aboutsummaryrefslogtreecommitdiff
path: root/bpkg/build.cxx
blob: b91609d87744e390939bf9c5b347e15e2046860e (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
// file      : bpkg/build.cxx -*- C++ -*-
// copyright : Copyright (c) 2014-2015 Code Synthesis Ltd
// license   : MIT; see accompanying LICENSE file

#include <bpkg/build>

#include <map>
#include <list>
#include <iterator>   // make_move_iterator()
#include <iostream>   // cout
#include <functional> // reference_wrapper

#include <butl/utility> // reverse_iterate()

#include <bpkg/types>
#include <bpkg/package>
#include <bpkg/package-odb>
#include <bpkg/utility>
#include <bpkg/database>
#include <bpkg/diagnostics>
#include <bpkg/satisfaction>
#include <bpkg/manifest-utility>

#include <bpkg/common-options>

#include <bpkg/pkg-verify>

using namespace std;
using namespace butl;

namespace bpkg
{
  // @@ TODO
  //
  //    - User-selected vs auto-selected packages.
  //

  // Try to find a package that optionally satisfies the specified
  // version constraint. Look in the specified repository, its
  // prerequisite repositories, and their complements, recursively
  // (note: recursivity applies to complements, not prerequisites).
  // Return the package and the repository in which it was found or
  // NULL for both if not found.
  //
  std::pair<shared_ptr<available_package>, shared_ptr<repository>>
  find_available (database& db,
                  const string& name,
                  const shared_ptr<repository>& r,
                  const optional<dependency_constraint>& c)
  {
    using query = query<available_package>;

    query q (query::id.name == name);
    const auto& vm (query::id.version);

    // If there is a constraint, then translate it to the query. Otherwise,
    // get the latest version.
    //
    bool order (true);
    if (c)
    {
      const version& v (c->version);

      // Note that the constraint's version is always rhs (libfoo >= 1.2.3).
      //
      switch (c->operation)
      {
      case comparison::eq: q = q && vm == v; order = false; break;
      case comparison::lt: q = q && vm <  v; break;
      case comparison::gt: q = q && vm >  v; break;
      case comparison::le: q = q && vm <= v; break;
      case comparison::ge: q = q && vm >= v; break;
      }
    }

    if (order)
      q += order_by_version_desc (vm);

    // Filter the result based on the repository to which each version
    // belongs.
    //
    return filter_one (r, db.query<available_package> (q));
  }

  // Create a transient (or fake, if you prefer) available_package
  // object corresponding to the specified selected object. Note
  // that the package locations list is left empty and that the
  // returned repository could be NULL if the package is an orphan.
  //
  std::pair<shared_ptr<available_package>, shared_ptr<repository>>
  make_available (const common_options& options,
                  const dir_path& cd,
                  database& db,
                  const shared_ptr<selected_package>& sp)
  {
    assert (sp != nullptr && sp->state != package_state::broken);

    // First see if we can find its repository.
    //
    shared_ptr<repository> ar (
      db.find<repository> (
        sp->repository.canonical_name ()));

    // The package is in at least fetched state, which means we should
    // be able to get its manifest.
    //
    const optional<path>& a (sp->archive);
    const optional<dir_path>& d (sp->src_root);

    package_manifest m (
      sp->state == package_state::fetched
      ? pkg_verify (options, a->absolute () ? *a : cd / *a)
      : pkg_verify (d->absolute () ? *d : cd / *d));

    return make_pair (make_shared<available_package> (move (m)), move (ar));
  }

  // A "dependency-ordered" list of packages and their prerequisites.
  // That is, every package on the list only possibly depending on the
  // ones after it. In a nutshell, the usage is as follows: we first
  // add one or more packages (the "initial selection"; for example, a
  // list of packages the user wants built). The list then satisfies all
  // the prerequisites of the packages that were added, recursively. At
  // the end of this process we have an ordered list of all the packages
  // that we have to build, from last to first, in order to build our
  // initial selection.
  //
  // This process is split into two phases: satisfaction of all the
  // dependencies (the collect() function) and ordering of the list
  // (the order() function).
  //
  // During the satisfaction phase, we collect all the packages, their
  // prerequisites (and so on, recursively) in a map trying to satisfy
  // any dependency constraints. Specifically, during this step, we may
  // "upgrade" or "downgrade" a package that is already in a map as a
  // result of another package depending on it and, for example requiring
  // a different version. One notable side-effect of this process is that
  // we may end up with a lot more packages in the map than we will have
  // on the list. This is because some of the prerequisites of upgraded
  // or downgraded packages may no longer need to be built.
  //
  // Note also that we don't try to do exhaustive constraint satisfaction
  // (i.e., there is no backtracking). Specifically, if we have two
  // candidate packages each satisfying a constraint of its dependent
  // package, then if neither of them satisfy both constraints, then we
  // give up and ask the user to resolve this manually by explicitly
  // specifying the version that will satisfy both constraints.
  //
  //
  struct satisfied_package
  {
    shared_ptr<selected_package>  selected;   // NULL if not selected.
    shared_ptr<available_package> available;  // Can be fake/transient.
    shared_ptr<bpkg::repository>  repository; // Can be NULL (orphan) or root.

    // Constraint value plus, normally, the dependent package name that
    // placed this constraint but can also be some other name for the
    // initial selection (e.g., package version specified by the user
    // on the command line).
    //
    struct constraint_type
    {
      string dependent;
      dependency_constraint value;

      constraint_type () = default;
      constraint_type (string d, dependency_constraint v)
          : dependent (move (d)), value (move (v)) {}
    };

    vector<constraint_type> constraints;
  };

  // Now that we have collected all the package versions that we need
  // to build, arrange them in the "dependency order", that is, with
  // every package on the list only possibly depending on the ones
  // after it. Iterate over the names we have collected on the previous
  // step in reverse so that when we iterate over the packages (also in
  // reverse), things will be built as close as possible to the order
  // specified by the user (it may still get altered if there are
  // dependencies between the specified packages).
  //
  struct satisfied_packages
  {
    using list_type = list<reference_wrapper<const satisfied_package>>;

    using iterator = list_type::iterator;
    using reverse_iterator = list_type::const_reverse_iterator;

    reverse_iterator rbegin () const {return list_.rbegin ();}
    reverse_iterator rend () const {return list_.rend ();}

    // Collect the package. Return true if this package version was,
    // in fact, added to the map and false if it was already there
    // or the existing version was preferred.
    //
    bool
    collect (const common_options& options,
             const dir_path& cd,
             database& db,
             satisfied_package&& pkg)
    {
      tracer trace ("collect");

      auto i (map_.find (pkg.available->id.name));

      // If we already have an entry for this package name, then we
      // have to pick one over the other.
      //
      if (i != map_.end ())
      {
        const string& n (i->first);

        // At the end we want p1 to point to the object that we keep
        // and p2 to the object whose constraints we should copy.
        //
        satisfied_package* p1 (&i->second.package);
        satisfied_package* p2 (&pkg);

        // If versions are the same, then all we have to do is copy the
        // constraint (p1/p2 already point to where we would want them to).
        //
        if (p1->available->version != p2->available->version)
        {
          using constraint_type = satisfied_package::constraint_type;

          // If the versions differ, we have to pick one. Start with the
          // newest version since if both satisfy, then that's the one we
          // should prefer. So get the first to try into p1 and the second
          // to try -- into p2.
          //
          if (p2->available->version > p1->available->version)
            swap (p1, p2);

          // See if pv's version satisfies pc's constraints. Return the
          // pointer to the unsatisfied constraint or NULL if all are
          // satisfied.
          //
          auto test = [] (satisfied_package* pv, satisfied_package* pc)
            -> const constraint_type*
          {
            for (const constraint_type& c: pc->constraints)
              if (!satisfies (pv->available->version, c.value))
                return &c;

            return nullptr;
          };

          // First see if p1 satisfies p2's constraints.
          //
          if (auto c2 = test (p1, p2))
          {
            // If not, try the other way around.
            //
            if (auto c1 = test (p2, p1))
            {
              const string& d1 (c1->dependent);
              const string& d2 (c2->dependent);

              fail << "unable to satisfy constraints on package " << n <<
                info << d1 << " depends on (" << n << " " << c1->value << ")" <<
                info << d2 << " depends on (" << n << " " << c2->value << ")" <<
                info << "available " << n << " " << p1->available->version <<
                info << "available " << n << " " << p2->available->version <<
                info << "explicitly specify " << n << " version to manually "
                   << "satisfy both constraints";
            }
            else
              swap (p1, p2);
          }

          level4 ([&]{trace << "pick " << n << " " << p1->available->version
                            << " over " << p2->available->version;});
        }

        // See if we are replacing the object. If not, then we don't
        // need to collect its prerequisites since that should have
        // already been done. Remember, p1 points to the object we
        // want to keep.
        //
        bool replace (p1 != &i->second.package);

        if (replace)
        {
          swap (*p1, *p2);
          swap (p1, p2); // Setup for constraints copying below.
        }

        p1->constraints.insert (p1->constraints.end (),
                                make_move_iterator (p2->constraints.begin ()),
                                make_move_iterator (p2->constraints.end ()));

        if (!replace)
          return false;
      }
      else
      {
        string n (pkg.available->id.name); // Note: copy; see emplace() below.

        level4 ([&]{trace << "add " << n << " " << pkg.available->version;});

        // This is the first time we are adding this package name to the
        // map. If it is already selected, then we need to make sure that
        // packages that already depend on it (called dependents) are ok
        // with the up/downgrade. We will also have to keep doing this
        // every time we choose a new available package above. So what
        // we are going to do is copy the dependents' constrains over to
        // our constraint list; this way they will be automatically taken
        // into account by the rest of the logic.
        //
        const shared_ptr<selected_package>& sp (pkg.selected);
        const shared_ptr<available_package>& ap (pkg.available);

        int r;
        if (sp != nullptr &&
            sp->state == package_state::configured &&
            (r = sp->version.compare (ap->version)) != 0)
        {
          using query = query<package_dependent>;

          for (const auto& pd: db.query<package_dependent> (query::name == n))
          {
            if (!pd.constraint)
              continue;

            const version& v (ap->version);
            const dependency_constraint& c (*pd.constraint);

            if (satisfies (v, c))
            {
              pkg.constraints.emplace_back (pd.name, c);
              continue;
            }

            const char* a (r < 0 ? "upgrade" : "downgrade");

            fail << "unable to " << a << " package " << n << " to " << v <<
              info << pd.name << " depends on (" << n << " " << c << ")" <<
              info << "explicitly specify " << n << " version to manually "
                 << "satisfy this constraint";
          }
        }

        i = map_.emplace (move (n),
                          data_type {list_.end (), move (pkg)}).first;
      }

      // Now collect all the prerequisites recursively. But first "prune"
      // this process if the package is already configured since that would
      // mean all its prerequisites are configured as well. Note that this
      // is not merely an optimization: the package could be an orphan in
      // which case the below logic will fail. By skipping the prerequisite
      // check we are able to gracefully handle configured orphans.
      //
      const satisfied_package& p (i->second.package);
      const shared_ptr<selected_package>& sp (p.selected);
      const shared_ptr<available_package>& ap (p.available);

      if (sp != nullptr &&
          sp->version == ap->version &&
          sp->state == package_state::configured)
        return true;

      const shared_ptr<repository>& ar (p.repository);
      const string& name (ap->id.name);

      // Show how we got here if things go wrong while recursively
      // collecting prerequisites.
      //
      auto g (
        make_exception_guard (
          [&ap] ()
          {
            info << "while satisfying " << ap->id.name << " " << ap->version;
          }));

      for (const dependency_alternatives& da: ap->dependencies)
      {
        if (da.conditional) // @@ TODO
          fail << "conditional dependencies are not yet supported";

        if (da.size () != 1) // @@ TODO
          fail << "multiple dependency alternatives not yet supported";

        const dependency& d (da.front ());

        // The first step is to always find the available package even
        // if, in the end, it won't be the one we select. If we cannot
        // find the package then that means the repository is broken.
        // And if we have no repository to look in, then that means the
        // package is an orphan (we delay this check until we actually
        // need the repository to allow orphans without prerequisites).
        //
        if (ar == nullptr)
          fail << "package " << name << " " << ap->version << " is orphaned" <<
            info << "explicitly upgrade it to a new version";

        auto rp (find_available (db, d.name, ar, d.constraint));

        if (rp.first == nullptr)
          fail << "unknown prerequisite " << d << " of package " << name <<
            info << "repository " << ar->location << " appears to be broken";

        // Next see if this package is already selected. If we already
        // have it in the configuraion and it satisfies our dependency
        // constraint, then we don't want to be forcing its upgrade (or,
        // worse, downgrade).
        //
        bool force (false);
        shared_ptr<selected_package> dsp (db.find<selected_package> (d.name));
        if (dsp != nullptr)
        {
          if (dsp->state == package_state::broken)
            fail << "unable to build broken package " << d.name <<
              info << "use 'pkg-purge --force' to remove";

          if (satisfies (dsp->version, d.constraint))
            rp = make_available (options, cd, db, dsp);
          else
            // Remember that we may be forcing up/downgrade; we will deal
            // with it below.
            //
            force = true;
        }

        satisfied_package dp {dsp, rp.first, rp.second, {}};

        // Add our constraint, if we have one.
        //
        if (d.constraint)
          dp.constraints.emplace_back (name, *d.constraint);

        // Now collect this prerequisite. If it was actually collected
        // (i.e., it wasn't already there) and we are forcing an upgrade,
        // then warn. Downgrade -- outright refuse.
        //
        if (collect (options, cd, db, move (dp)) && force)
        {
          const version& v (rp.first->version);

          bool u (v > dsp->version);
          bool c (d.constraint);
          diag_record dr;

          (u ? dr << warn : dr << fail)
            << "package " << name << " dependency on "
            << (c ? "(" : "") << d << (c ? ")" : "") << " is forcing "
            << (u ? "up" : "down") << "grade of " << d.name << " to " << v;

          if (!u)
            dr << info << "explicitly specify version downgrade to continue";
        }
      }

      return true;
    }

    // Order the previously-collected package with the specified name
    // returning its positions. If reorder is true, then reorder this
    // package to be considered as "early" as possible.
    //
    iterator
    order (const string& name, bool reorder = true)
    {
      // Every package that we order should have already be collected.
      //
      auto mi (map_.find (name));
      assert (mi != map_.end ());

      // If this package is already in the list, then that would also
      // mean all its prerequisites are in the list and we can just
      // return its position. Unless we want it reordered.
      //
      iterator& pos (mi->second.position);
      if (pos != list_.end ())
      {
        if (reorder)
          list_.erase (pos);
        else
          return pos;
      }

      const satisfied_package& p (mi->second.package);

      // Unless this package needs something to be before it, add it to
      // the end of the list.
      //
      iterator i (list_.end ());

      // Order all the prerequisites of this package and compute the
      // position of its "earliest" prerequisite -- this is where it
      // will be inserted. Similar to collect(), prune if configured
      // package (we don't have its prerequisites in the map).
      //
      if (p.selected == nullptr ||
          p.selected->version != p.available->version ||
          p.selected->state != package_state::configured)
      {
        // We are iterating in reverse so that when we iterate over
        // the dependency list (also in reverse), prerequisites will
        // be built in the order that is as close to the manifest as
        // possible.
        //
        for (const dependency_alternatives& da:
               reverse_iterate (p.available->dependencies))
        {
          assert (!da.conditional && da.size () == 1); // @@ TODO
          const dependency& d (da.front ());

          iterator j (order (d.name, false));

          // Figure out if j is before i, in which case set i to j. The
          // goal here is to find the position of our first prerequisite.
          //
          for (iterator k (j); i != j && k != list_.end ();)
            if (++k == i)
              i = j;
        }
      }

      return pos = list_.insert (i, p);
    }

  private:
    struct data_type
    {
      iterator position;         // Note: can be end(), see collect().
      satisfied_package package;
    };

    using map_type = map<string, data_type>;

    list_type list_;
    map_type map_;
  };

  void
  build (const build_options& o, cli::scanner& args)
  {
    tracer trace ("build");

    const dir_path& c (o.directory ());
    level4 ([&]{trace << "configuration: " << c;});

    if (!args.more ())
      fail << "package name argument expected" <<
        info << "run 'bpkg help build' for more information";

    database db (open (c, trace));
    transaction t (db.begin ());
    session s;

    shared_ptr<repository> root (db.load<repository> (""));

    // Start assembling the list of packages we will need to build by
    // first collecting the user's selection and its prerequisites.
    //
    satisfied_packages pkgs;
    vector<string> names;

    while (args.more ())
    {
      const char* s (args.next ());

      // Reduce all the potential variations (archive, directory, package
      // name, package name/version) to a single available_package object.
      //
      string n;
      version v;

      shared_ptr<repository> ar;
      shared_ptr<available_package> ap;

      // Is this a package archive?
      //
      try
      {
        path a (s);
        if (exists (a))
        {
          package_manifest m (pkg_verify (o, a, false));

          // This is a package archive (note that we shouldn't throw
          // failed from here on).
          //
          level4 ([&]{trace << "archive " << a;});
          n = m.name;
          v = m.version;
          ar = root;
          ap = make_shared<available_package> (move (m));
          ap->locations.push_back (package_location {root, move (a)});
        }
      }
      catch (const invalid_path&)
      {
        // Not a valid path so cannot be an archive.
      }
      catch (const failed&)
      {
        // Not a valid package archive.
      }

      // Is this a package directory?
      //
      try
      {
        dir_path d (s);
        if (exists (d))
        {
          package_manifest m (pkg_verify (d, false));

          // This is a package directory (note that we shouldn't throw
          // failed from here on).
          //
          level4 ([&]{trace << "directory " << d;});
          n = m.name;
          v = m.version;
          ap = make_shared<available_package> (move (m));
          ar = root;
          ap->locations.push_back (package_location {root, move (d)});
        }
      }
      catch (const invalid_path&)
      {
        // Not a valid path so cannot be an archive.
      }
      catch (const failed&)
      {
        // Not a valid package archive.
      }

      // Then it got to be a package name with optional version.
      //
      if (ap == nullptr)
      {
        n = parse_package_name (s);
        v = parse_package_version (s);
        level4 ([&]{trace << "package " << n << "; version " << v;});

        // Either get the user-specified version or the latest.
        //
        auto rp (
          v.empty ()
          ? find_available (db, n, root, nullopt)
          : find_available (db, n, root,
                            dependency_constraint {comparison::eq, v}));

        ap = rp.first;
        ar = rp.second;
      }

      // Load the package that may have already been selected and
      // figure out what exactly we need to do here. The end goal
      // is the available_package object corresponding to the actual
      // package that we will be building (which may or may not be
      // the same as the selected package).
      //
      shared_ptr<selected_package> sp (db.find<selected_package> (n));

      if (sp != nullptr && sp->state == package_state::broken)
        fail << "unable to build broken package " << n <<
          info << "use 'pkg-purge --force' to remove";

      bool found (true);

      // If the user asked for a specific version, then that's what
      // we ought to be building.
      //
      if (!v.empty ())
      {
        for (;;)
        {
          if (ap != nullptr) // Must be that version, see above.
            break;

          // Otherwise, our only chance is that the already selected
          // object is that exact version.
          //
          if (sp != nullptr && sp->version == v)
            break; // Derive ap from sp below.

          found = false;
          break;
        }
      }
      //
      // No explicit version was specified by the user.
      //
      else
      {
        if (ap != nullptr)
        {
          // Even if this package is already in the configuration, should
          // we have a newer version, we treat it as an upgrade request;
          // otherwise, why specify the package in the first place? We just
          // need to check if what we already have is "better" (i.e., newer).
          //
          if (sp != nullptr && ap->id.version < sp->version)
            ap = nullptr; // Derive ap from sp below.
        }
        else
        {
          if (sp == nullptr)
            found = false;

          // Otherwise, derive ap from sp below.
        }
      }

      if (!found)
      {
        diag_record dr;

        dr << fail << "unknown package " << n;
        if (!v.empty ())
          dr << " " << v;

        // Let's help the new user out here a bit.
        //
        if (db.query_value<repository_count> () == 0)
          dr << info << "configuration " << c << " has no repositories"
             << info << "use 'bpkg rep-add' to add a repository";
        else if (db.query_value<available_package_count> () == 0)
          dr << info << "configuration " << c << " has no available packages"
             << info << "use 'bpkg rep-fetch' to fetch available packages list";
      }

      // If the available_package object is still NULL, then it means
      // we need to get one corresponding to the selected package.
      //
      if (ap == nullptr)
      {
        assert (sp != nullptr);

        auto rp (make_available (o, c, db, sp));
        ap = rp.first;
        ar = rp.second; // Could be NULL (orphan).
      }

      // Finally add this package to the list.
      //
      level4 ([&]{trace << "collect " << ap->id.name << " " << ap->version;});

      satisfied_package p {move (sp), move (ap), move (ar), {}};

      // "Fix" the version the user asked for by adding the '==' constraint.
      //
      if (!v.empty ())
        p.constraints.emplace_back (
          "command line",
          dependency_constraint {comparison::eq, v});

      pkgs.collect (o, c, db, move (p));
      names.push_back (n);
    }

    // Now that we have collected all the package versions that we need
    // to build, arrange them in the "dependency order", that is, with
    // every package on the list only possibly depending on the ones
    // after it. Iterate over the names we have collected on the previous
    // step in reverse so that when we iterate over the packages (also in
    // reverse), things will be built as close as possible to the order
    // specified by the user (it may still get altered if there are
    // dependencies between the specified packages).
    //
    for (const string& n: reverse_iterate (names))
      pkgs.order (n);

    // Print what we are going to do, then ask for the user's confirmation.
    //
    for (const satisfied_package& p: reverse_iterate (pkgs))
    {
      const shared_ptr<selected_package>& sp (p.selected);
      const shared_ptr<available_package>& ap (p.available);

      const char* a;

      // Even if we already have this package selected, we have to
      // make sure it is configured and updated.
      //
      if (sp == nullptr || sp->version == ap->version)
        a = "build";
      else
        a = sp->version < ap->version ? "upgrade" : "downgrade";

      if (o.print_only ())
        cout << a << " " << ap->id.name << " " << ap->version << endl;
      else
        text << a << " " << ap->id.name << " " << ap->version;
    }

    if (o.print_only ())
    {
      t.commit ();
      return;
    }

    t.commit ();
  }
}