PathSet roots;
    PathSet tempRoots;
    PathSet deleted;
    PathSet live;
    PathSet busy;
    bool gcKeepOutputs;
    bool gcKeepDerivations;
    GCState(GCResults & results_) : results(results_)
    {
    }
};


static bool doDelete(GCOptions::GCAction action)
{
    return action == GCOptions::gcDeleteDead
        || action == GCOptions::gcDeleteSpecific;
}


bool LocalStore::isActiveTempFile(const GCState & state,
    const Path & path, const string & suffix)
{
    return hasSuffix(path, suffix)
        && state.tempRoots.find(string(path, 0, path.size() - suffix.size())) != state.tempRoots.end();
}

    
bool LocalStore::tryToDelete(GCState & state, const Path & path)
{
    if (!pathExists(path)) return true;
    if (state.deleted.find(path) != state.deleted.end()) return true;
    if (state.live.find(path) != state.live.end()) return false;

    startNest(nest, lvlDebug, format("considering whether to delete `%1%'") % path);

    if (state.roots.find(path) != state.roots.end()) {
        printMsg(lvlDebug, format("cannot delete `%1%' because it's a root") % path);
        goto isLive;
    }

    if (isValidPath(path)) {

        /* Recursively try to delete the referrers of this path.  If
           any referrer can't be deleted, then this path can't be
           deleted either. */
        PathSet referrers;
        queryReferrers(path, referrers);
        foreach (PathSet::iterator, i, referrers)
            if (*i != path && !tryToDelete(state, *i)) {
                printMsg(lvlDebug, format("cannot delete `%1%' because it has live referrers") % path);
                goto isLive;
            }

        /* If gc-keep-derivations is set and this is a derivation,
           then don't delete the derivation if any of the outputs are
           live. */
        if (state.gcKeepDerivations && isDerivation(path)) {
            PathSet outputs = queryDerivationOutputs(path);
            foreach (PathSet::iterator, i, outputs)
                if (!tryToDelete(state, *i)) {
                    printMsg(lvlDebug, format("cannot delete derivation `%1%' because its output `%2%' is alive") % path % *i);
                    goto isLive;
                }
        }

        /* If gc-keep-derivations and gc-keep-outputs are both set,
           it's possible that the path has already been deleted (due
           to the recursion below), so bail out. */
        if (!pathExists(path)) return true;

        /* If gc-keep-outputs is set, then don't delete this path if
           there are derivers of this path that are not garbage. */
        if (state.gcKeepOutputs) {
            PathSet derivers = queryValidDerivers(path);
            foreach (PathSet::iterator, deriver, derivers) {
                /* Break an infinite recursion if gc-keep-derivations
                   and gc-keep-outputs are both set by tentatively
                   assuming that this path is garbage.  This is a safe
                   assumption because at this point, the only thing
                   that can prevent it from being garbage is the
                   deriver.  Since tryToDelete() works "upwards"
                   through the dependency graph, it won't encouter
                   this path except in the call to tryToDelete() in
                   the gc-keep-derivation branch. */
                state.deleted.insert(path);
                if (!tryToDelete(state, *deriver)) {
                    state.deleted.erase(path);
                    printMsg(lvlDebug, format("cannot delete `%1%' because its deriver `%2%' is alive") % path % *deriver);
                    goto isLive;
                }
            }
        }
    }

    else {

        /* A lock file belonging to a path that we're building right
           now isn't garbage. */
        if (isActiveTempFile(state, path, ".lock")) return false;

        /* Don't delete .chroot directories for derivations that are
           currently being built. */
        if (isActiveTempFile(state, path, ".chroot")) return false;

    }

    /* The path is garbage, so delete it. */
    if (doDelete(state.options.action)) {
        printMsg(lvlInfo, format("deleting `%1%'") % path);

        unsigned long long bytesFreed, blocksFreed;
        deleteFromStore(path, bytesFreed, blocksFreed);
        state.results.bytesFreed += bytesFreed;
        state.results.blocksFreed += blocksFreed;

        if (state.options.maxFreed && state.results.bytesFreed > state.options.maxFreed) {
            printMsg(lvlInfo, format("deleted more than %1% bytes; stopping") % state.options.maxFreed);
            throw GCLimitReached();
        }

        if (state.options.maxLinks) {
            struct stat st;
            if (stat(nixStore.c_str(), &st) == -1)
                throw SysError(format("statting `%1%'") % nixStore);
            if (st.st_nlink < state.options.maxLinks) {
                printMsg(lvlInfo, format("link count on the store has dropped below %1%; stopping") % state.options.maxLinks);
                throw GCLimitReached();
            }
        }
        
    } else
        printMsg(lvlTalkative, format("would delete `%1%'") % path);
    
    state.deleted.insert(path);
    if (state.options.action != GCOptions::gcReturnLive)
        state.results.paths.insert(path);
    return true;

 isLive:
    state.live.insert(path);
    if (state.options.action == GCOptions::gcReturnLive)
        state.results.paths.insert(path);
    return false;
}


void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
{
    GCState state(results);
    state.options = options;    
    
    state.gcKeepOutputs = queryBoolSetting("gc-keep-outputs", false);
    state.gcKeepDerivations = queryBoolSetting("gc-keep-derivations", true);

    /* Using `--ignore-liveness' with `--delete' can have unintended
       consequences if `gc-keep-outputs' or `gc-keep-derivations' are
       true (the garbage collector will recurse into deleting the
       outputs or derivers, respectively).  So disable them. */
    if (options.action == GCOptions::gcDeleteSpecific && options.ignoreLiveness) {
        state.gcKeepOutputs = false;
        state.gcKeepDerivations = false;
    }
    
    /* Acquire the global GC root.  This prevents
       a) New roots from being added.
       b) Processes from creating new temporary root files. */
    AutoCloseFD fdGCLock = openGCLock(ltWrite);

    /* Find the roots.  Since we've grabbed the GC lock, the set of
       permanent roots cannot increase now. */
    printMsg(lvlError, format("finding garbage collector roots..."));
    Roots rootMap = options.ignoreLiveness ? Roots() : nix::findRoots(true);

    foreach (Roots::iterator, i, rootMap) state.roots.insert(i->second);

    /* Add additional roots returned by the program specified by the
       NIX_ROOT_FINDER environment variable.  This is typically used
       to add running programs to the set of roots (to prevent them
       from being garbage collected). */
    if (!options.ignoreLiveness)
        addAdditionalRoots(state.roots);

    /* Read the temporary roots.  This acquires read locks on all
       per-process temporary root files.  So after this point no paths
       can be added to the set of temporary roots. */
    FDs fds;
    readTempRoots(state.tempRoots, fds);
    state.roots.insert(state.tempRoots.begin(), state.tempRoots.end());

    /* After this point the set of roots or temporary roots cannot
       increase, since we hold locks on everything.  So everything
       that is not reachable from `roots'. */

    /* Now either delete all garbage paths, or just the specified
       paths (for gcDeleteSpecific). */

    if (options.action == GCOptions::gcDeleteSpecific) {

        foreach (PathSet::iterator, i, options.pathsToDelete) {
            assertStorePath(*i);
            if (!tryToDelete(state, *i))
                throw Error(format("cannot delete path `%1%' since it is still alive") % *i);
        }
        
    } else {
        
        printMsg(lvlError, format("reading the Nix store..."));
        Paths entries = readDirectory(nixStore);

        /* Randomise the order in which we delete entries to make the
           collector less biased towards deleting paths that come
           alphabetically first (e.g. /nix/store/000...).  This
           matters when using --max-freed etc. */
        vector<Path> entries_(entries.begin(), entries.end());
        random_shuffle(entries_.begin(), entries_.end());

        if (doDelete(state.options.action))
            printMsg(lvlError, format("deleting garbage..."));
        else
            printMsg(lvlError, format("determining live/dead paths..."));
    
        try {
            foreach (vector<Path>::iterator, i, entries_)
                tryToDelete(state, canonPath(nixStore + "/" + *i));
        } catch (GCLimitReached & e) {
        }
    }        
}


}