summary refs log tree commit diff
path: root/src/libstore/gc.cc
blob: 4385e4456fec807d1d9daefe15edae924d9a4efa (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
#include "globals.hh"
#include "misc.hh"
#include "local-store.hh"
#include "immutable.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. */
int LocalStore::openGCLock(LockType lockType)
{
    Path fnGCLock = (format("%1%/%2%")
        % settings.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);
    closeOnExec(fdGCLock);

    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)
{
    /* Create directories up to `gcRoot'. */
    createDirs(dirOf(link));

    /* Create the new symlink. */
    Path tempLink = (format("%1%.tmp-%2%-%3%")
        % link % getpid() % rand()).str();
    if (symlink(target.c_str(), tempLink.c_str()) == -1)
        throw SysError(format("symlinking `%1%' to `%2%'")
            % tempLink % target);

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


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%")
        % settings.nixStateDir % gcRootsDir % hash).str());
    createSymlink(realRoot, path);
}


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

    else {
        if (!allowOutsideRootsDir) {
            Path rootsDir = canonPath((format("%1%/%2%") % settings.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);
    }

    /* 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 (settings.checkRootReachability) {
        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%") % settings.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. */
                unlink(fnTempRoots.c_str());

            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.data(), 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());
    }
}


/* Automatically clean up the temporary roots file when we exit. */
struct RemoveTempRoots
{
    ~RemoveTempRoots()
    {
        removeTempRoots();
    }
};

static RemoveTempRoots autoRemoveTempRoots __attribute__((unused));


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%") % settings.nixStateDir % tempRootsDir).str());

    foreach (Strings::iterator, i, tempRootFiles) {
        Path path = (format("%1%/%2%/%3%") % settings.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(StoreAPI & store, 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(store, 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(store, 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(StoreAPI & store, bool deleteStale)
{
    Roots roots;
    Path rootsDir = canonPath((format("%1%/%2%") % settings.nixStateDir % gcRootsDir).str());
    findRoots(store, rootsDir, true, deleteStale, roots);
    return roots;
}


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


static void addAdditionalRoots(StoreAPI & store, PathSet & roots)
{
    Path rootFinder = getEnv("NIX_ROOT_FINDER",
        settings.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<Strings>(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);
            }
        }
    }
}


struct GCLimitReached { };


struct LocalStore::GCState
{
    GCOptions options;
    GCResults & results;
    PathSet roots;
    PathSet tempRoots;
    PathSet dead;
    PathSet alive;
    bool gcKeepOutputs;
    bool gcKeepDerivations;
    unsigned long long bytesInvalidated;
    Path trashDir;
    bool shouldDelete;
    GCState(GCResults & results_) : results(results_), bytesInvalidated(0) { }
};


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();
}


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


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

    unsigned long long size = 0;

    if (isValidPath(path)) {
        PathSet referrers;
        queryReferrers(path, referrers);
        foreach (PathSet::iterator, i, referrers)
            if (*i != path) deletePathRecursive(state, *i);
        size = queryPathInfo(path).narSize;
        invalidatePathChecked(path);
    }

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

    printMsg(lvlInfo, format("deleting `%1%'") % path);

    /* If the path is not a regular file or symlink, move it to the
       trash directory.  The move is to ensure that later (when we're
       not holding the global GC lock) we can delete the path without
       being afraid that the path has become alive again.  Otherwise
       delete it right away. */
    if (S_ISDIR(st.st_mode)) {
        // Estimate the amount freed using the narSize field.  FIXME:
        // if the path was not valid, need to determine the actual
        // size.
        state.bytesInvalidated += size;
        makeMutable(path.c_str());
        // Mac OS X cannot rename directories if they are read-only.
        if (chmod(path.c_str(), st.st_mode | S_IWUSR) == -1)
            throw SysError(format("making `%1%' writable") % path);
        Path tmp = state.trashDir + "/" + baseNameOf(path);
        if (rename(path.c_str(), tmp.c_str()))
            throw SysError(format("unable to rename `%1%' to `%2%'") % path % tmp);
    } else
        deleteGarbage(state, path);

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


bool LocalStore::canReachRoot(GCState & state, PathSet & visited, const Path & path)
{
    if (visited.find(path) != visited.end()) return false;

    if (state.alive.find(path) != state.alive.end()) {
        return true;
    }

    if (state.dead.find(path) != state.dead.end()) {
        return false;
    }

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

    visited.insert(path);

    if (!isValidPath(path)) return false;

    PathSet incoming;

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

    /* If gc-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 && isDerivation(path)) {
        PathSet outputs = queryDerivationOutputs(path);
        foreach (PathSet::iterator, i, outputs)
            if (isValidPath(*i) && queryDeriver(*i) == path)
                incoming.insert(*i);
    }

    /* 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, i, derivers)
            incoming.insert(*i);
    }

    foreach (PathSet::iterator, 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();

    if (path == linksDir || path == state.trashDir) return;

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

    if (!isValidPath(path)) {
        /* 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;
    }

    PathSet visited;

    if (canReachRoot(state, visited, path)) {
        printMsg(lvlDebug, format("cannot delete `%1%' 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. */
        state.dead.insert(visited.begin(), visited.end());
        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(format("opening directory `%1%'") % linksDir);

    long long actualSize = 0, unsharedSize = 0;

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

        struct stat st;
        if (lstat(path.c_str(), &st) == -1)
            throw SysError(format("statting `%1%'") % path);

        if (st.st_nlink != 1) {
            unsigned long long size = st.st_blocks * 512ULL;
            actualSize += size;
            unsharedSize += (st.st_nlink - 1) * size;
            continue;
        }

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

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

        state.results.bytesFreed += st.st_blocks * 512;
    }

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

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


void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
{
    GCState state(results);
    state.options = options;
    state.trashDir = settings.nixStore + "/trash";
    state.gcKeepOutputs = settings.gcKeepOutputs;
    state.gcKeepDerivations = settings.gcKeepDerivations;

    /* 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;
    }

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

    /* 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(*this, 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(*this, 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'. */

    if (state.shouldDelete) createDirs(state.trashDir);

    /* 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);
            tryToDelete(state, *i);
            if (state.dead.find(*i) == state.dead.end())
                throw Error(format("cannot delete path `%1%' since it is still alive") % *i);
        }

    } else if (options.maxFreed > 0) {

        if (state.shouldDelete)
            printMsg(lvlError, format("deleting garbage..."));
        else
            printMsg(lvlError, format("determining live/dead paths..."));

        try {

            AutoCloseDir dir = opendir(settings.nixStore.c_str());
            if (!dir) throw SysError(format("opening directory `%1%'") % settings.nixStore);

            /* 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)) {
                checkInterrupt();
                string name = dirent->d_name;
                if (name == "." || name == "..") continue;
                Path path = settings.nixStore + "/" + name;
                if (isValidPath(path))
                    entries.push_back(path);
                else
                    tryToDelete(state, path);
            }

            dir.close();

            /* 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());
            random_shuffle(entries_.begin(), entries_.end());

            foreach (vector<Path>::iterator, i, entries_)
                tryToDelete(state, *i);

        } catch (GCLimitReached & e) {
        }
    }

    if (state.options.action == GCOptions::gcReturnLive) {
        state.results.paths = state.alive;
        return;
    }

    if (state.options.action == GCOptions::gcReturnDead) {
        state.results.paths = state.dead;
        return;
    }

    /* Allow other processes to add to the store from here on. */
    fdGCLock.close();

    /* Delete the trash directory. */
    printMsg(lvlInfo, format("deleting `%1%'") % state.trashDir);
    deleteGarbage(state, state.trashDir);

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

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


}