aboutsummaryrefslogtreecommitdiff
path: root/libbuild2/rule.cxx
blob: 7ea6e68c40fabc229c63cd1ac82f7b6a0276e785 (plain)
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
// file      : libbuild2/rule.cxx -*- C++ -*-
// license   : MIT; see accompanying LICENSE file

#include <libbuild2/rule.hxx>

#include <sstream>

#include <libbuild2/file.hxx>
#include <libbuild2/depdb.hxx>
#include <libbuild2/scope.hxx>
#include <libbuild2/target.hxx>
#include <libbuild2/context.hxx>
#include <libbuild2/algorithm.hxx>
#include <libbuild2/filesystem.hxx>
#include <libbuild2/diagnostics.hxx>

#include <libbuild2/parser.hxx> // attributes

#include <libbuild2/build/script/parser.hxx>
#include <libbuild2/build/script/runner.hxx>

using namespace std;
using namespace butl;

namespace build2
{
  // rule (vtable)
  //
  rule::
  ~rule ()
  {
  }

  // file_rule
  //
  // Note that this rule is special. It is the last, fallback rule. If
  // it doesn't match, then no other rule can possibly match and we have
  // an error. It also cannot be ambigious with any other rule. As a
  // result the below implementation bends or ignores quite a few rules
  // that normal implementations should follow. So you probably shouldn't
  // use it as a guide to implement your own, normal, rules.
  //
  bool file_rule::
  match (action a, target& t, const string&) const
  {
    tracer trace ("file_rule::match");

    // While strictly speaking we should check for the file's existence
    // for every action (because that's the condition for us matching),
    // for some actions this is clearly a waste. Say, perform_clean: we
    // are not doing anything for this action so not checking if the file
    // exists seems harmless.
    //
    switch (a)
    {
    case perform_clean_id:
      return true;
    default:
      {
        // While normally we shouldn't do any of this in match(), no other
        // rule should ever be ambiguous with the fallback one and path/mtime
        // access is atomic. In other words, we know what we are doing but
        // don't do this in normal rules.

        // First check the timestamp. This takes care of the special "trust
        // me, this file exists" situations (used, for example, for installed
        // stuff where we know it's there, just not exactly where).
        //
        mtime_target& mt (t.as<mtime_target> ());

        timestamp ts (mt.mtime ());

        if (ts != timestamp_unknown)
          return ts != timestamp_nonexistent;

        // Otherwise, if this is not a path_target, then we don't match.
        //
        path_target* pt (mt.is_a<path_target> ());
        if (pt == nullptr)
          return false;

        const path* p (&pt->path ());

        // Assign the path.
        //
        if (p->empty ())
        {
          // Since we cannot come up with an extension, ask the target's
          // derivation function to treat this as a prerequisite (just like in
          // search_existing_file()).
          //
          if (pt->derive_extension (true) == nullptr)
          {
            l4 ([&]{trace << "no default extension for target " << *pt;});
            return false;
          }

          p = &pt->derive_path ();
        }

        ts = mtime (*p);
        pt->mtime (ts);

        if (ts != timestamp_nonexistent)
          return true;

        l4 ([&]{trace << "no existing file for target " << *pt;});
        return false;
      }
    }
  }

  recipe file_rule::
  apply (action a, target& t) const
  {
    // Update triggers the update of this target's prerequisites so it would
    // seem natural that we should also trigger their cleanup. However, this
    // possibility is rather theoretical so until we see a real use-case for
    // this functionality, we simply ignore the clean operation.
    //
    if (a.operation () == clean_id)
      return noop_recipe;

    // If we have no prerequisites, then this means this file is up to date.
    // Return noop_recipe which will also cause the target's state to be set
    // to unchanged. This is an important optimization on which quite a few
    // places that deal with predominantly static content rely.
    //
    if (!t.has_group_prerequisites ()) // Group as in match_prerequisites().
      return noop_recipe;

    // Match all the prerequisites.
    //
    match_prerequisites (a, t);

    // Note that we used to provide perform_update() which checked that this
    // target is not older than any of its prerequisites. However, later we
    // realized this is probably wrong: consider a script with a testscript as
    // a prerequisite; chances are the testscript will be newer than the
    // script and there is nothing wrong with that.
    //
    return default_recipe;
  }

  const file_rule file_rule::instance;

  // alias_rule
  //
  bool alias_rule::
  match (action, target&, const string&) const
  {
    return true;
  }

  recipe alias_rule::
  apply (action a, target& t) const
  {
    // Inject dependency on our directory (note: not parent) so that it is
    // automatically created on update and removed on clean.
    //
    inject_fsdir (a, t, false);

    match_prerequisites (a, t);
    return default_recipe;
  }

  const alias_rule alias_rule::instance;

  // fsdir_rule
  //
  bool fsdir_rule::
  match (action, target&, const string&) const
  {
    return true;
  }

  recipe fsdir_rule::
  apply (action a, target& t) const
  {
    // Inject dependency on the parent directory. Note that it must be first
    // (see perform_update_direct()).
    //
    inject_fsdir (a, t);

    match_prerequisites (a, t);

    switch (a)
    {
    case perform_update_id: return &perform_update;
    case perform_clean_id: return &perform_clean;
    default: assert (false); return default_recipe;
    }
  }

  static bool
  fsdir_mkdir (const target& t, const dir_path& d)
  {
    // Even with the exists() check below this can still be racy so only print
    // things if we actually did create it (similar to build2::mkdir()).
    //
    auto print = [&t, &d] ()
    {
      if (verb >= 2)
        text << "mkdir " << d;
      else if (verb && t.ctx.current_diag_noise)
        text << "mkdir " << t;
    };

    // Note: ignoring the dry_run flag.
    //
    mkdir_status ms;

    try
    {
      ms = try_mkdir (d);
    }
    catch (const system_error& e)
    {
      print ();
      fail << "unable to create directory " << d << ": " << e << endf;
    }

    if (ms == mkdir_status::success)
    {
      print ();
      return true;
    }

    return false;
  }

  target_state fsdir_rule::
  perform_update (action a, const target& t)
  {
    target_state ts (target_state::unchanged);

    // First update prerequisites (e.g. create parent directories) then create
    // this directory.
    //
    // @@ outer: should we assume for simplicity its only prereqs are fsdir{}?
    //
    if (!t.prerequisite_targets[a].empty ())
      ts = straight_execute_prerequisites (a, t);

    // The same code as in perform_update_direct() below.
    //
    const dir_path& d (t.dir); // Everything is in t.dir.

    // Generally, it is probably correct to assume that in the majority of
    // cases the directory will already exist. If so, then we are going to get
    // better performance by first checking if it indeed exists. See
    // butl::try_mkdir() for details.
    //
    // @@ Also skip prerequisites? Can't we return noop in apply?
    //
    if (!exists (d) && fsdir_mkdir (t, d))
      ts |= target_state::changed;

    return ts;
  }

  void fsdir_rule::
  perform_update_direct (action a, const target& t)
  {
    // First create the parent directory. If present, it is always first.
    //
    const target* p (t.prerequisite_targets[a].empty ()
                     ? nullptr
                     : t.prerequisite_targets[a][0]);

    if (p != nullptr && p->is_a<fsdir> ())
      perform_update_direct (a, *p);

    // The same code as in perform_update() above.
    //
    const dir_path& d (t.dir);

    if (!exists (d))
      fsdir_mkdir (t, d);
  }

  target_state fsdir_rule::
  perform_clean (action a, const target& t)
  {
    // The reverse order of update: first delete this directory, then clean
    // prerequisites (e.g., delete parent directories).
    //
    // Don't fail if we couldn't remove the directory because it is not empty
    // (or is current working directory). In this case rmdir() will issue a
    // warning when appropriate.
    //
    target_state ts (rmdir (t.dir, t, t.ctx.current_diag_noise ? 1 : 2)
                     ? target_state::changed
                     : target_state::unchanged);

    if (!t.prerequisite_targets[a].empty ())
      ts |= reverse_execute_prerequisites (a, t);

    return ts;
  }

  const fsdir_rule fsdir_rule::instance;

  // noop_rule
  //
  bool noop_rule::
  match (action, target&, const string&) const
  {
    return true;
  }

  recipe noop_rule::
  apply (action, target&) const
  {
    return noop_recipe;
  }

  const noop_rule noop_rule::instance;

  // adhoc_rule
  //
  const dir_path adhoc_rule::recipes_build_dir ("recipes.out");

  bool adhoc_rule::
  match (action a, target& t, const string& h, optional<action> fallback) const
  {
    return !fallback && match (a, t, h);
  }

  bool adhoc_rule::
  match (action, target&, const string&) const
  {
    return true;
  }

  // Scope operation callback that cleans up recipe builds.
  //
  target_state adhoc_rule::
  clean_recipes_build (action, const scope& rs, const dir&)
  {
    context& ctx (rs.ctx);

    const dir_path& out_root (rs.out_path ());

    dir_path d (out_root / rs.root_extra->build_dir / recipes_build_dir);

    if (exists (d))
    {
      if (rmdir_r (ctx, d))
      {
        // Clean up build/ if it also became empty (e.g., in case of a build
        // with a transient configuration).
        //
        d = out_root / rs.root_extra->build_dir;
        if (empty (d))
          rmdir (ctx, d);

        return target_state::changed;
      }
    }

    return target_state::unchanged;
  }

  // adhoc_script_rule
  //
  bool adhoc_script_rule::
  recipe_text (context& ctx, const target& tg, string&& t, attributes& as)
  {
    // Handle and erase recipe-specific attributes.
    //
    optional<string> diag;
    for (auto i (as.begin ()); i != as.end (); )
    {
      attribute& a (*i);
      const string& n (a.name);

      if (n == "diag")
      try
      {
        diag = convert<string> (move (a.value));
      }
      catch (const invalid_argument& e)
      {
        fail (as.loc) << "invalid " << n << " attribute value: " << e;
      }
      else
      {
        ++i;
        continue;
      }

      i = as.erase (i);
    }

    checksum = sha256 (t).string ();

    istringstream is (move (t));
    build::script::parser p (ctx);

    script = p.pre_parse (tg,
                          is, loc.file, loc.line + 1,
                          move (diag), as.loc);

    return false;
  }

  void adhoc_script_rule::
  dump (ostream& os, string& ind) const
  {
    // Do we need the header?
    //
    // @@ TODO: for now we dump it as an attribute whether it was specified or
    //    derived from the script. Maybe that's ok?
    //
    if (script.diag_name)
    {
      os << ind << '%';

      if (script.diag_name)
      {
        os << " [";
        os << "diag=";
        to_stream (os, name (*script.diag_name), true /* quote */, '@');
        os << ']';
      }

      os << endl;
    }

    os << ind << string (braces, '{') << endl;
    ind += "  ";
    script::dump (os, ind, script.lines);
    ind.resize (ind.size () - 2);
    os << ind << string (braces, '}');
  }

  bool adhoc_script_rule::
  match (action a, target& t, const string&, optional<action> fb) const
  {
    if (!fb)
      ;
    // If this is clean for a file target and we are supplying the update,
    // then we will also supply the standard clean.
    //
    else if (a   == perform_clean_id  &&
             *fb == perform_update_id &&
             t.is_a<file> ())
      ;
    else
      return false;

    // It's unfortunate we have to resort to this but we need to remember this
    // in apply().
    //
    t.data (fb.has_value ());

    return true;
  }

  recipe adhoc_script_rule::
  apply (action a, target& t) const
  {
    // Derive file names for the target and its ad hoc group members, if any.
    //
    for (target* m (&t); m != nullptr; m = m->adhoc_member)
    {
      if (auto* p = m->is_a<path_target> ())
        p->derive_path ();
    }

    // Inject dependency on the output directory.
    //
    // We do it always instead of only if one of the targets is path-based in
    // case the recipe creates temporary files or some such.
    //
    inject_fsdir (a, t);

    // Match prerequisites.
    //
    match_prerequisite_members (a, t);

    // See if we are providing the standard clean as a fallback.
    //
    if (t.data<bool> ())
      return &perform_clean_depdb;

    if (a == perform_update_id && t.is_a<file> ())
    {
      return [this] (action a, const target& t)
      {
        return perform_update_file (a, t);
      };
    }
    else
    {
      return [this] (action a, const target& t)
      {
        return default_action (a, t);
      };
    }
  }

  target_state adhoc_script_rule::
  perform_update_file (action a, const target& xt) const
  {
    tracer trace ("adhoc_rule::perform_update_file");

    context& ctx (xt.ctx);

    const file& t (xt.as<file> ());
    const path& tp (t.path ());

    // The script can reference a program in one of four ways:
    //
    // 1. As an (imported) target (e.g., $cli)
    //
    // 2. As a process_path_ex (e.g., $cxx.path).
    //
    // 3. As a builtin (e.g., sed)
    //
    // 4. As a program path/name.
    //
    // When it comes to change tracking, there is nothing we can do for (4)
    // and there is nothing to do for (3) (assuming builtin semantics is
    // stable/backwards-compatible). The (2) case is handled automatically by
    // hashing all the variable values referenced by the script (see below),
    // which in case of process_path_ex includes the checksum, if available.
    //
    // This leaves the (1) case, which itself splits into two sub-cases: the
    // target comes with the dependency information (e.g., imported from a
    // project via an export stub) or it does not (e.g., imported as
    // installed). We don't need to do anything extra for the first sub-case
    // since the target's state/mtime can be relied upon like any other
    // prerequisite. Which cannot be said about the second sub-case, where we
    // reply on checksum that may be included as part of the target metadata.
    //
    // So what we are going to do here is hash checksum metadata of every
    // executable prerequisite target that has it. We do it before executing
    // in order to include ad hoc prerequisites (which feels like the right
    // thing to do; the user may mark tools as ad hoc in order to omit them
    // from $<).
    //
    sha256 prog_cs;
    for (const target* pt: t.prerequisite_targets[a])
    {
      if (pt != nullptr)
      {
        if (auto* e = pt->is_a<exe> ())
        {
          if (auto* c = e->lookup_metadata<string> ("checksum"))
          {
            prog_cs.append (*c);
          }
        }
      }
    }

    // Update prerequisites and determine if any of them render this target
    // out-of-date.
    //
    timestamp mt (t.load_mtime ());
    optional<target_state> ps (execute_prerequisites (a, t, mt));

    bool update (!ps);

    // We use depdb to track changes to the script itself, input/output file
    // names, tools, etc.
    //
    depdb dd (tp + ".d");
    {
      // First should come the rule name/version.
      //
      if (dd.expect ("adhoc 1") != nullptr)
        l4 ([&]{trace << "rule mismatch forcing update of " << t;});

      // Then the script checksum.
      //
      // Ideally, to detect changes to the script semantics, we would hash the
      // text with all the variables expanded but without executing any
      // commands. In practice, this is easier said than done (think the set
      // builtin that receives output of a command that modifies the
      // filesystem).
      //
      // So as the next best thing we are going to hash the unexpanded text as
      // well as values of all the variables expanded in it (which we get as a
      // side effect of pre-parsing the script). This approach has a number of
      // drawbacks:
      //
      // - We can't handle computed variable names (e.g., $($x ? X : Y)).
      //
      // - We may "overhash" by including variables that are actually
      //   script-local.
      //
      // - There are functions like $install.resolve() with result based on
      //   external (to the script) information.
      //
      if (dd.expect (checksum) != nullptr)
        l4 ([&]{trace << "recipe text change forcing update of " << t;});

      // For each variable hash its name, undefined/null/non-null indicator,
      // and the value if non-null.
      //
      // Note that this excludes the special $< and $> variables which we
      // handle below.
      //
      {
        sha256 cs;
        names storage;

        for (const string& n: script.vars)
        {
          cs.append (n);

          lookup l;

          if (const variable* var = ctx.var_pool.find (n))
            l = t[var];

          cs.append (!l.defined () ? '\x1' : l->null ? '\x2' : '\x3');

          if (l)
          {
            storage.clear ();
            names_view ns (reverse (*l, storage));

            for (const name& n: ns)
              to_checksum (cs, n);
          }
        }

        if (dd.expect (cs.string ()) != nullptr)
          l4 ([&]{trace << "recipe variable change forcing update of " << t;});
      }

      // Target and prerequisite sets ($> and $<).
      //
      // How should we hash them? We could hash them as target names (i.e.,
      // the same as the $>/< content) or as paths (only for path-based
      // targets). While names feel more general, they are also more expensive
      // to compute. And for path-based targets, path is generally a good
      // proxy for the target name. Since the bulk of the ad hoc recipes will
      // presumably be operating exclusively on path-based targets, let's do
      // it both ways.
      //
      {
        auto hash = [ns = names ()] (sha256& cs, const target& t) mutable
        {
          if (const path_target* pt = t.is_a<path_target> ())
            cs.append (pt->path ().string ());
          else
          {
            ns.clear ();
            t.as_name (ns);
            for (const name& n: ns)
              to_checksum (cs, n);
          }
        };

        sha256 tcs;
        for (const target* m (&t); m != nullptr; m = m->adhoc_member)
          hash (tcs, *m);

        if (dd.expect (tcs.string ()) != nullptr)
          l4 ([&]{trace << "target set change forcing update of " << t;});

        sha256 pcs;
        for (const target* pt: t.prerequisite_targets[a])
          if (pt != nullptr)
            hash (pcs, *pt);

        if (dd.expect (pcs.string ()) != nullptr)
          l4 ([&]{trace << "prerequisite set change forcing update of " << t;});
      }

      // Finally the programs checksum.
      //
      if (dd.expect (prog_cs.string ()) != nullptr)
        l4 ([&]{trace << "program checksum change forcing update of " << t;});
    }

    // Update if depdb mismatch.
    //
    if (dd.writing () || dd.mtime > mt)
      update = true;

    dd.close ();

    // If nothing changed, then we are done.
    //
    if (!update)
      return *ps;

    if (!ctx.dry_run || verb != 0)
    {
      const scope& bs (t.base_scope ());
      const scope& rs (*bs.root_scope ());

      build::script::environment e (a, t, script.temp_dir);
      build::script::parser p (ctx);

      if (verb == 1)
      {
        if (script.diag_line)
        {
          text << p.execute_special (rs, bs, e, *script.diag_line);
        }
        else
        {
          // @@ TODO (and below):
          //
          // - we are printing target, not source (like in most other places)
          //
          // - printing of ad hoc target group (the {hxx cxx}{foo} idea)
          //
          // - if we are printing prerequisites, should we print all of them
          //   (including tools)?
          //
          text << *script.diag_name << ' ' << t;
        }
      }

      if (!ctx.dry_run || verb >= 2)
      {
        build::script::default_runner r;
        p.execute (rs, bs, e, script, r);

        if (!ctx.dry_run)
          dd.check_mtime (tp);
      }
    }

    t.mtime (system_clock::now ());
    return target_state::changed;
  }

  target_state adhoc_script_rule::
  default_action (action a, const target& t) const
  {
    tracer trace ("adhoc_rule::default_action");

    context& ctx (t.ctx);

    execute_prerequisites (a, t);

    if (!ctx.dry_run || verb != 0)
    {
      const scope& bs (t.base_scope ());
      const scope& rs (*bs.root_scope ());

      build::script::environment e (a, t, script.temp_dir);
      build::script::parser p (ctx);

      if (verb == 1)
      {
        if (script.diag_line)
        {
          text << p.execute_special (rs, bs, e, *script.diag_line);
        }
        else
        {
          // @@ TODO: as above
          //
          text << *script.diag_name << ' ' << t;
        }
      }

      if (!ctx.dry_run || verb >= 2)
      {
        build::script::default_runner r;
        p.execute (rs, bs, e, script, r);
      }
    }

    return target_state::changed;
  }

  // cxx_rule_v1
  //
  bool cxx_rule_v1::
  match (action, target&, const string&) const
  {
    return true;
  }

  // adhoc_cxx_rule
  //
  adhoc_cxx_rule::
  adhoc_cxx_rule (const location& l, size_t b, uint64_t v)
      : adhoc_rule (l, b), version (v), impl (nullptr)
  {
    if (v != 1)
      fail (l) << "unsupported c++ recipe version " << v;
  }

  bool adhoc_cxx_rule::
  recipe_text (context&, const target&, string&& t, attributes&)
  {
    code = move (t);
    return true;
  }

  adhoc_cxx_rule::
  ~adhoc_cxx_rule ()
  {
    delete impl.load (memory_order_relaxed); // Serial execution.
  }

  void adhoc_cxx_rule::
  dump (ostream& os, string& ind) const
  {
    // @@ TODO: indentation is multi-line recipes is off (would need to insert
    //          indentation after every newline).
    //
    os << ind << string (braces, '{') << " c++ " << version << endl
       << ind << code
       << ind << string (braces, '}');
  }

  // From module.cxx.
  //
  void
  create_module_context (context&, const location&);

  const target&
  update_in_module_context (context&, const scope&, names tgt,
                            const location&, const path& bf);

  pair<void*, void*>
  load_module_library (const path& lib, const string& sym, string& err);

  bool adhoc_cxx_rule::
  match (action a, target& t, const string& hint) const
  {
    tracer trace ("adhoc_cxx_rule::match");

    context& ctx (t.ctx);
    const scope& rs (t.root_scope ());

    // The plan is to reduce this to the build system module case as much as
    // possible. Specifically, we switch to the load phase, create a module-
    // like library with the recipe text as a rule implementation, then build
    // and load it.
    //
    // Since the recipe can be shared among multiple targets, several threads
    // can all be trying to do this in parallel.
    //
    // We use the relaxed memory order here because any change must go through
    // the serial load phase. In other words, all we need here is atomicity
    // with ordering/visibility provided by the phase mutex.
    //
    cxx_rule* impl (this->impl.load (memory_order_relaxed));

    while (impl == nullptr) // Breakout loop.
    {
      // Switch the phase to (serial) load and re-check.
      //
      phase_switch ps (ctx, run_phase::load);

      if ((impl = this->impl.load (memory_order_relaxed)) != nullptr)
        break;

      using create_function = cxx_rule_v1* (const location&, target_state);
      using load_function = create_function* ();

      // The only way to guarantee that the name of our module matches its
      // implementation is to based the name on the implementation hash (plus
      // the language, in case we support other compiled implementations in
      // the future).
      //
      // Unfortunately, this means we will be creating a new project (and
      // leaving behind the old one as garbage) for every change to the
      // recipe. On the other hand, if the recipe is moved around unchanged,
      // we will reuse the same project. In fact, two different recipes (e.g.,
      // in different buildfiles) with the same text will share the project.
      //
      // The fact that we don't incorporate the recipe location into the hash
      // but include it in the source (in the form of the #line directive; see
      // below) has its own problems. If we do nothing extra here, then if a
      // "moved" but otherwise unchanged recipe is updated (for example,
      // because of changes in the build system core), then we may end up with
      // bogus location in the diagnostics.
      //
      // The straightforward solution would be to just update the location in
      // the source code if it has changed. This, however, will lead to
      // unnecessary and probably surprising recompilations since any line
      // count change before the recipe will trigger this update. One key
      // observation here is that we need accurate location information only
      // if we are going to recompile the recipe but the change to location
      // itself does not render the recipe out of date. So what we going to do
      // is factor the location information into its own small header and then
      // keep it up-to-date without changing its modification time.
      //
      // This works well if the project is not shared by multiple recipes.
      // However, if we have recipes in several buildfiles with identical
      // text, then the location information may end up yo-yo'ing depending on
      // which recipe got here first.
      //
      // There doesn't seem to be much we can do about it without incurring
      // other drawbacks/overheads. So the answer is for the user to use an ad
      // hoc rule with the common implementation instead of a bunch of
      // duplicate recipes.
      //
      string id;
      {
        sha256 cs;
        cs.append ("c++");
        cs.append (code);
        id = cs.abbreviated_string (12);
      }

      dir_path pd (rs.out_path () /
                   rs.root_extra->build_dir /
                   recipes_build_dir /= id);

      path bf (pd / std_buildfile_file);

      string sym ("load_" + id);

      // Check whether the file exists and its last line matches the specified
      // signature.
      //
      // Note: we use the last instead of the first line for extra protection
      // against incomplete writes.
      //
      auto check_sig = [] (const path& f, const string& s) -> bool
      {
        try
        {
          if (!file_exists (f))
            return false;

          ifdstream ifs (f);

          string l;
          while (ifs.peek () != ifdstream::traits_type::eof ())
            getline (ifs, l);

          return l == s;
        }
        catch (const io_error& e)
        {
          fail << "unable to read " << f << ": " << e << endf;
        }
        catch (const system_error& e)
        {
          fail << "unable to access " << f << ": " << e << endf;
        }
      };

      bool nested (ctx.module_context == &ctx);

      // Create the build context if necessary.
      //
      if (ctx.module_context == nullptr)
      {
        if (!ctx.module_context_storage)
          fail (loc) << "unable to update ad hoc recipe for target " << t <<
            info << "building of ad hoc recipes is disabled";

        create_module_context (ctx, loc);
      }

      // "Switch" to the module context.
      //
      context& ctx (*t.ctx.module_context);

      const uint16_t verbosity (3); // Project creation command verbosity.

      // Project and location signatures.
      //
      // Specifically, we update the project version when changing anything
      // which would make the already existing projects unusable.
      //
      const string& lf (!loc.file.path.empty ()
                        ? loc.file.path.string ()
                        : loc.file.name ? *loc.file.name : string ());

      const string psig ("# c++ " + to_string (version));
      const string lsig ("// " + lf + ':' + to_string (loc.line));

      // Check whether we need to (re)create the project.
      //
      optional<bool> altn (false); // Standard naming scheme.
      bool create (!is_src_root (pd, altn));

      if (!create && (create = !check_sig (bf, psig)))
        rmdir_r (ctx, pd, false, verbosity); // Never dry-run.

      path of;
      ofdstream ofs;

      if (create)
      try
      {
        // Write ad hoc config.build that loads the ~build2 configuration.
        // This way the configuration will be always in sync with ~build2
        // and we can update the recipe manually (e.g., for debugging).
        //
        create_project (
          pd,
          dir_path (),                             /* amalgamation */
          {},                                      /* boot_modules */
          "cxx.std = latest",                      /* root_pre */
          {"cxx."},                                /* root_modules */
          "",                                      /* root_post */
          string ("config"),                       /* config_module */
          string ("config.config.load = ~build2"), /* config_file */
          false,                                   /* buildfile */
          "build2 core",                           /* who */
          verbosity);                              /* verbosity */


        // Write the rule source file.
        //
        of = path (pd / "rule.cxx");

        if (verb >= verbosity)
          text << (verb >= 2 ? "cat >" : "save ") << of;

        ofs.open (of);

        ofs << "#include \"location.hxx\""                              << '\n'
            << '\n';

        // Include every header that can plausibly be needed by a rule.
        //
        // @@ TMP: any new headers to add? [Keep this note for review.]
        //
        ofs << "#include <libbuild2/types.hxx>"                         << '\n'
            << "#include <libbuild2/forward.hxx>"                       << '\n'
            << "#include <libbuild2/utility.hxx>"                       << '\n'
            << '\n'
            << "#include <libbuild2/file.hxx>"                          << '\n'
            << "#include <libbuild2/rule.hxx>"                          << '\n'
            << "#include <libbuild2/depdb.hxx>"                         << '\n'
            << "#include <libbuild2/scope.hxx>"                         << '\n'
            << "#include <libbuild2/target.hxx>"                        << '\n'
            << "#include <libbuild2/context.hxx>"                       << '\n'
            << "#include <libbuild2/variable.hxx>"                      << '\n'
            << "#include <libbuild2/algorithm.hxx>"                     << '\n'
            << "#include <libbuild2/filesystem.hxx>"                    << '\n'
            << "#include <libbuild2/diagnostics.hxx>"                   << '\n'
            << '\n';

        // Normally the recipe code will have one level of indentation so
        // let's not indent the namespace level to match.
        //
        ofs << "namespace build2"                                       << '\n'
            << "{"                                                      << '\n'
            << '\n';

        // If we want the user to be able to supply a custom constuctor, then
        // we have to give the class a predictable name (i.e., we cannot use
        // id as part of its name) and put it into an unnamed namespace. One
        // clever idea is to call the class `constructor` but the name could
        // also be used for a custom destructor (still could work) or for name
        // qualification (would definitely look bizarre).
        //
        // In this light the most natural name is probable `rule`. The issue
        // is we already have this name in the build2 namespace (and its our
        // indirect base). In fact, any name that we choose could in the
        // future conflict with something in that namespace so maybe it makes
        // sense to bite the bullet and pick a name that is least likely to be
        // used by the user directly (can always use cxx_rule instead).
        //
        ofs << "namespace"                                              << '\n'
            << "{"                                                      << '\n'
            << "class rule: public cxx_rule_v1"                         << '\n'
            << "{"                                                      << '\n'
            << "public:"                                                << '\n'
            << '\n';

        // Inherit base constructor. This way the user may provide their own
        // but don't have to.
        //
        ofs << "  using cxx_rule_v1::cxx_rule_v1;"                      << '\n'
            << '\n';

        // An extern "C" function cannot throw which can happen in case of a
        // user-defined constructor. So we need an extra level of indirection.
        // We incorporate id to make sure it doesn't conflict with anything
        // user-defined.
        //
        ofs << "  static cxx_rule_v1*"                                  << '\n'
            << "  create_" << id << " (const location& l, target_state s)" << '\n'
            << "  {"                                                    << '\n'
            << "    return new rule (l, s);"                            << '\n'
            << "  }"                                                    << '\n'
            << '\n';

        // Use the #line directive to point diagnostics to the code in the
        // buildfile. Note that there is no easy way to restore things to
        // point back to the source file (other than another #line with a line
        // and a file). Seeing that we don't have much after, let's not bother
        // for now.
        //
        ofs << "#line RECIPE_LINE RECIPE_FILE"                          << '\n';

        // Note that the code always includes trailing newline.
        //
        ofs << code
            << "};"                                                     << '\n'
            << '\n';

        // Add an alias that we can use unambiguously in the load function.
        //
        ofs << "using rule_" << id << " = rule;"                        << '\n'
            << "}"                                                      << '\n'
            << '\n';

        // Entry point.
        //
        ofs << "extern \"C\""                                           << '\n'
            << "#ifdef _WIN32"                                          << '\n'
            << "__declspec(dllexport)"                                  << '\n'
            << "#endif"                                                 << '\n'
            << "cxx_rule_v1* (*" << sym << " ()) (const location&, target_state)" << '\n'
            << "{"                                                      << '\n'
            << "  return &rule_" << id << "::create_" << id << ";"      << '\n'
            << "}"                                                      << '\n'
            << '\n';

        ofs << "}"                                                      << '\n';

        ofs.close ();


        // Write buildfile.
        //
        of = bf;

        if (verb >= verbosity)
          text << (verb >= 2 ? "cat >" : "save ") << of;

        ofs.open (of);

        ofs << "import imp_libs += build2%lib{build2}"                  << '\n'
            << "libs{" << id << "}: cxx{rule} hxx{location} $imp_libs"  << '\n'
            << '\n'
            << psig                                                     << '\n';

        ofs.close ();
      }
      catch (const io_error& e)
      {
        fail << "unable to write to " << of << ": " << e;
      }

      // Update the library target in the module context.
      //
      const target* l (nullptr);
      do // Breakout loop.
      {
        // Load the project in the module context.
        //
        // Note that it's possible it has already been loaded (see above about
        // the id calculation).
        //
        scope& rs (load_project (ctx, pd, pd, false /* forwarded */));

        auto find_target = [&ctx, &rs, &pd, &id] ()
        {
          const target_type* tt (rs.find_target_type ("libs"));
          assert (tt != nullptr);

          const target* t (
            ctx.targets.find (*tt, pd, dir_path () /* out */, id));
          assert (t != nullptr);

          return t;
        };

        // If the project has already been loaded then, as an optimization,
        // check if the target has already been updated (this will make a
        // difference we if we have identical recipes in several buildfiles,
        // especially to the location update that comes next).
        //
        if (!source_once (rs, rs, bf))
        {
          l = find_target ();

          if (l->executed_state (perform_update_id) != target_state::unknown)
            break;
        }

        // Create/update the recipe location header.
        //
        // For update, preserve the file timestamp in order not to render the
        // recipe out of date.
        //
        of = path (pd / "location.hxx");
        if (!check_sig (of, lsig))
        try
        {
          entry_time et (file_time (of));

          if (verb >= verbosity)
            text << (verb >= 2 ? "cat >" : "save ") << of;

          ofs.open (of);

          // Recipe file and line for the #line directive above. Note that the
          // code starts from the next line thus +1. We also need to escape
          // backslashes (Windows paths).
          //
          ofs << "#define RECIPE_FILE \"" << sanitize_strlit (lf) << '"'<< '\n'
              << "#define RECIPE_LINE "   << loc.line + 1               << '\n'
              << '\n'
              << lsig                                                   << '\n';

          ofs.close ();

          if (et.modification != timestamp_nonexistent)
            file_time (of, et);
        }
        catch (const io_error& e)
        {
          fail << "unable to write to " << of << ": " << e;
        }
        catch (const system_error& e)
        {
          fail << "unable to get/set timestamp for " << of << ": " << e;
        }

        if (nested)
        {
          // This means there is a perform update action already in progress
          // in this context. So we are going to switch the phase and
          // perform direct match and update (similar how we do this for
          // generated headers).
          //
          // Note that since neither match nor execute are serial phases, it
          // means other targets in this context can be matched and executed
          // in paralellel with us.
          //
          if (l == nullptr)
            l = find_target ();

          phase_switch mp (ctx, run_phase::match);
          if (build2::match (perform_update_id, *l) != target_state::unchanged)
          {
            phase_switch ep (ctx, run_phase::execute);
            execute (a, *l);
          }
        }
        else
        {
          // Cutoff the existing diagnostics stack and push our own entry.
          //
          diag_frame::stack_guard diag_cutoff (nullptr);

          auto df = make_diag_frame (
            [this, &t] (const diag_record& dr)
            {
              dr << info (loc) << "while updating ad hoc recipe for target "
                 << t;
            });

          l = &update_in_module_context (
            ctx, rs, names {name (pd, "libs", id)},
            loc, bf);
        }
      } while (false);

      // Load the library.
      //
      const path& lib (l->as<file> ().path ());

      // Note again that it's possible the library has already been loaded
      // (see above about the id calculation).
      //
      string err;
      pair<void*, void*> hs (load_module_library (lib, sym, err));

      // These normally shouldn't happen unless something is seriously broken.
      //
      if (hs.first == nullptr)
        fail (loc) << "unable to load recipe library " << lib << ": " << err;

      if (hs.second == nullptr)
        fail (loc) << "unable to lookup " << sym << " in recipe library "
                   << lib << ": " << err;

      {
        auto df = make_diag_frame (
          [this](const diag_record& dr)
          {
            if (verb != 0)
              dr << info (loc) << "while initializing ad hoc recipe";
          });

        load_function* lf (function_cast<load_function*> (hs.second));
        create_function* cf (lf ());

        impl = cf (loc, l->executed_state (perform_update_id));
        this->impl.store (impl, memory_order_relaxed); // Still in load phase.
      }
    }

    return impl->match (a, t, hint);
  }

  recipe adhoc_cxx_rule::
  apply (action a, target& t) const
  {
    return impl.load (memory_order_relaxed)->apply (a, t);
  }
}