about summary refs log tree commit diff
path: root/src/libstore/build.cc
blob: 6d0498968d0dee6e4c1b5f981bf497f63fee9ae6 (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
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
#include <map>
#include <iostream>
#include <sstream>
#include <boost/shared_ptr.hpp>
#include <boost/weak_ptr.hpp>
#include <boost/enable_shared_from_this.hpp>

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

#ifdef __CYGWIN__
#include <windows.h>
#endif

#include <pwd.h>
#include <grp.h>

#include "build.hh"
#include "references.hh"
#include "pathlocks.hh"
#include "misc.hh"
#include "globals.hh"
#include "gc.hh"


/* !!! TODO derivationFromPath shouldn't be used here */


static string pathNullDevice = "/dev/null";


static const uid_t rootUserId = 0;


/* 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>
{
public:
    typedef enum {ecBusy, ecSuccess, ecFailed} ExitCode;
    
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;

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

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

    Goal(Worker & worker) : worker(worker)
    {
        nrFailed = 0;
        exitCode = ecBusy;
    }

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

public:
    virtual void work() = 0;

    void addWaitee(GoalPtr waitee);

    virtual void waiteeDone(GoalPtr waitee, bool success);

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

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

    void trace(const format & f);

    string getName()
    {
        return name;
    }
    
    ExitCode getExitCode()
    {
        return exitCode;
    }

protected:
    void amDone(bool success = true);
};


/* A mapping used to remember for each child process to what goal it
   belongs, and file descriptors for receiving log data and output
   path creation commands. */
struct Child
{
    WeakGoalPtr goal;
    set<int> fds;
    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 derivation / 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,
        const set<int> & fds, 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 goals have finished. */
    void 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);

    trace(format("waitee `%1%' done; %2% left") %
        waitee->name % waitees.size());
    
    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(exitCode == ecBusy);
    exitCode = success ? ecSuccess : ecFailed;
    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 session (and thus a separate
       process group) so that it has no controlling terminal (meaning
       that e.g. ssh cannot open /dev/tty) and it doesn't receive
       terminal signals. */
    if (setsid() == -1)
        throw SysError(format("creating a new session"));
    
    /* Dup the write side of the logger pipe into stderr. */
    if (dup2(logPipe.writeSide, STDERR_FILENO) == -1)
        throw SysError("cannot pipe standard error into log file");
    logPipe.readSide.close();
            
    /* Dup stderr to stdout. */
    if (dup2(STDERR_FILENO, STDOUT_FILENO) == -1)
        throw SysError("cannot dup stderr into stdout");

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


/* 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;
}


/* Hack for Cygwin: _exit() doesn't seem to work quite right, since
   some Berkeley DB code appears to be called when a child exits
   through _exit() (e.g., because execve() failed).  So call the
   Windows API directly. */
#define _exit(n) ExitProcess(n)



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


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

    Path fnUserLock;
    AutoCloseFD fdUserLock;

    uid_t uid;
    
public:
    UserLock();
    ~UserLock();

    void acquire();
    void release();

    uid_t getUID();
};


PathSet UserLock::lockedPaths;


UserLock::UserLock()
{
    uid = 0;
}


UserLock::~UserLock()
{
    release();
}


void UserLock::acquire()
{
    assert(uid == 0);
    
    Strings buildUsers = querySetting("build-users", Strings());

    if (buildUsers.empty())
        throw Error(
            "cannot build as `root'; you must define "
            "one or more users for building in `build-users' "
            "in the Nix configuration file");

    for (Strings::iterator i = buildUsers.begin(); i != buildUsers.end(); ++i) {
        printMsg(lvlError, format("trying user `%1%'") % *i);

        struct passwd * pw = getpwnam(i->c_str());
        if (!pw)
            throw Error(format("the user `%1%' listed in `build-users' does not exist") % *i);

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

        if (lockedPaths.find(fnUserLock) != lockedPaths.end())
            /* We already have a lock on this one. */
            continue;
        
        AutoCloseFD fd = open(fnUserLock.c_str(), O_RDWR | O_CREAT, 0600);
        if (fd == -1)
            throw SysError(format("opening user lock `%1%'") % fnUserLock);

        if (lockFile(fd, ltWrite, false)) {
            fdUserLock = fd.borrow();
            lockedPaths.insert(fnUserLock);
            uid = pw->pw_uid;
            return;
        }
    }

    throw Error("all build users are currently in use; "
        "consider expanding the `build-users' field "
        "in the Nix configuration file");
}


void UserLock::release()
{
    if (uid == 0) return;
    fdUserLock.close(); /* releases lock */
    assert(lockedPaths.find(fnUserLock) != lockedPaths.end());
    lockedPaths.erase(fnUserLock);
    fnUserLock = "";
    uid = 0;
}


uid_t UserLock::getUID()
{
    return uid;
}


static void killUser(uid_t uid)
{
    debug(format("killing all processes running under uid `%1%'") % uid);
    
    assert(uid != rootUserId); /* just to be safe... */

    /* The system call kill(-1, sig) sends the signal `sig' to all
       users to which the current process can send signals.  So we
       fork a process, switch to uid, and send a mass kill. */

    Pid pid;
    pid = fork();
    switch (pid) {

    case -1:
        throw SysError("unable to fork");

    case 0:
        try { /* child */

            if (setuid(uid) == -1) abort();

            if (kill(-1, SIGKILL) == -1)
                throw SysError(format("cannot kill processes for UID `%1%'") % uid);
        
        } catch (exception & e) {
            cerr << format("build error: %1%\n") % e.what();
            _exit(1);
        }
        _exit(0);
    }
    
    /* parent */
    if (pid.wait(true) != 0)
        throw Error(format("cannot kill processes for UID `%1%'") % uid);

    /* !!! We should really do some check to make sure that there are
       no processes left running under `uid', but there is no portable
       way to do so (I think).  The most reliable way may be `ps -eo
       uid | grep -q $uid'. */
}



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


class DerivationGoal : public Goal
{
private:
    /* The path of the derivation. */
    Path drvPath;

    /* The derivation 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;

    /* User selected for running the builder. */
    UserLock buildUser;

    /* 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();

    Path getDrvPath()
    {
        return drvPath;
    }
    
private:
    /* The states. */
    void init();
    void haveStoreExpr();
    void outputsSubstituted();
    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 handleChildOutput(int fd, const string & data);
    void handleEOF(int fd);

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


DerivationGoal::DerivationGoal(const Path & drvPath, Worker & worker)
    : Goal(worker)
{
    this->drvPath = drvPath;
    state = &DerivationGoal::init;
    name = (format("building of `%1%'") % drvPath).str();
    trace("created");
}


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 derivation
       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);

    for (DerivationOutputs::iterator i = drv.outputs.begin();
         i != drv.outputs.end(); ++i)
        addTempRoot(i->second.path);

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

    /* If they are all valid, then we're done. */
    if (invalidOutputs.size() == 0) {
        amDone(true);
        return;
    }

    /* We are first going to try to create the invalid output paths
       through substitutes.  If that doesn't work, we'll build
       them. */
    for (PathSet::iterator i = invalidOutputs.begin();
         i != invalidOutputs.end(); ++i)
        /* Don't bother creating a substitution goal if there are no
           substitutes. */
        if (querySubstitutes(noTxn, *i).size() > 0)
            addWaitee(worker.makeSubstitutionGoal(*i));

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


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

    if (nrFailed > 0 && !tryFallback)
        throw Error(format("some substitutes for the outputs of derivation `%1%' failed; try `--fallback'") % drvPath);

    nrFailed = 0;

    if (checkPathValidity(false).size() == 0) {
        amDone(true);
        return;
    }

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

    /* The inputs must be built before we can build this goal. */
    /* !!! but if possible, only install the paths that we need */
    for (DerivationInputs::iterator i = drv.inputDrvs.begin();
         i != drv.inputDrvs.end(); ++i)
        addWaitee(worker.makeDerivationGoal(i->first));

    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! security problem! solution: kill the
       child */
    pid_t savedPid = pid;
    int status = pid.wait(true);

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

    /* When running under a build user, make sure that all processes
       running under that uid are gone.  This is to prevent a
       malicious user from leaving behind a process that keeps files
       open and modifies them after they have been chown'ed to
       root. */
    if (buildUser.getUID() != 0)
        killUser(buildUser.getUID());

    /* 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;
    }

    /* Release the build user, if applicable. */
    buildUser.release();

    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(), NULL);
            
            throw SysError(format("executing `%1%'") % buildHook);
            
        } 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, singleton<set<int> >(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, the set of output
           paths, and the references (pointer graph) in the input
           paths. */
        
        Path inputListFN = tmpDir + "/inputs";
        Path outputListFN = tmpDir + "/outputs";
        Path referencesFN = tmpDir + "/references";

        /* The `inputs' file lists all inputs that have to be copied
           to the remote system.  This unfortunately has to contain
           the entire derivation closure to ensure that the validity
           invariant holds on the remote system.  (I.e., it's
           unfortunate that we have to list it since the remote system
           *probably* already has it.) */
        PathSet allInputs;
        allInputs.insert(inputPaths.begin(), inputPaths.end());
        computeFSClosure(drvPath, allInputs);
        
        string s;
        for (PathSet::iterator i = allInputs.begin();
             i != allInputs.end(); ++i)
            s += *i + "\n";
        
        writeStringToFile(inputListFN, s);

        /* The `outputs' file lists all outputs that have to be copied
           from the remote system. */
        s = "";
        for (DerivationOutputs::iterator i = drv.outputs.begin();
             i != drv.outputs.end(); ++i)
            s += i->second.path + "\n";
        writeStringToFile(outputListFN, s);

        /* The `references' file has exactly the format accepted by
           `nix-store --register-validity'. */
        s = "";
        for (PathSet::iterator i = allInputs.begin();
             i != allInputs.end(); ++i)
        {
            s += *i + "\n";
            
            Path deriver = queryDeriver(noTxn, *i);
            s += deriver + "\n";

            PathSet references;
            queryReferences(noTxn, *i, references);

            s += (format("%1%\n") % references.size()).str();
            
            for (PathSet::iterator j = references.begin();
                 j != references.end(); ++j)
                s += *j + "\n";
        }
        
        writeStringToFile(referencesFN, 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),
        (format("waiting for lock on %1%") % showPaths(outputPaths(drv.outputs))).str());

    /* Now check again whether the outputs are valid.  This is because
       another process may have started building in parallel.  After
       it has finished and released the locks, we can (and should)
       reuse its results.  (Strictly speaking the first check can be
       omitted, but that would be less efficient.)  Note that since we
       now hold the locks on the output paths, no other process can
       build this derivation, so no further checks are necessary. */
    PathSet validPaths = checkPathValidity(true);
    if (validPaths.size() == drv.outputs.size()) {
        debug(format("skipping build of derivation `%1%', someone beat us to it")
            % drvPath);
        outputLocks.setDeletion(true);
        return false;
    }

    if (validPaths.size() > 0) {
        /* !!! fix this; try to delete valid paths */
        throw Error(
            format("derivation `%1%' is blocked by its output paths")
            % drvPath);
    }

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

    /* Second, the input sources. */
    for (PathSet::iterator i = drv.inputSrcs.begin();
         i != drv.inputSrcs.end(); ++i)
        computeFSClosure(*i, inputPaths);

    debug(format("added input paths %1%") % showPaths(inputPaths));

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

    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. */
    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;

    /* Compatibility hack with Nix <= 0.7: if this is a fixed-output
       derivation, tell the builder, so that for instance `fetchurl'
       can skip checking the output.  On older Nixes, this environment
       variable won't be set, so `fetchurl' will do the check. */
    bool fixedOutput = true;
    for (DerivationOutputs::iterator i = drv.outputs.begin(); 
         i != drv.outputs.end(); ++i)
        if (i->second.hash == "") fixedOutput = false;
    if (fixedOutput) 
        env["NIX_OUTPUT_CHECKED"] = "1";

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

    
    /* If we are running as root, and the `build-allow-root' setting
       is `false', then we have to build as one of the users listed in
       `build-users'. */
    if (!queryBoolSetting("build-allow-root", true) &&
        getuid() == rootUserId)
    {
        buildUser.acquire();
        assert(buildUser.getUID() != 0);

        /* Make sure that no other processes are executing under this
           uid. */
        killUser(buildUser.getUID());
        
        /* Change ownership of the temporary build directory.  !!! gid */
        if (chown(tmpDir.c_str(), buildUser.getUID(), (gid_t) -1) == -1)
            throw SysError(format("cannot change ownership of `%1%'") % tmpDir);

        /* Check that the Nix store has the appropriate permissions,
           i.e., owned by root and mode 1777 (sticky bit on so that
           the builder can create its output but not mess with the
           outputs of other processes). */
        struct stat st;
        if (stat(nixStore.c_str(), &st) == -1)
            throw SysError(format("cannot stat `%1%'") % nixStore);
        if (st.st_uid != rootUserId)
            throw Error(format("`%1%' is not owned by root") % nixStore);
        if (!(st.st_mode & S_ISVTX) ||
            ((st.st_mode & S_IRWXO) != S_IRWXO))
            throw Error(format(
                "builder does not have write permission to `%1%'; "
                "try `chmod 1777 %1%'") % nixStore);
    }

    
    /* 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);

            /* If we are running as root and `build-allow-root' is
               `false', then switch to the user we allocated above.
               Make sure that we drop all root privileges.  Note that
               initChild() above has closed all file descriptors
               except std*, so that's safe.  Also note that setuid()
               when run as root sets the real, effective and saved
               UIDs. */
            if (buildUser.getUID() != 0) {
                printMsg(lvlError, format("switching to uid `%1%'") % buildUser.getUID());
                
                /* !!! setgid also */
                if (setgroups(0, 0) == -1)
                    throw SysError("cannot clear the set of supplementary groups");
                setuid(buildUser.getUID());
                assert(getuid() == buildUser.getUID());
                assert(geteuid() == buildUser.getUID());

            }
            
            /* 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,
        singleton<set<int> >(logPipe.readSide), true);
}


void DerivationGoal::computeClosure()
{
    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);
        }

        struct stat st;
        if (lstat(path.c_str(), &st))
            throw SysError(format("getting attributes of path `%1%'") % path);
            
        startNest(nest, lvlTalkative,
            format("scanning for references inside `%1%'") % path);

        /* Check that fixed-output derivations produced the right
           outputs (i.e., the content hash should match the specified
           hash). */ 
        if (i->second.hash != "") {

            bool recursive = false;
            string algo = i->second.hashAlgo;
            
            if (string(algo, 0, 2) == "r:") {
                recursive = true;
                algo = string(algo, 2);
            }

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

            /* Check the hash. */
            HashType ht = parseHashType(algo);
            if (ht == htUnknown)
                throw Error(format("unknown hash algorithm `%1%'") % algo);
            Hash h = parseHash(ht, i->second.hash);
            Hash h2 = recursive ? hashPath(ht, path) : hashFile(ht, path);
            if (h != h2)
                throw Error(
                    format("output path `%1%' should have %2% hash `%3%', instead has `%4%'")
                    % path % algo % printHash(h) % printHash(h2));
        }

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

        /* Get rid of all weird permissions. */
	canonicalisePathMetaData(path);

	/* For this output path, find the references to other paths contained
	   in it. */
        PathSet references;
        if (!pathExists(path + "/nix-support/no-scan")) {
            references = scanForReferences(path, allPaths);

            /* 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);
            }
        }

        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],
            allReferences[i->second.path],
            drvPath);
    }
    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);
}


static string drvsLogDir = "drvs";


void DerivationGoal::openLogFile()
{
    /* Create a log file. */
    Path dir = (format("%1%/%2%") % nixLogDir % drvsLogDir).str();
    createDirs(dir);
    
    Path logFileName = (format("%1%/%2%") % dir % baseNameOf(drvPath)).str();
    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::handleChildOutput(int fd, const string & data)
{
    if (fd == logPipe.readSide) {
        if (verbosity >= buildVerbosity)
            writeFull(STDERR_FILENO, (unsigned char *) data.c_str(), data.size());
        writeFull(fdLogFile, (unsigned char *) data.c_str(), data.size());
    }

    else abort();
}


void DerivationGoal::handleEOF(int fd)
{
    if (fd == logPipe.readSide) worker.wakeUp(shared_from_this());
}


PathSet DerivationGoal::checkPathValidity(bool returnValid)
{
    PathSet result;
    for (DerivationOutputs::iterator i = drv.outputs.begin();
         i != drv.outputs.end(); ++i)
        if (isValidPath(i->second.path)) {
            if (returnValid) result.insert(i->second.path);
        } else {
            if (!returnValid) result.insert(i->second.path);
        }
    return result;
}



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


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;

    /* Outgoing references for this path. */
    PathSet references;

    /* 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 referencesValid();
    void tryNext();
    void tryToRun();
    void finished();

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


SubstitutionGoal::SubstitutionGoal(const Path & storePath, Worker & worker)
    : Goal(worker)
{
    this->storePath = storePath;
    state = &SubstitutionGoal::init;
    name = (format("substitution of `%1%'") % storePath).str();
    trace("created");
}


SubstitutionGoal::~SubstitutionGoal()
{
    if (pid != -1) worker.childTerminated(pid);
}


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


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

    addTempRoot(storePath);
    
    /* If the path already exists we're done. */
    if (isValidPath(storePath)) {
        amDone();
        return;
    }

    /* Read the substitutes. */
    subs = querySubstitutes(noTxn, storePath);

    /* To maintain the closure invariant, we first have to realise the
       paths referenced by this one. */
    queryReferences(noTxn, storePath, references);

    for (PathSet::iterator i = references.begin();
         i != references.end(); ++i)
        if (*i != storePath) /* ignore self-references */
            addWaitee(worker.makeSubstitutionGoal(*i));

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


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

    if (nrFailed > 0)
        throw Error(format("some references of path `%1%' could not be realised") % storePath);

    for (PathSet::iterator i = references.begin();
         i != references.end(); ++i)
        if (*i != storePath) /* ignore self-references */
            assert(isValidPath(*i));
    
    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. */
    outputLock = shared_ptr<PathLocks>(new PathLocks);
    outputLock->lockPaths(singleton<PathSet>(storePath),
        (format("waiting for lock on `%1%'") % storePath).str());

    /* 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, singleton<set<int> >(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,
        references, sub.deriver);
    txn.commit();

    outputLock->setDeletion(true);
    
    printMsg(lvlChatty,
        format("substitution of path `%1%' succeeded") % storePath);

    amDone();
}


void SubstitutionGoal::handleChildOutput(int fd, const string & data)
{
    assert(fd == logPipe.readSide);
    if (verbosity >= buildVerbosity)
        writeFull(STDERR_FILENO, (unsigned char *) data.c_str(), data.size());
    /* Don't write substitution output to a log file for now.  We
       probably should, though. */
}


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



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


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)
{
    /* !!! inefficient */
    for (WeakGoalMap::iterator i = goalMap.begin();
         i != goalMap.end(); )
        if (i->second.lock() == goal) {
            WeakGoalMap::iterator j = i; ++j;
            goalMap.erase(i);
            i = j;
        }
        else ++i;
}


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


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, const set<int> & fds, bool inBuildSlot)
{
    Child child;
    child.goal = goal;
    child.fds = fds;
    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);
}


void Worker::run(const Goals & _topGoals)
{
    for (Goals::iterator i = _topGoals.begin();
         i != _topGoals.end(); ++i)
        topGoals.insert(*i);
    
    startNest(nest, lvlDebug, format("entered goal loop"));

    while (1) {

        checkInterrupt();

        /* Call every wake goal. */
        while (!awake.empty() && !topGoals.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;
            }
        }

        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());
}


void Worker::waitForInput()
{
    printMsg(lvlVomit, "waiting for children");

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

    /* 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)
    {
        for (set<int>::iterator j = i->second.fds.begin();
             j != i->second.fds.end(); ++j)
        {
            FD_SET(*j, &fds);
            if (*j >= fdMax) fdMax = *j + 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);
        set<int> fds2(i->second.fds);
        for (set<int>::iterator j = fds2.begin(); j != fds2.end(); ++j)
        {
            if (FD_ISSET(*j, &fds)) {
                unsigned char buffer[4096];
                ssize_t rd = read(*j, buffer, sizeof(buffer));
                if (rd == -1) {
                    if (errno != EINTR)
                        throw SysError(format("reading from %1%")
                            % goal->getName());
                } else if (rd == 0) {
                    debug(format("%1%: got EOF") % goal->getName());
                    goal->handleEOF(*j);
                    i->second.fds.erase(*j);
                } else {
                    printMsg(lvlVomit, format("%1%: read %2% bytes")
                        % goal->getName() % rd);
                    string data((char *) buffer, rd);
                    goal->handleChildOutput(*j, data);
                }
            }
        }
    }
}



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


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));

    worker.run(goals);

    PathSet failed;
    for (Goals::iterator i = goals.begin(); i != goals.end(); ++i)
        if ((*i)->getExitCode() == Goal::ecFailed) {
            DerivationGoal * i2 = dynamic_cast<DerivationGoal *>(i->get());
            assert(i2);
            failed.insert(i2->getDrvPath());
        }
            
    if (!failed.empty())
        throw Error(format("build of %1% failed") % showPaths(failed));
}


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

    Worker worker;
    GoalPtr goal = worker.makeSubstitutionGoal(path);
    Goals goals = singleton<Goals>(goal);

    worker.run(goals);

    if (goal->getExitCode() != Goal::ecSuccess)
        throw Error(format("path `%1%' does not exist and cannot be created") % path);
}