aboutsummaryrefslogtreecommitdiff
path: root/libbuild2/adhoc-rule-buildscript.cxx
blob: e8bc20d96681e7fcb51908f63b70261e11fc9d89 (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
// file      : libbuild2/adhoc-rule-buildscript.cxx -*- C++ -*-
// license   : MIT; see accompanying LICENSE file

#include <libbuild2/adhoc-rule-buildscript.hxx>

#include <sstream>

#include <libbuild2/depdb.hxx>
#include <libbuild2/scope.hxx>
#include <libbuild2/target.hxx>
#include <libbuild2/dyndep.hxx>
#include <libbuild2/context.hxx>
#include <libbuild2/algorithm.hxx>
#include <libbuild2/filesystem.hxx>  // path_perms(), auto_rmfile
#include <libbuild2/diagnostics.hxx>

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

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

using namespace std;

namespace build2
{
  static inline void
  hash_script_vars (sha256& cs,
                    const build::script::script& s,
                    const target& t,
                    names& storage)
  {
    context& ctx (t.ctx);

    for (const string& n: s.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);
      }
    }
  }

  // How should we hash target and prerequisite sets ($> and $<)? 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.
  //
  static inline void
  hash_target (sha256& cs, const target& t, names& storage)
  {
    if (const path_target* pt = t.is_a<path_target> ())
      cs.append (pt->path ().string ());
    else
    {
      storage.clear ();
      t.as_name (storage);
      for (const name& n: storage)
        to_checksum (cs, n);
    }
  };

  // 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) (the
  // user can track its environment manually with depdb-env) 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 checksums (both executable and environment),
  // 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 is hash checksum metadata of every executable
  // prerequisite target that has it (we do it here 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 $<).
  //
  static inline void
  hash_prerequisite_target (sha256& cs, sha256& exe_cs, sha256& env_cs,
                            const target& pt,
                            names& storage)
  {
    hash_target (cs, pt, storage);

    if (const exe* et = pt.is_a<exe> ())
    {
      if (const string* c = et->lookup_metadata<string> ("checksum"))
      {
        exe_cs.append (*c);
      }

      if (const strings* e = et->lookup_metadata<strings> ("environment"))
      {
        hash_environment (env_cs, *e);
      }
    }
  }

  bool adhoc_buildscript_rule::
  recipe_text (const scope& s,
               const target_type& tt,
               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 ();
    ttype = &tt;

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

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

    return false;
  }

  void adhoc_buildscript_rule::
  dump_attributes (ostream& os) const
  {
    // For now we dump it as an attribute whether it was specified or derived
    // from the script. Maybe that's ok (we use this in tests)?
    //
    if (script.diag_name)
    {
      os << " [";
      os << "diag=";
      to_stream (os, name (*script.diag_name), true /* quote */, '@');
      os << ']';
    }
  }

  void adhoc_buildscript_rule::
  dump_text (ostream& os, string& ind) const
  {
    os << ind << string (braces, '{') << endl;
    ind += "  ";

    if (script.depdb_clear)
      os << ind << "depdb clear" << endl;

    script::dump (os, ind, script.depdb_preamble);

    if (script.diag_line)
    {
      os << ind; script::dump (os, *script.diag_line, true /* newline */);
    }

    script::dump (os, ind, script.body);
    ind.resize (ind.size () - 2);
    os << ind << string (braces, '}');
  }

  bool adhoc_buildscript_rule::
  reverse_fallback (action a, const target_type& tt) const
  {
    // We can provide clean for a file target if we are providing update.
    //
    return a == perform_clean_id && tt.is_a<file> () &&
      find (actions.begin (), actions.end (),
            perform_update_id) != actions.end ();
  }

  struct adhoc_buildscript_rule::match_data
  {
    match_data (action a, const target& t, bool temp_dir)
        : env (a, t, temp_dir) {}

    build::script::environment env;
    build::script::default_runner run;

    path dd;
    const scope* bs;
    timestamp mt;
    bool deferred_failure;
  };

  bool adhoc_buildscript_rule::
  match (action a, target& t, const string& h, match_extra& me) const
  {
    // We pre-parsed the script with the assumption it will be used on a
    // non/file-based target. Note that this should not be possible with
    // patterns.
    //
    if (pattern == nullptr)
    {
      if ((t.is_a<file> () != nullptr) != ttype->is_a<file> ())
      {
        fail (loc) << "incompatible target types used with shared recipe" <<
          info << "all targets must be file-based or non-file-based";
      }
    }

    return adhoc_rule::match (a, t, h, me);
  }

  recipe adhoc_buildscript_rule::
  apply (action a, target& t, match_extra& me) const
  {
    return apply (a, t, me, nullopt);
  }

  recipe adhoc_buildscript_rule::
  apply (action a,
         target& xt,
         match_extra& me,
         const optional<timestamp>& d) const
  {
    tracer trace ("adhoc_buildscript_rule::apply");

    // We don't support deadlines for any of these cases (see below).
    //
    if (d && (a.outer ()  ||
              me.fallback ||
              (a == perform_update_id && xt.is_a<file> ())))
      return empty_recipe;

    // If this is an outer operation (e.g., update-for-test), then delegate to
    // the inner.
    //
    if (a.outer ())
    {
      match_inner (a, xt);
      return execute_inner;
    }

    // Inject pattern's ad hoc group members, if any.
    //
    if (pattern != nullptr)
      pattern->apply_adhoc_members (a, xt, me);

    // Derive file names for the target and its ad hoc group members, if any.
    //
    if (a == perform_update_id || a == perform_clean_id)
    {
      for (target* m (&xt); 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.
    //
    const fsdir* dir (inject_fsdir (a, xt));

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

    // Inject pattern's prerequisites, if any.
    //
    if (pattern != nullptr)
      pattern->apply_prerequisites (a, xt, me);

    // See if we are providing the standard clean as a fallback.
    //
    if (me.fallback)
      return &perform_clean_depdb;

    // See if this is not update or not on a file-based target.
    //
    if (a != perform_update_id || !xt.is_a<file> ())
    {
      return [d, this] (action a, const target& t)
      {
        return default_action (a, t, d);
      };
    }

    // See if this is the simple case with only static dependencies.
    //
    if (!script.depdb_dyndep)
    {
      return [this] (action a, const target& t)
      {
        return perform_update_file (a, t);
      };
    }

    // This is a perform update on a file target with extraction of dynamic
    // dependency information in the depdb preamble (depdb-dyndep).
    //
    // This means we may need to add additional prerequisites (or even target
    // group members). We also have to save any such additional prerequisites
    // in depdb so that we can check if any of them have changed on subsequent
    // updates. So all this means that have to take care of depdb here in
    // apply() instead of perform_*() like we normally do. We also do things
    // in slightly different order due to the restrictions impose by the match
    // phase.
    //
    // Note that the C/C++ header dependency extraction is the canonical
    // example and all this logic is based on the prior work in the cc module
    // where you can often find more detailed rationale for some of the steps
    // performed (like the fsdir update below).
    //
    context& ctx (xt.ctx);

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

    if (dir != nullptr)
      fsdir_rule::perform_update_direct (a, t);

    // Because the depdb preamble can access $<, we have to blank out all the
    // ad hoc prerequisites. Since we will still need them later, we "move"
    // them to the auxiliary data member in prerequisite_target (which also
    // means we cannot use the standard execute_prerequisites()).
    //
    auto& pts (t.prerequisite_targets[a]);
    for (prerequisite_target& p: pts)
    {
      // Note that fsdir{} injected above is adhoc.
      //
      if (p.target != nullptr && p.adhoc)
      {
        p.data = reinterpret_cast<uintptr_t> (p.target);
        p.target = nullptr;
      }
    }

    // NOTE: see the "static dependencies" version (with comments) below.
    //
    depdb dd (tp + ".d");

    if (dd.expect ("<ad hoc buildscript recipe> 1") != nullptr)
      l4 ([&]{trace << "rule mismatch forcing update of " << t;});

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

    if (!script.depdb_clear)
    {
      names storage;

      sha256 prq_cs, exe_cs, env_cs;

      for (const prerequisite_target& p: pts)
      {
        if (const target* pt =
            (p.target != nullptr ? p.target :
             p.data   != 0       ? reinterpret_cast<target*> (p.data) :
             nullptr))
        {
          hash_prerequisite_target (prq_cs, exe_cs, env_cs, *pt, storage);
        }
      }

      {
        sha256 cs;
        hash_script_vars (cs, script, t, storage);

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

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

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

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

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

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

    const scope& bs (t.base_scope ());

    unique_ptr<match_data> md (
      new match_data (a, t, script.depdb_preamble_temp_dir));

    build::script::environment& env (md->env);
    build::script::default_runner& run (md->run);

    run.enter (env, script.start_loc);

    // Run the first half of the preamble (before depdb-dyndep).
    //
    {
      build::script::parser p (ctx);
      p.execute_depdb_preamble (a, bs, t, env, script, run, dd);
    }

    // Determine if we need to do an update based on the above checks.
    //
    bool update;
    timestamp mt;

    if (dd.writing ())
      update = true;
    else
    {
      if ((mt = t.mtime ()) == timestamp_unknown)
        t.mtime (mt = mtime (tp)); // Cache.

      update = dd.mtime > mt;
    }

    if (update)
      mt = timestamp_nonexistent;

    // Update our prerequisite targets. While strictly speaking we only need
    // to update those that are referenced by depdb-dyndep, communicating
    // this is both tedious and error-prone. So we update them all.
    //
    for (const prerequisite_target& p: pts)
    {
      if (const target* pt =
          (p.target != nullptr ? p.target :
           p.data   != 0       ? reinterpret_cast<target*> (p.data) : nullptr))
      {
        update = dyndep_rule::update (
          trace, a, *pt, update ? timestamp_unknown : mt) || update;
      }
    }

    // Run the second half of the preamble (depdb-dyndep commands) to extract
    // dynamic dependencies.
    //
    // Note that this should be the last update to depdb (the invalidation
    // order semantics).
    //
    bool deferred_failure (false);
    {
      build::script::parser p (ctx);
      p.execute_depdb_preamble_dyndep (a, bs, t,
                                       env, script, run,
                                       dd,
                                       update,
                                       deferred_failure,
                                       mt);
    }

    if (update && dd.reading () && !ctx.dry_run)
      dd.touch = timestamp_unknown;

    dd.close ();
    md->dd = move (dd.path);

    // Pass on base scope and update/mtime.
    //
    md->bs = &bs;
    md->mt = update ? timestamp_nonexistent : mt;
    md->deferred_failure = deferred_failure;

    // @@ TMP: re-enable once recipe becomes move_only_function.
    //
#if 0
    return [this, md = move (md)] (action a, const target& t) mutable
    {
      auto r (perform_update_file_dyndep (a, t, *md));
      md.reset (); // @@ TMP: is this really necessary (+mutable)?
      return r;
    };
#else
    t.data (move (md));
    return recipe ([this] (action a, const target& t) mutable
    {
      auto md (move (t.data<unique_ptr<match_data>> ()));
      return perform_update_file_dyndep (a, t, *md);
    });
#endif
  }

  target_state adhoc_buildscript_rule::
  perform_update_file_dyndep (action a, const target& xt, match_data& md) const
  {
    tracer trace ("adhoc_buildscript_rule::perform_update_file_dyndep");

    context& ctx (xt.ctx);

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

    // While we've updated all our prerequisites in apply(), we still need to
    // execute them here to keep the dependency counts straight.
    //
    for (const prerequisite_target& p: t.prerequisite_targets[a])
    {
      if (const target* pt =
          (p.target != nullptr ? p.target :
           p.data   != 0       ? reinterpret_cast<target*> (p.data) : nullptr))
      {
        target_state ts (execute_wait (a, *pt));
        assert (ts == target_state::unchanged || ts == target_state::changed);
      }
    }

    build::script::environment& env (md.env);
    build::script::default_runner& run (md.run);

    // Force update in case of a deferred failure even if nothing changed.
    //
    if (md.mt != timestamp_nonexistent && !md.deferred_failure)
    {
      run.leave (env, script.end_loc);
      return target_state::unchanged;
    }

    // Sequence start time for mtime checks below.
    //
    timestamp start (!ctx.dry_run && depdb::mtime_check ()
                     ? system_clock::now ()
                     : timestamp_unknown);

    if (!ctx.dry_run || verb != 0)
    {
      if (execute_update_file (*md.bs, a, t, env, run, md.deferred_failure))
        ;
      else
        run.leave (env, script.end_loc);
    }
    else
      run.leave (env, script.end_loc);

    timestamp now (system_clock::now ());

    if (!ctx.dry_run)
      depdb::check_mtime (start, md.dd, tp, now);

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

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

    context& ctx (xt.ctx);

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

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

    names storage;

    sha256 prq_cs, exe_cs, env_cs;
    {
      // This is essentially ps=execute_prerequisites(a, t, mt) which we
      // cannot use because we need to see ad hoc prerequisites.
      //
      size_t busy (ctx.count_busy ());
      size_t exec (ctx.count_executed ());

      target_state rs (target_state::unchanged);

      wait_guard wg (ctx, busy, t[a].task_count);

      auto& pts (t.prerequisite_targets[a]);

      for (const target*& pt: pts)
      {
        if (pt == nullptr) // Skipped.
          continue;

        target_state s (execute_async (a, *pt, busy, t[a].task_count));

        if (s == target_state::postponed)
        {
          rs |= s;
          pt = nullptr;
        }
      }

      wg.wait ();

      bool e (mt == timestamp_nonexistent);
      for (prerequisite_target& p: pts)
      {
        if (p == nullptr)
          continue;

        const target& pt (*p.target);

        ctx.sched.wait (exec, pt[a].task_count, scheduler::work_none);

        target_state s (pt.executed_state (a));
        rs |= s;

        // Compare our timestamp to this prerequisite's.
        //
        if (!e)
        {
          // If this is an mtime-based target, then compare timestamps.
          //
          if (const mtime_target* mpt = pt.is_a<mtime_target> ())
          {
            if (mpt->newer (mt, s))
              e = true;
          }
          else
          {
            // Otherwise we assume the prerequisite is newer if it was
            // changed.
            //
            if (s == target_state::changed)
              e = true;
          }
        }

        if (p.adhoc)
          p.target = nullptr; // Blank out.

        // As part of this loop calculate checksums that need to include ad
        // hoc prerequisites (unless the script tracks changes itself).
        //
        if (!script.depdb_clear)
          hash_prerequisite_target (prq_cs, exe_cs, env_cs, pt, storage);
      }

      if (!e)
        ps = rs;
    }

    bool update (!ps);

    // We use depdb to track changes to the script itself, input/output file
    // names, tools, etc.
    //
    // NOTE: see the "dynamic dependencies" version above.
    //
    depdb dd (tp + ".d");

    // First should come the rule name/version.
    //
    if (dd.expect ("<ad hoc buildscript recipe> 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;});

    // Track the variables, targets, and prerequisites changes, unless the
    // script tracks the dependency changes itself.
    //
    if (!script.depdb_clear)
    {
      // 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.
      //
      // @@ TODO: maybe detect and decompose process_path_ex in order to
      //    properly attribute checksum and environment changes?
      //
      {
        sha256 cs;
        hash_script_vars (cs, script, t, storage);

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

      // Target and prerequisite sets ($> and $<).
      //
      {
        sha256 tcs;
        for (const target* m (&t); m != nullptr; m = m->adhoc_member)
          hash_target (tcs, *m, storage);

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

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

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

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

    const scope* bs (nullptr);

    // Execute the custom dependency change tracking commands, if present.
    //
    // Note that we share the environment between the execute_depdb_preamble()
    // and execute_body() calls, which is not merely an optimization since
    // variables set in the preamble must be available in the body.
    //
    // Creating the environment instance is not cheap so optimize for the
    // common case where we don't have the depdb preamble and nothing to
    // update.
    //
    bool depdb_preamble (!script.depdb_preamble.empty ());

    if (!depdb_preamble)
    {
      if (dd.writing () || dd.mtime > mt)
        update = true;

      if (!update)
      {
        dd.close ();
        return *ps;
      }
    }

    build::script::environment env (a, t, false /* temp_dir */);
    build::script::default_runner run;

    if (depdb_preamble)
    {
      bs = &t.base_scope ();

      if (script.depdb_preamble_temp_dir)
        env.set_temp_dir_variable ();

      build::script::parser p (ctx);

      run.enter (env, script.start_loc);
      p.execute_depdb_preamble (a, *bs, t, env, script, run, dd);
    }

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

    dd.close ();

    // If nothing changed, then we are done.
    //
    if (!update)
    {
      // Note that if we execute the depdb preamble but not the script body,
      // we need to call the runner's leave() function explicitly (here and
      // below).
      //
      if (depdb_preamble)
        run.leave (env, script.end_loc);

      return *ps;
    }

    if (!ctx.dry_run || verb != 0)
    {
      // Prepare to execute the script diag line and/or body.
      //
      if (bs == nullptr)
        bs = &t.base_scope ();

      if (execute_update_file (*bs, a, t, env, run))
      {
        if (!ctx.dry_run)
          dd.check_mtime (tp);
      }
      else if (depdb_preamble)
        run.leave (env, script.end_loc);
    }
    else if (depdb_preamble)
      run.leave (env, script.end_loc);

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

  bool adhoc_buildscript_rule::
  execute_update_file (const scope& bs,
                       action, const file& t,
                       build::script::environment& env,
                       build::script::default_runner& run,
                       bool deferred_failure) const
  {
    context& ctx (t.ctx);

    const scope& rs (*bs.root_scope ());

    // Note that it doesn't make much sense to use the temporary directory
    // variable ($~) in the 'diag' builtin call, so we postpone setting it
    // until the script body execution, that can potentially be omitted.
    //
    build::script::parser p (ctx);

    if (verb == 1)
    {
      if (script.diag_line)
      {
        text << p.execute_special (rs, bs, env, *script.diag_line);
      }
      else
      {
        // @@ TODO (and in default_action() 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)
    {
      // On failure remove the target files that may potentially exist but
      // be invalid.
      //
      small_vector<auto_rmfile, 8> rms;

      if (!ctx.dry_run)
      {
        for (const target* m (&t); m != nullptr; m = m->adhoc_member)
        {
          if (auto* f = m->is_a<file> ())
            rms.emplace_back (f->path ());
        }
      }

      if (script.body_temp_dir && !script.depdb_preamble_temp_dir)
        env.set_temp_dir_variable ();

      p.execute_body (rs, bs, env, script, run, script.depdb_preamble.empty ());

      if (!ctx.dry_run)
      {
        if (deferred_failure)
          fail << "expected error exit status from recipe body";

        // If this is an executable, let's be helpful to the user and set
        // the executable bit on POSIX.
        //
#ifndef _WIN32
        auto chmod = [] (const path& p)
        {
          path_perms (p,
                      (path_perms (p)  |
                       permissions::xu |
                       permissions::xg |
                       permissions::xo));
        };

        for (const target* m (&t); m != nullptr; m = m->adhoc_member)
        {
          if (auto* p = m->is_a<exe> ())
            chmod (p->path ());
        }
#endif
        for (auto& rm: rms)
          rm.cancel ();
      }

      return true;
    }
    else
      return false;
  }

  target_state adhoc_buildscript_rule::
  default_action (action a,
                  const target& t,
                  const optional<timestamp>& deadline) const
  {
    tracer trace ("adhoc_buildscript_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.body_temp_dir, deadline);
      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 (execute_update_file()).
          //
          text << *script.diag_name << ' ' << t;
        }
      }

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

    return target_state::changed;
  }
}