aboutsummaryrefslogtreecommitdiff
path: root/build/file.cxx
blob: 970957b4cf52e8688b169431704a7cfaf1ab45e6 (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
// file      : build/file.cxx -*- C++ -*-
// copyright : Copyright (c) 2014-2015 Code Synthesis Ltd
// license   : MIT; see accompanying LICENSE file

#include <build/file>

#include <fstream>
#include <utility> // move()
#include <system_error>

#include <butl/filesystem>

#include <build/scope>
#include <build/context>
#include <build/parser>
#include <build/prerequisite>
#include <build/diagnostics>

#include <build/token>
#include <build/lexer>

using namespace std;
using namespace butl;

namespace build
{
  const dir_path build_dir ("build");
  const dir_path bootstrap_dir ("build/bootstrap");

  const path root_file ("build/root.build");
  const path bootstrap_file ("build/bootstrap.build");
  const path src_root_file ("build/bootstrap/src-root.build");

  bool
  is_src_root (const dir_path& d)
  {
    // @@ Can we have root without bootstrap? I don't think so.
    //
    return file_exists (d / bootstrap_file) || file_exists (d / root_file);
  }

  bool
  is_out_root (const dir_path& d)
  {
    return file_exists (d / src_root_file);
  }

  dir_path
  find_src_root (const dir_path& b)
  {
    for (dir_path d (b); !d.root () && d != home; d = d.directory ())
    {
      if (is_src_root (d))
        return d;
    }

    return dir_path ();
  }

  dir_path
  find_out_root (const dir_path& b, bool* src)
  {
    for (dir_path d (b); !d.root () && d != home; d = d.directory ())
    {
      bool s (false);
      if ((s = is_src_root (d)) || is_out_root (d)) // Order is important!
      {
        if (src != nullptr)
          *src = s;

        return d;
      }
    }

    return dir_path ();
  }

  void
  source (const path& bf, scope& root, scope& base)
  {
    tracer trace ("source");

    try
    {
      ifstream ifs (bf.string ());
      if (!ifs.is_open ())
        fail << "unable to open " << bf;

      ifs.exceptions (ifstream::failbit | ifstream::badbit);

      level5 ([&]{trace << "sourcing " << bf;});

      parser p;
      p.parse_buildfile (ifs, bf, root, base);
    }
    catch (const ifstream::failure&)
    {
      fail << "unable to read buildfile " << bf;
    }
  }

  void
  source_once (const path& bf, scope& root, scope& base, scope& once)
  {
    tracer trace ("source_once");

    if (!once.buildfiles.insert (bf).second)
    {
      level5 ([&]{trace << "skipping already sourced " << bf;});
      return;
    }

    source (bf, root, base);
  }

  scope&
  create_root (const dir_path& out_root, const dir_path& src_root)
  {
    auto i (scopes.insert (out_root, nullptr, true, true));
    scope& rs (*i->second);

    // Set out_path. src_path is set in setup_root() below.
    //
    if (rs.out_path_ != &i->first)
    {
      assert (rs.out_path_ == nullptr);
      rs.out_path_ = &i->first;
    }

    // Enter built-in meta-operation and operation names. Loading of
    // modules (via the src bootstrap; see below) can result in
    // additional meta/operations being added.
    //
    if (rs.meta_operations.empty ())
    {
      rs.meta_operations.insert (perform_id, perform);

      rs.operations.insert (default_id, default_);
      rs.operations.insert (update_id, update);
      rs.operations.insert (clean_id, clean);
    }

    // If this is already a root scope, verify that things are
    // consistent.
    //
    {
      value& v (rs.assign ("out_root"));

      if (!v)
        v = out_root;
      else
      {
        const dir_path& p (as<dir_path> (v));

        if (p != out_root)
          fail << "new out_root " << out_root << " does not match "
               << "existing " << p;
      }
    }

    if (!src_root.empty ())
    {
      value& v (rs.assign ("src_root"));

      if (!v)
        v = src_root;
      else
      {
        const dir_path& p (as<dir_path> (v));

        if (p != src_root)
          fail << "new src_root " << src_root << " does not match "
               << "existing " << p;
      }
    }

    return rs;
  }

  void
  setup_root (scope& s)
  {
    value& v (s.assign ("src_root"));
    assert (v);

    // Register and set src_path.
    //
    if (s.src_path_ == nullptr)
      s.src_path_ = &scopes.insert (as<dir_path> (v), &s, false, true)->first;
  }

  scope&
  setup_base (scope_map::iterator i,
              const dir_path& out_base,
              const dir_path& src_base)
  {
    scope& s (*i->second);

    // Set src/out_path. The key (i->first) can be either out_base
    // or src_base.
    //
    if (s.out_path_ == nullptr)
    {
      s.out_path_ =
        i->first == out_base
        ? &i->first
        : &scopes.insert (out_base, &s, true, false)->first;
    }

    if (s.src_path_ == nullptr)
    {
      s.src_path_ =
        i->first == src_base
        ? &i->first
        : &scopes.insert (src_base, &s, false, false)->first;
    }

    // Set src/out_base variables.
    //
    {
      value& v (s.assign ("out_base"));

      if (!v)
        v = out_base;
      else
        assert (as<dir_path> (v) == out_base);
    }

    {
      value& v (s.assign ("src_base"));

      if (!v)
        v = src_base;
      else
        assert (as<dir_path> (v) == src_base);
    }

    return s;
  }

  void
  bootstrap_out (scope& root)
  {
    path bf (root.out_path () / path ("build/bootstrap/src-root.build"));

    if (!file_exists (bf))
      return;

    //@@ TODO: if bootstrap files can source other bootstrap files
    //   (the way to express dependecies), then we need a way to
    //   prevent multiple sourcing. We handle it here but we still
    //   need something like source_once (once [scope] source).
    //
    source_once (bf, root, root);
  }

  // Extract the specified variable value from a buildfile. It is
  // expected to be the first non-comment line and not to rely on
  // any variable expansion other than those from the global scope.
  //
  static value
  extract_variable (const path& bf, const char* var)
  {
    try
    {
      ifstream ifs (bf.string ());
      if (!ifs.is_open ())
        fail << "unable to open " << bf;

      ifs.exceptions (ifstream::failbit | ifstream::badbit);

      path rbf (diag_relative (bf));

      lexer lex (ifs, rbf.string ());
      token t (lex.next ());
      token_type tt;

      if (t.type != token_type::name || t.value != var ||
          ((tt = lex.next ().type) != token_type::equal &&
           tt != token_type::plus_equal))
      {
        error << "variable '" << var << "' expected as first line in " << rbf;
        throw failed (); // Suppress "used uninitialized" warning.
      }

      parser p;
      temp_scope tmp (*global_scope);
      p.parse_variable (lex, tmp, t.value, tt);

      auto l (tmp.vars[var]);
      assert (l.defined ());
      value& v (*l);
      return move (v); // Steal the value, the scope is going away.
    }
    catch (const ifstream::failure&)
    {
      fail << "unable to read buildfile " << bf;
    }

    return value (); // Never reaches.
  }

  // Extract the project name from bootstrap.build.
  //
  static string
  find_project_name (const dir_path& out_root,
                     const dir_path& fallback_src_root,
                     bool* src_hint = nullptr)
  {
    tracer trace ("find_project_name");

    // Load the project name. If this subdirectory is the subproject's
    // src_root, then we can get directly to that. Otherwise, we first
    // have to discover its src_root.
    //
    const dir_path* src_root;
    value src_root_v; // Need it to live until the end.

    if (src_hint != nullptr ? *src_hint : is_src_root (out_root))
      src_root = &out_root;
    else
    {
      path f (out_root / src_root_file);

      if (!fallback_src_root.empty () && !file_exists (f))
        src_root = &fallback_src_root;
      else
      {
        src_root_v = extract_variable (f, "src_root");
        src_root = &as<dir_path> (src_root_v);
        level5 ([&]{trace << "extracted src_root " << *src_root << " for "
                          << out_root;});
      }
    }

    string name;
    {
      value v (extract_variable (*src_root / bootstrap_file, "project"));
      name = move (as<string> (v));
    }

    level5 ([&]{trace << "extracted project name '" << name << "' for "
                      << *src_root;});
    return name;
  }

  // Scan the specified directory for any subprojects. If a subdirectory
  // is a subproject, then enter it into the map, handling the duplicates.
  // Otherwise, scan the subdirectory recursively.
  //
  static void
  find_subprojects (subprojects& sps,
                    const dir_path& d,
                    const dir_path& root,
                    bool out)
  {
    tracer trace ("find_subprojects");

    for (const dir_entry& de: dir_iterator (d))
    {
      // If this is a link, then type() will try to stat() it. And if
      // the link is dangling or points to something inaccessible, it
      // will fail.
      //
      try
      {
        if (de.type () != entry_type::directory)
          continue;
      }
      catch (const system_error& e)
      {
        continue;
      }

      dir_path sd (d / path_cast<dir_path> (de.path ()));

      bool src (false);
      if (!((out && is_out_root (sd)) || (src = is_src_root (sd))))
      {
        find_subprojects (sps, sd, root, out);
        continue;
      }

      // Calculate relative subdirectory for this subproject.
      //
      dir_path dir (sd.leaf (root));
      level5 ([&]{trace << "subproject " << sd << " as " << dir;});

      // Load its name. Note that here we don't use fallback src_root
      // since this function is used to scan both out_root and src_root.
      //
      string name (find_project_name (sd, dir_path (), &src));

      // If the name is empty, then is is an unnamed project. While the
      // 'project' variable stays empty, here we come up with a surrogate
      // name for a key. The idea is that such a key should never conflict
      // with a real project name. We ensure this by using the project's
      // sub-directory and appending trailing '/' to it.
      //
      if (name.empty ())
        name = dir.posix_string () + '/';

      // @@ Can't use move() because we may need the values in diagnostics
      // below. Looks like C++17 try_emplace() is what we need.
      //
      auto rp (sps.emplace (name, dir));

      // Handle duplicates.
      //
      if (!rp.second)
      {
        const dir_path& dir1 (rp.first->second);

        if (dir != dir1)
          fail << "inconsistent subproject directories for " << name <<
            info << "first alternative: " << dir1 <<
            info << "second alternative: " << dir;

        level6 ([&]{trace << "skipping duplicate";});
      }
    }
  }

  bool
  bootstrap_src (scope& root)
  {
    tracer trace ("bootstrap_src");

    bool r (false);

    const dir_path& out_root (root.out_path ());
    const dir_path& src_root (root.src_path ());

    path bf (src_root / path ("build/bootstrap.build"));

    if (file_exists (bf))
    {
      // We assume that bootstrap out cannot load this file explicitly. It
      // feels wrong to allow this since that makes the whole bootstrap
      // process hard to reason about. But we may try to bootstrap the
      // same root scope multiple time.
      //
      source_once (bf, root, root);
      r = true;
    }

    // See if we are a part of an amalgamation. There are two key
    // players: the outer root scope which may already be present
    // (i.e., we were loaded as part of an amalgamation) and the
    // amalgamation variable that may or may not be set by the
    // user (in bootstrap.build) or by an earlier call to this
    // function for the same scope. When set by the user, the
    // empty special value means that the project shall not be
    // amalgamated (and which we convert to NULL below). When
    // calculated, the NULL value indicates that we are not
    // amalgamated.
    //
    {
      auto rp (root.vars.assign ("amalgamation")); // Set NULL by default.
      value& v (rp.first);

      if (v && v.empty ()) // Convert empty to NULL.
        v = nullptr;

      if (scope* aroot = root.parent_scope ()->root_scope ())
      {
        const dir_path& ad (aroot->out_path ());
        dir_path rd (ad.relative (out_root));

        // If we already have the amalgamation variable set, verify
        // that aroot matches its value.
        //
        if (!rp.second)
        {
          if (!v)
          {
            fail << out_root << " cannot be amalgamated" <<
              info << "amalgamated by " << ad;
          }
          else
          {
            const dir_path& vd (as<dir_path> (v));

            if (vd != rd)
            {
              fail << "inconsistent amalgamation of " << out_root <<
                info << "specified: " << vd <<
                info << "actual: " << rd << " by " << ad;
            }
          }
        }
        else
        {
          // Otherwise, use the outer root as our amalgamation.
          //
          level5 ([&]{trace << out_root << " amalgamated as " << rd;});
          v = move (rd);
        }
      }
      else if (rp.second)
      {
        // If there is no outer root and the amalgamation variable
        // hasn't been set, then we need to check if any of the
        // outer directories is a project's out_root. If so, then
        // that's our amalgamation.
        //
        const dir_path& ad (find_out_root (out_root.directory ()));

        if (!ad.empty ())
        {
          dir_path rd (ad.relative (out_root));
          level5 ([&]{trace << out_root << " amalgamated as " << rd;});
          v = move (rd);
        }
      }
    }

    // See if we have any subprojects. In a sense, this is the other
    // side/direction of the amalgamation logic above. Here, the
    // subprojects variable may or may not be set by the user (in
    // bootstrap.build) or by an earlier call to this function for
    // the same scope. When set by the user, the empty special value
    // means that there are no subproject and none should be searched
    // for (and which we convert to NULL below). Otherwise, it is a
    // list of directory[=project] pairs. The directory must be
    // relative to our out_root. If the project name is not specified,
    // then we have to figure it out. When subprojects are calculated,
    // the NULL value indicates that we found no subprojects.
    //
    {
      const variable& var (variable_pool.find ("subprojects"));
      auto rp (root.vars.assign(var)); // Set NULL by default.
      value& v (rp.first);

      if (rp.second)
      {
        // No subprojects set so we need to figure out if there are any.
        //
        // First we are going to scan our out_root and find all the
        // pre-configured subprojects. Then, if out_root != src_root,
        // we are going to do the same for src_root. Here, however,
        // we need to watch out for duplicates.
        //
        subprojects sps;

        if (dir_exists (out_root))
          find_subprojects (sps, out_root, out_root, true);

        if (out_root != src_root)
          find_subprojects (sps, src_root, src_root, false);

        if (!sps.empty ()) // Keep it NULL if no subprojects.
          v = move (sps);
      }
      else if (v)
      {
        // Convert empty to NULL.
        //
        if (v.empty ())
          v = nullptr;
        else
        {
          // Pre-scan the value and convert it to the "canonical" form,
          // that is, a list of simple=dir pairs.
          //
          for (auto i (v.data_.begin ()); i != v.data_.end (); ++i)
          {
            bool p (i->pair != '\0');

            if (p)
            {
              // Project name.
              //
              if (!assign<string> (*i) || as<string> (*i).empty ())
                fail << "expected project name instead of '" << *i << "' in "
                     << "the subprojects variable";

              ++i; // Got to have the second half of the pair.
            }

            if (!assign<dir_path> (*i))
              fail << "expected directory instead of '" << *i << "' in the "
                   << "subprojects variable";

            auto& d (as<dir_path> (*i));

            // Figure out the project name if the user didn't specify one.
            //
            if (!p)
            {
              // Pass fallback src_root since this is a subproject that
              // was specified by the user so it is most likely in our
              // src.
              //
              string n (find_project_name (out_root / d, src_root / d));

              // See find_subprojects() for details on unnamed projects.
              //
              if (n.empty ())
                n = d.posix_string () + '/';

              i = v.data_.emplace (i, move (n));

              i->pair = '=';
              ++i;
            }
          }

          // Make it of the map type.
          //
          assign<subprojects> (v, var);
        }
      }
    }

    return r;
  }

  void
  create_bootstrap_outer (scope& root)
  {
    auto l (root.vars["amalgamation"]);

    if (!l)
      return;

    const dir_path& d (as<dir_path> (*l));
    dir_path out_root (root.out_path () / d);
    out_root.normalize ();

    // src_root is a bit more complicated. Here we have three cases:
    //
    // 1. Amalgamation's src_root is "parallel" to the sub-project's.
    // 2. Amalgamation's src_root is the same as its out_root.
    // 3. Some other pre-configured (via src-root.build) src_root.
    //
    // So we need to try all these cases in some sensible order.
    // #3 should probably be tried first since that src_root was
    // explicitly configured by the user. After that, #2 followed
    // by #1 seems reasonable.
    //
    scope& rs (create_root (out_root, dir_path ()));
    bootstrap_out (rs); // #3 happens here, if at all.

    value& v (rs.assign ("src_root"));

    if (!v)
    {
      if (is_src_root (out_root)) // #2
        v = out_root;
      else // #1
      {
        dir_path src_root (root.src_path () / d);
        src_root.normalize ();
        v = move (src_root);
      }
    }

    setup_root (rs);

    bootstrap_src (rs);
    create_bootstrap_outer (rs);

    // Check if we are strongly amalgamated by this outer root scope.
    //
    if (root.src_path ().sub (rs.src_path ()))
      root.strong_ = rs.strong_scope (); // Itself or some outer scope.
  }

  scope&
  create_bootstrap_inner (scope& root, const dir_path& out_base)
  {
    if (auto l = root.vars["subprojects"])
    {
      for (const name& n: *l)
      {
        if (n.pair != '\0')
          continue; // Skip project names.

        dir_path out_root (root.out_path () / n.dir);

        if (!out_base.sub (out_root))
          continue;

        // The same logic to src_root as in create_bootstrap_outer().
        //
        scope& rs (create_root (out_root, dir_path ()));
        bootstrap_out (rs);

        value& v (rs.assign ("src_root"));

        if (!v)
          v = is_src_root (out_root)
            ? out_root
            : (root.src_path () / n.dir);

        setup_root (rs);

        bootstrap_src (rs);

        // Check if we strongly amalgamated this inner root scope.
        //
        if (rs.src_path ().sub (root.src_path ()))
          rs.strong_ = root.strong_scope (); // Itself or some outer scope.

        // See if there are more inner roots.
        //
        return create_bootstrap_inner (rs, out_base);
      }
    }

    return root;
  }

  void
  load_root_pre (scope& root)
  {
    tracer trace ("root_pre");

    // First load outer roots, if any.
    //
    if (scope* rs = root.parent_scope ()->root_scope ())
      load_root_pre (*rs);

    path bf (root.src_path () / path ("build/root.build"));

    if (file_exists (bf))
      source_once (bf, root, root);
  }

  names
  import (scope& ibase, name target, const location& loc)
  {
    tracer trace ("import");

    // If there is no project specified for this target, then our
    // run will be short and sweet: we simply return it as empty-
    // project-qualified and let someone else (e.g., a rule) take
    // a stab at it.
    //
    if (target.unqualified ())
    {
      target.proj = &project_name_pool.find ("");
      return names {move (target)};
    }

    // Otherwise, get the project name and convert the target to
    // unqualified.
    //
    const string& project (*target.proj);
    target.proj = nullptr;

    scope& iroot (*ibase.root_scope ());

    // Figure out this project's out_root.
    //
    dir_path out_root;
    dir_path fallback_src_root; // We have seen this already, haven't we..?

    // First search subprojects, starting with our root and then trying
    // outer roots for as long as we are inside an amalgamation.
    //
    for (scope* r (&iroot);; r = r->parent_scope ()->root_scope ())
    {
      // First check the amalgamation itself.
      //
      if (r != &iroot && as<string> (*r->vars["project"]) == project)
      {
        out_root = r->out_path ();
        fallback_src_root = r->src_path ();
        break;
      }

      if (auto l = r->vars["subprojects"])
      {
        const auto& m (as<subprojects> (*l));
        auto i (m.find (project));

        if (i != m.end ())
        {
          const dir_path& d ((*i).second);
          out_root = r->out_path () / d;
          fallback_src_root = r->src_path () / d;
          break;
        }
      }

      if (!r->vars["amalgamation"])
        break;
    }

    // Then try the config.import.* mechanism.
    //
    if (out_root.empty ())
    {
      const variable& var (
        variable_pool.find ("config.import." + project,
                            dir_path_type));

      if (auto l = iroot[var])
      {
        out_root = as<dir_path> (*l);

        if (l.belongs (*global_scope)) // A value from command line.
        {
          // Process the path by making it absolute and normalized.
          //
          if (out_root.relative ())
            out_root = work / out_root;

          out_root.normalize ();

          // Set on our root scope (part of our configuration).
          //
          iroot.assign (var) = out_root;

          // Also update the command-line value. This is necessary to avoid
          // a warning issued by the config module about global/root scope
          // value mismatch. Not very clean.
          //
          dir_path& d (as<dir_path> (const_cast<value&> (*l)));
          if (d != out_root)
            d = out_root;
        }
      }
      else
      {
        // If we can't find the project, convert it back into qualified
        // target and return to let someone else (e.g., a rule) to take
        // a stab at it.
        //
        target.proj = &project;
        level5 ([&]{trace << "postponing " << target;});
        return names {move (target)};
      }
    }

    // Bootstrap the imported root scope. This is pretty similar to
    // what we do in main() except that here we don't try to guess
    // src_root.
    //
    dir_path src_root (is_src_root (out_root) ? out_root : dir_path ());
    scope& root (create_root (out_root, src_root));

    bootstrap_out (root);

    // Check that the bootstrap process set src_root.
    //
    if (auto l = root.vars["src_root"])
    {
      const dir_path& p (as<dir_path> (*l));

      if (!src_root.empty () && p != src_root)
        fail (loc) << "bootstrapped src_root " << p << " does not match "
                   << "discovered " << src_root;
    }
    // Otherwise, use fallback if available.
    //
    else if (!fallback_src_root.empty ())
    {
      value& v (root.assign ("src_root"));
      v = move (fallback_src_root);
    }
    else
      fail (loc) << "unable to determine src_root for imported " << project <<
        info << "consider configuring " << out_root;

    setup_root (root);

    bootstrap_src (root);

    // Bootstrap outer roots if any. Loading will be done by
    // load_root_pre() below.
    //
    create_bootstrap_outer (root);

    // Load the imported root scope.
    //
    load_root_pre (root);

    // Create a temporary scope so that the export stub does not mess
    // up any of our variables.
    //
    temp_scope ts (ibase);

    // "Pass" the imported project's roots to the stub.
    //
    ts.assign ("out_root") = move (out_root);
    ts.assign ("src_root") = move (src_root);

    // Also pass the target being imported.
    //
    {
      value& v (ts.assign ("target"));

      if (!target.empty ()) // Otherwise leave NULL.
        v = move (target);
    }

    // Load the export stub. Note that it is loaded in the context
    // of the importing project, not the imported one. The export
    // stub will normally switch to the imported root scope at some
    // point.
    //
    path es (root.src_path () / path ("build/export.build"));

    try
    {
      ifstream ifs (es.string ());
      if (!ifs.is_open ())
        fail (loc) << "unable to open " << es;

      ifs.exceptions (ifstream::failbit | ifstream::badbit);

      level5 ([&]{trace << "importing " << es;});

      // @@ Should we verify these are all unqualified names? Or maybe
      // there is a use-case for the export stub to return a qualified
      // name?
      //
      parser p;
      return p.parse_export_stub (ifs, es, iroot, ts);
    }
    catch (const ifstream::failure&)
    {
      fail (loc) << "unable to read buildfile " << es;
    }

    return names (); // Never reached.
  }

  target&
  import (const prerequisite_key& pk)
  {
    assert (*pk.proj != nullptr);
    const string& p (**pk.proj);

    // @@ We no longer have location. This is especially bad for the
    //    empty case, i.e., where do I need to specify the project
    //    name)? Looks like the only way to do this is to keep location
    //    in name and then in prerequisite. Perhaps one day...
    //
    if (!p.empty ())
      fail << "unable to import target " << pk <<
        info << "consider explicitly specifying its project out_root via the "
           << "config.import." << p << " command line variable";
    else
      fail << "unable to import target " << pk <<
        info << "consider adding its installation location" <<
        info << "or explicitly specifying its project name";

    throw failed (); // No return.
  }
}