lix/src/libstore/gc.cc

#include "derivations.hh"
#include "globals.hh"
#include "local-store.hh"
#include "local-fs-store.hh"
#include "finally.hh"

#include <functional>
#include <queue>
#include <algorithm>
#include <regex>
#include <random>

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

namespace nix {


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


/* 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. */
AutoCloseFD LocalStore::openGCLock(LockType lockType)
{
    Path fnGCLock = (format("%1%/%2%")
        % stateDir % gcLockName).str();

    debug(format("acquiring global GC lock '%1%'") % fnGCLock);

    AutoCloseFD fdGCLock = open(fnGCLock.c_str(), O_RDWR | O_CREAT | O_CLOEXEC, 0600);
    if (!fdGCLock)
        throw SysError("opening global GC lock '%1%'", fnGCLock);

    if (!lockFile(fdGCLock.get(), lockType, false)) {
        printInfo("waiting for the big garbage collector lock...");
        lockFile(fdGCLock.get(), 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;
}


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

    /* Create the new symlink. */
    Path tempLink = (format("%1%.tmp-%2%-%3%")
        % link % getpid() % random()).str();
    createSymlink(target, tempLink);

    /* Atomically replace the old one. */
    if (rename(tempLink.c_str(), link.c_str()) == -1)
        throw SysError("cannot rename '%1%' to '%2%'",
            tempLink , link);
}


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


void LocalStore::addIndirectRoot(const Path & path)
{
    string hash = hashString(htSHA1, path).to_string(Base32, false);
    Path realRoot = canonPath((format("%1%/%2%/auto/%3%")
        % stateDir % gcRootsDir % hash).str());
    makeSymlink(realRoot, path);
}


Path LocalFSStore::addPermRoot(const StorePath & storePath, const Path & _gcRoot)
{
    Path gcRoot(canonPath(_gcRoot));

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

    /* Don't clobber the link if it already exists and doesn't
       point to the Nix store. */
    if (pathExists(gcRoot) && (!isLink(gcRoot) || !isInStore(readLink(gcRoot))))
        throw Error("cannot create symlink '%1%'; already exists", gcRoot);
    makeSymlink(gcRoot, printStorePath(storePath));
    addIndirectRoot(gcRoot);

    /* 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. */
    syncWithGC();

    return gcRoot;
}


void LocalStore::addTempRoot(const StorePath & path)
{
    auto state(_state.lock());

    /* Create the temporary roots file for this process. */
    if (!state->fdTempRoots) {

        while (1) {
            AutoCloseFD fdGCLock = openGCLock(ltRead);

            if (pathExists(fnTempRoots))
                /* It *must* be stale, since there can be no two
                   processes with the same pid. */
                unlink(fnTempRoots.c_str());

            state->fdTempRoots = openLockFile(fnTempRoots, true);

            fdGCLock = -1;

            debug(format("acquiring read lock on '%1%'") % fnTempRoots);
            lockFile(state->fdTempRoots.get(), ltRead, true);

            /* Check whether the garbage collector didn't get in our
               way. */
            struct stat st;
            if (fstat(state->fdTempRoots.get(), &st) == -1)
                throw SysError("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(state->fdTempRoots.get(), ltWrite, true);

    string s = printStorePath(path) + '\0';
    writeFull(state->fdTempRoots.get(), s);

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


static std::string censored = "{censored}";


void LocalStore::findTempRoots(FDs & fds, Roots & tempRoots, bool censor)
{
    /* Read the `temproots' directory for per-process temporary root
       files. */
    for (auto & i : readDirectory(tempRootsDir)) {
        if (i.name[0] == '.') {
            // Ignore hidden files. Some package managers (notably portage) create
            // those to keep the directory alive.
            continue;
        }
        Path path = tempRootsDir + "/" + i.name;

        pid_t pid = std::stoi(i.name);

        debug(format("reading temporary root file '%1%'") % path);
        FDPtr fd(new AutoCloseFD(open(path.c_str(), O_CLOEXEC | O_RDWR, 0666)));
        if (!*fd) {
            /* It's okay if the file has disappeared. */
            if (errno == ENOENT) continue;
            throw SysError("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->get(), ltWrite, false)) {
            printInfo("removing stale temporary roots file '%1%'", path);
            unlink(path.c_str());
            writeFull(fd->get(), "d");
            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->get(), ltRead, true);

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

        /* 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("got temporary root '%s'", root);
            tempRoots[parseStorePath(root)].emplace(censor ? censored : fmt("{temp:%d}", pid));
            pos = end + 1;
        }

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


void LocalStore::findRoots(const Path & path, unsigned char type, Roots & roots)
{
    auto foundRoot = [&](const Path & path, const Path & target) {
        try {
            auto storePath = toStorePath(target).first;
            if (isValidPath(storePath))
                roots[std::move(storePath)].emplace(path);
            else
                printInfo("skipping invalid root from '%1%' to '%2%'", path, target);
        } catch (BadStorePath &) { }
    };

    try {

        if (type == DT_UNKNOWN)
            type = getFileType(path);

        if (type == DT_DIR) {
            for (auto & i : readDirectory(path))
                findRoots(path + "/" + i.name, i.type, roots);
        }

        else if (type == DT_LNK) {
            Path target = readLink(path);
            if (isInStore(target))
                foundRoot(path, target);

            /* Handle indirect roots. */
            else {
                target = absPath(target, dirOf(path));
                if (!pathExists(target)) {
                    if (isInDir(path, stateDir + "/" + gcRootsDir + "/auto")) {
                        printInfo(format("removing stale link from '%1%' to '%2%'") % path % target);
                        unlink(path.c_str());
                    }
                } else {
                    struct stat st2 = lstat(target);
                    if (!S_ISLNK(st2.st_mode)) return;
                    Path target2 = readLink(target);
                    if (isInStore(target2)) foundRoot(target, target2);
                }
            }
        }

        else if (type == DT_REG) {
            auto storePath = maybeParseStorePath(storeDir + "/" + std::string(baseNameOf(path)));
            if (storePath && isValidPath(*storePath))
                roots[std::move(*storePath)].emplace(path);
        }

    }

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


void LocalStore::findRootsNoTemp(Roots & roots, bool censor)
{
    /* Process direct roots in {gcroots,profiles}. */
    findRoots(stateDir + "/" + gcRootsDir, DT_UNKNOWN, roots);
    findRoots(stateDir + "/profiles", DT_UNKNOWN, roots);

    /* Add additional roots returned by different platforms-specific
       heuristics.  This is typically used to add running programs to
       the set of roots (to prevent them from being garbage collected). */
    findRuntimeRoots(roots, censor);
}


Roots LocalStore::findRoots(bool censor)
{
    Roots roots;
    findRootsNoTemp(roots, censor);

    FDs fds;
    findTempRoots(fds, roots, censor);

    return roots;
}

typedef std::unordered_map<Path, std::unordered_set<std::string>> UncheckedRoots;

static void readProcLink(const string & file, UncheckedRoots & roots)
{
    /* 64 is the starting buffer size gnu readlink uses... */
    auto bufsiz = ssize_t{64};
try_again:
    char buf[bufsiz];
    auto res = readlink(file.c_str(), buf, bufsiz);
    if (res == -1) {
        if (errno == ENOENT || errno == EACCES || errno == ESRCH)
            return;
        throw SysError("reading symlink");
    }
    if (res == bufsiz) {
        if (SSIZE_MAX / 2 < bufsiz)
            throw Error("stupidly long symlink");
        bufsiz *= 2;
        goto try_again;
    }
    if (res > 0 && buf[0] == '/')
        roots[std::string(static_cast<char *>(buf), res)]
            .emplace(file);
}

static string quoteRegexChars(const string & raw)
{
    static auto specialRegex = std::regex(R"([.^$\\*+?()\[\]{}|])");
    return std::regex_replace(raw, specialRegex, R"(\$&)");
}

static void readFileRoots(const char * path, UncheckedRoots & roots)
{
    try {
        roots[readFile(path)].emplace(path);
    } catch (SysError & e) {
        if (e.errNo != ENOENT && e.errNo != EACCES)
            throw;
    }
}

void LocalStore::findRuntimeRoots(Roots & roots, bool censor)
{
    UncheckedRoots unchecked;

    auto procDir = AutoCloseDir{opendir("/proc")};
    if (procDir) {
        struct dirent * ent;
        auto digitsRegex = std::regex(R"(^\d+$)");
        auto mapRegex = std::regex(R"(^\s*\S+\s+\S+\s+\S+\s+\S+\s+\S+\s+(/\S+)\s*$)");
        auto storePathRegex = std::regex(quoteRegexChars(storeDir) + R"(/[0-9a-z]+[0-9a-zA-Z\+\-\._\?=]*)");
        while (errno = 0, ent = readdir(procDir.get())) {
            checkInterrupt();
            if (std::regex_match(ent->d_name, digitsRegex)) {
                readProcLink(fmt("/proc/%s/exe" ,ent->d_name), unchecked);
                readProcLink(fmt("/proc/%s/cwd", ent->d_name), unchecked);

                auto fdStr = fmt("/proc/%s/fd", ent->d_name);
                auto fdDir = AutoCloseDir(opendir(fdStr.c_str()));
                if (!fdDir) {
                    if (errno == ENOENT || errno == EACCES)
                        continue;
                    throw SysError("opening %1%", fdStr);
                }
                struct dirent * fd_ent;
                while (errno = 0, fd_ent = readdir(fdDir.get())) {
                    if (fd_ent->d_name[0] != '.')
                        readProcLink(fmt("%s/%s", fdStr, fd_ent->d_name), unchecked);
                }
                if (errno) {
                    if (errno == ESRCH)
                        continue;
                    throw SysError("iterating /proc/%1%/fd", ent->d_name);
                }
                fdDir.reset();

                try {
                    auto mapFile = fmt("/proc/%s/maps", ent->d_name);
                    auto mapLines = tokenizeString<std::vector<string>>(readFile(mapFile), "\n");
                    for (const auto & line : mapLines) {
                        auto match = std::smatch{};
                        if (std::regex_match(line, match, mapRegex))
                            unchecked[match[1]].emplace(mapFile);
                    }

                    auto envFile = fmt("/proc/%s/environ", ent->d_name);
                    auto envString = readFile(envFile);
                    auto env_end = std::sregex_iterator{};
                    for (auto i = std::sregex_iterator{envString.begin(), envString.end(), storePathRegex}; i != env_end; ++i)
                        unchecked[i->str()].emplace(envFile);
                } catch (SysError & e) {
                    if (errno == ENOENT || errno == EACCES || errno == ESRCH)
                        continue;
                    throw;
                }
            }
        }
        if (errno)
            throw SysError("iterating /proc");
    }

#if !defined(__linux__)
    // lsof is really slow on OS X. This actually causes the gc-concurrent.sh test to fail.
    // See: https://github.com/NixOS/nix/issues/3011
    // Because of this we disable lsof when running the tests.
    if (getEnv("_NIX_TEST_NO_LSOF") != "1") {
        try {
            std::regex lsofRegex(R"(^n(/.*)$)");
            auto lsofLines =
                tokenizeString<std::vector<string>>(runProgram(LSOF, true, { "-n", "-w", "-F", "n" }), "\n");
            for (const auto & line : lsofLines) {
                std::smatch match;
                if (std::regex_match(line, match, lsofRegex))
                    unchecked[match[1]].emplace("{lsof}");
            }
        } catch (ExecError & e) {
            /* lsof not installed, lsof failed */
        }
    }
#endif

#if defined(__linux__)
    readFileRoots("/proc/sys/kernel/modprobe", unchecked);
    readFileRoots("/proc/sys/kernel/fbsplash", unchecked);
    readFileRoots("/proc/sys/kernel/poweroff_cmd", unchecked);
#endif

    for (auto & [target, links] : unchecked) {
        if (!isInStore(target)) continue;
        try {
            auto path = toStorePath(target).first;
            if (!isValidPath(path)) continue;
            debug("got additional root '%1%'", printStorePath(path));
            if (censor)
                roots[path].insert(censored);
            else
                roots[path].insert(links.begin(), links.end());
        } catch (BadStorePath &) { }
    }
}


struct GCLimitReached { };


struct LocalStore::GCState
{
    const GCOptions & options;
    GCResults & results;
    StorePathSet roots;
    StorePathSet tempRoots;
    StorePathSet dead;
    StorePathSet alive;
    bool gcKeepOutputs;
    bool gcKeepDerivations;
    bool shouldDelete;
    GCState(const GCOptions & options, GCResults & results)
        : options(options), results(results) { }
};


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


void LocalStore::deleteGarbage(GCState & state, const Path & path)
{
    uint64_t bytesFreed;
    deletePath(path, bytesFreed);
    state.results.bytesFreed += bytesFreed;
}


void LocalStore::deletePathRecursive(GCState & state, const Path & path)
{
    checkInterrupt();

    auto storePath = maybeParseStorePath(path);
    if (storePath && isValidPath(*storePath)) {
        StorePathSet referrers;
        queryReferrers(*storePath, referrers);
        for (auto & i : referrers)
            if (printStorePath(i) != path) deletePathRecursive(state, printStorePath(i));
        invalidatePathChecked(*storePath);
    }

    Path realPath = realStoreDir + "/" + std::string(baseNameOf(path));

    struct stat st;
    if (lstat(realPath.c_str(), &st)) {
        if (errno == ENOENT) return;
        throw SysError("getting status of %1%", realPath);
    }

    printInfo(format("deleting '%1%'") % path);

    state.results.paths.insert(path);

    deleteGarbage(state, realPath);

    if (state.results.bytesFreed > state.options.maxFreed) {
        printInfo("deleted more than %d bytes; stopping", state.options.maxFreed);
        throw GCLimitReached();
    }
}


bool LocalStore::canReachRoot(GCState & state, StorePathSet & visited, const StorePath & path)
{
    if (visited.count(path)) return false;

    if (state.alive.count(path)) return true;

    if (state.dead.count(path)) return false;

    if (state.roots.count(path)) {
        debug("cannot delete '%1%' because it's a root", printStorePath(path));
        state.alive.insert(path);
        return true;
    }

    visited.insert(path);

    if (!isValidPath(path)) return false;

    StorePathSet incoming;

    /* Don't delete this path if any of its referrers are alive. */
    queryReferrers(path, incoming);

    /* If keep-derivations is set and this is a derivation, then
       don't delete the derivation if any of the outputs are alive. */
    if (state.gcKeepDerivations && path.isDerivation()) {
        for (auto & [name, maybeOutPath] : queryPartialDerivationOutputMap(path))
            if (maybeOutPath &&
                isValidPath(*maybeOutPath) &&
                queryPathInfo(*maybeOutPath)->deriver == path
                )
                incoming.insert(*maybeOutPath);
    }

    /* If keep-outputs is set, then don't delete this path if there
       are derivers of this path that are not garbage. */
    if (state.gcKeepOutputs) {
        auto derivers = queryValidDerivers(path);
        for (auto & i : derivers)
            incoming.insert(i);
    }

    for (auto & i : incoming)
        if (i != path)
            if (canReachRoot(state, visited, i)) {
                state.alive.insert(path);
                return true;
            }

    return false;
}


void LocalStore::tryToDelete(GCState & state, const Path & path)
{
    checkInterrupt();

    auto realPath = realStoreDir + "/" + std::string(baseNameOf(path));
    if (realPath == linksDir) return;

    //Activity act(*logger, lvlDebug, format("considering whether to delete '%1%'") % path);

    auto storePath = maybeParseStorePath(path);

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

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

        /* Don't delete .check directories for derivations that are
           currently being built, because we may need to run
           diff-hook. */
        if (isActiveTempFile(state, path, ".check")) return;
    }

    StorePathSet visited;

    if (storePath && canReachRoot(state, visited, *storePath)) {
        debug("cannot delete '%s' because it's still reachable", path);
    } else {
        /* No path we visited was a root, so everything is garbage.
           But we only delete ‘path’ and its referrers here so that
           ‘nix-store --delete’ doesn't have the unexpected effect of
           recursing into derivations and outputs. */
        for (auto & i : visited)
            state.dead.insert(i);
        if (state.shouldDelete)
            deletePathRecursive(state, path);
    }
}


/* Unlink all files in /nix/store/.links that have a link count of 1,
   which indicates that there are no other links and so they can be
   safely deleted.  FIXME: race condition with optimisePath(): we
   might see a link count of 1 just before optimisePath() increases
   the link count. */
void LocalStore::removeUnusedLinks(const GCState & state)
{
    AutoCloseDir dir(opendir(linksDir.c_str()));
    if (!dir) throw SysError("opening directory '%1%'", linksDir);

    int64_t actualSize = 0, unsharedSize = 0;

    struct dirent * dirent;
    while (errno = 0, dirent = readdir(dir.get())) {
        checkInterrupt();
        string name = dirent->d_name;
        if (name == "." || name == "..") continue;
        Path path = linksDir + "/" + name;

        auto st = lstat(path);

        if (st.st_nlink != 1) {
            actualSize += st.st_size;
            unsharedSize += (st.st_nlink - 1) * st.st_size;
            continue;
        }

        printMsg(lvlTalkative, format("deleting unused link '%1%'") % path);

        if (unlink(path.c_str()) == -1)
            throw SysError("deleting '%1%'", path);

        state.results.bytesFreed += st.st_size;
    }

    struct stat st;
    if (stat(linksDir.c_str(), &st) == -1)
        throw SysError("statting '%1%'", linksDir);
    int64_t overhead = st.st_blocks * 512ULL;

    printInfo("note: currently hard linking saves %.2f MiB",
        ((unsharedSize - actualSize - overhead) / (1024.0 * 1024.0)));
}


void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
{
    GCState state(options, results);
    state.gcKeepOutputs = settings.gcKeepOutputs;
    state.gcKeepDerivations = settings.gcKeepDerivations;

    /* Using `--ignore-liveness' with `--delete' can have unintended
       consequences if `keep-outputs' or `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;
    }

    state.shouldDelete = options.action == GCOptions::gcDeleteDead || options.action == GCOptions::gcDeleteSpecific;

    if (state.shouldDelete)
        deletePath(reservedPath);

    /* Acquire the global GC root. */
    AutoCloseFD fdGCLock = openGCLock(ltWrite);

    /* Find the roots.  Since we've grabbed the GC lock, the set of
       permanent roots cannot increase now. */
    printInfo("finding garbage collector roots...");
    Roots rootMap;
    if (!options.ignoreLiveness)
        findRootsNoTemp(rootMap, true);

    for (auto & i : rootMap) state.roots.insert(i.first);

    /* 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;
    Roots tempRoots;
    findTempRoots(fds, tempRoots, true);
    for (auto & root : tempRoots) {
        state.tempRoots.insert(root.first);
        state.roots.insert(root.first);
    }

    /* 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' is garbage. */

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

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

        for (auto & i : options.pathsToDelete) {
            tryToDelete(state, printStorePath(i));
            if (state.dead.find(i) == state.dead.end())
                throw Error(
                    "cannot delete path '%1%' since it is still alive. "
                    "To find out why use: "
                    "nix-store --query --roots",
                    printStorePath(i));
        }

    } else if (options.maxFreed > 0) {

        if (state.shouldDelete)
            printInfo("deleting garbage...");
        else
            printInfo("determining live/dead paths...");

        try {

            AutoCloseDir dir(opendir(realStoreDir.get().c_str()));
            if (!dir) throw SysError("opening directory '%1%'", realStoreDir);

            /* Read the store and immediately delete all paths that
               aren't valid.  When using --max-freed etc., deleting
               invalid paths is preferred over deleting unreachable
               paths, since unreachable paths could become reachable
               again.  We don't use readDirectory() here so that GCing
               can start faster. */
            Paths entries;
            struct dirent * dirent;
            while (errno = 0, dirent = readdir(dir.get())) {
                checkInterrupt();
                string name = dirent->d_name;
                if (name == "." || name == "..") continue;
                Path path = storeDir + "/" + name;
                auto storePath = maybeParseStorePath(path);
                if (storePath && isValidPath(*storePath))
                    entries.push_back(path);
                else
                    tryToDelete(state, path);
            }

            dir.reset();

            /* Now delete the unreachable valid paths.  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());
            std::mt19937 gen(1);
            std::shuffle(entries_.begin(), entries_.end(), gen);

            for (auto & i : entries_)
                tryToDelete(state, i);

        } catch (GCLimitReached & e) {
        }
    }

    if (state.options.action == GCOptions::gcReturnLive) {
        for (auto & i : state.alive)
            state.results.paths.insert(printStorePath(i));
        return;
    }

    if (state.options.action == GCOptions::gcReturnDead) {
        for (auto & i : state.dead)
            state.results.paths.insert(printStorePath(i));
        return;
    }

    /* Clean up the links directory. */
    if (options.action == GCOptions::gcDeleteDead || options.action == GCOptions::gcDeleteSpecific) {
        printInfo("deleting unused links...");
        removeUnusedLinks(state);
    }

    /* While we're at it, vacuum the database. */
    //if (options.action == GCOptions::gcDeleteDead) vacuumDB();
}


void LocalStore::autoGC(bool sync)
{
    static auto fakeFreeSpaceFile = getEnv("_NIX_TEST_FREE_SPACE_FILE");

    auto getAvail = [this]() -> uint64_t {
        if (fakeFreeSpaceFile)
            return std::stoll(readFile(*fakeFreeSpaceFile));

        struct statvfs st;
        if (statvfs(realStoreDir.get().c_str(), &st))
            throw SysError("getting filesystem info about '%s'", realStoreDir);

        return (uint64_t) st.f_bavail * st.f_frsize;
    };

    std::shared_future<void> future;

    {
        auto state(_state.lock());

        if (state->gcRunning) {
            future = state->gcFuture;
            debug("waiting for auto-GC to finish");
            goto sync;
        }

        auto now = std::chrono::steady_clock::now();

        if (now < state->lastGCCheck + std::chrono::seconds(settings.minFreeCheckInterval)) return;

        auto avail = getAvail();

        state->lastGCCheck = now;

        if (avail >= settings.minFree || avail >= settings.maxFree) return;

        if (avail > state->availAfterGC * 0.97) return;

        state->gcRunning = true;

        std::promise<void> promise;
        future = state->gcFuture = promise.get_future().share();

        std::thread([promise{std::move(promise)}, this, avail, getAvail]() mutable {

            try {

                /* Wake up any threads waiting for the auto-GC to finish. */
                Finally wakeup([&]() {
                    auto state(_state.lock());
                    state->gcRunning = false;
                    state->lastGCCheck = std::chrono::steady_clock::now();
                    promise.set_value();
                });

                GCOptions options;
                options.maxFreed = settings.maxFree - avail;

                printInfo("running auto-GC to free %d bytes", options.maxFreed);

                GCResults results;

                collectGarbage(options, results);

                _state.lock()->availAfterGC = getAvail();

            } catch (...) {
                // FIXME: we could propagate the exception to the
                // future, but we don't really care.
                ignoreException();
            }

        }).detach();
    }

 sync:
    // Wait for the future outside of the state lock.
    if (sync) future.get();
}


}