Use backtracking when solving dependency constraints.

utils/pub/version_solver.dart

Index: utils/pub/version_solver.dart
diff --git a/utils/pub/version_solver.dart b/utils/pub/version_solver.dart
deleted file mode 100644
index 36569bab117366b6b7651e985ddaf16e533d49c2..0000000000000000000000000000000000000000
--- a/utils/pub/version_solver.dart
+++ /dev/null
@@ -1,714 +0,0 @@
-// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
-// for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
-
-/// Attempts to resolve a set of version constraints for a package dependency
-/// graph and select an appropriate set of best specific versions for all
-/// dependent packages. It works iteratively and tries to reach a stable
-/// solution where the constraints of all dependencies are met. If it fails to
-/// reach a solution after a certain number of iterations, it assumes the
-/// dependency graph is unstable and reports and error.
-///
-/// There are two fundamental operations in the process of iterating over the
-/// graph:
-///
-/// 1.  Changing the selected concrete version of some package. (This includes
-///     adding and removing a package too, which is considering changing the
-///     version to or from "none".) In other words, a node has changed.
-/// 2.  Changing the version constraint that one package places on another. In
-///     other words, and edge has changed.
-///
-/// Both of these events have a corresponding (potentional) async operation and
-/// roughly cycle back and forth between each other. When we change the version
-/// of package changes, we asynchronously load the pubspec for the new version.
-/// When that's done, we compare the dependencies of the new version versus the
-/// old one. For everything that differs, we change those constraints between
-/// this package and that dependency.
-///
-/// When a constraint on a package changes, we re-calculate the overall
-/// constraint on that package. I.e. with a shared dependency, we intersect all
-/// of the constraints that its depending packages place on it. If that overall
-/// constraint changes (say from "<3.0.0" to "<2.5.0"), then the currently
-/// picked version for that package may fall outside of the new constraint. If
-/// that happens, we find the new best version that meets the updated constraint
-/// and then the change the package to use that version. That cycles back up to
-/// the beginning again.
-library version_solver;
-
-import 'dart:async';
-import 'dart:collection' show Queue;
-import 'dart:json' as json;
-import 'dart:math';
-import 'lock_file.dart';
-import 'log.dart' as log;
-import 'package.dart';
-import 'pubspec.dart';
-import 'source.dart';
-import 'source_registry.dart';
-import 'utils.dart';
-import 'version.dart';
-
-/// Attempts to select the best concrete versions for all of the transitive
-/// dependencies of [root] taking into account all of the [VersionConstraint]s
-/// that those dependencies place on each other and the requirements imposed by
-/// [lockFile]. If successful, completes to a [Map] that maps package names to
-/// the selected version for that package. If it fails, the future will complete
-/// with a [NoVersionException], [DisjointConstraintException], or
-/// [CouldNotSolveException].
-Future<List<PackageId>> resolveVersions(SourceRegistry sources, Package root,
-    LockFile lockFile) {
-  log.message('Resolving dependencies...');
-  return new VersionSolver(sources, root, lockFile).solve();
-}
-
-class VersionSolver {
-  final SourceRegistry _sources;
-  final Package _root;
-  final LockFile lockFile;
-  final PubspecCache _pubspecs;
-  final Map<String, Dependency> _packages;
-  final Queue<WorkItem> _work;
-  int _numIterations = 0;
-
-  VersionSolver(SourceRegistry sources, this._root, this.lockFile)
-      : _sources = sources,
-        _pubspecs = new PubspecCache(sources),
-        _packages = <String, Dependency>{},
-        _work = new Queue<WorkItem>();
-
-  /// Tell the version solver to use the most recent version of [package] that
-  /// exists in whatever source it's installed from. If that version violates
-  /// constraints imposed by other dependencies, an error will be raised when
-  /// solving the versions, even if an earlier compatible version exists.
-  void useLatestVersion(String package) {
-    // TODO(nweiz): How do we want to detect and handle unknown dependencies
-    // here?
-    getDependency(package).useLatestVersion = true;
-    lockFile.packages.remove(package);
-  }
-
-  Future<List<PackageId>> solve() {
-    // Kick off the work by adding the root package at its concrete version to
-    // the dependency graph.
-    var ref = new PackageRef.root(_root);
-    enqueue(new AddConstraint('(entrypoint)', ref));
-    _pubspecs.cache(ref.atVersion(_root.version), _root.pubspec);
-
-    Future processNextWorkItem(_) {
-      while (true) {
-        // Stop if we are done.
-        if (_work.isEmpty) return new Future.value(buildResults());
-
-        // If we appear to be stuck in a loop, then we probably have an unstable
-        // graph, bail. We guess this based on a rough heuristic that it should
-        // only take a certain number of steps to solve a graph with a given
-        // number of connections.
-        // TODO(rnystrom): These numbers here are magic and arbitrary. Tune
-        // when we have a better picture of real-world package topologies.
-        _numIterations++;
-        if (_numIterations > max(50, _packages.length * 5)) {
-          throw new CouldNotSolveException();
-        }
-
-        // Run the first work item.
-        var future = _work.removeFirst().process(this);
-
-        // If we have an async operation to perform, chain the loop to resume
-        // when it's done. Otherwise, just loop synchronously.
-        if (future != null) {
-          return future.then(processNextWorkItem);
-        }
-      }
-    }
-
-    return processNextWorkItem(null);
-  }
-
-  void enqueue(WorkItem work) {
-    _work.add(work);
-  }
-
-  Dependency getDependency(String package) {
-    // There can be unused dependencies in the graph, so just create an empty
-    // one if needed.
-    _packages.putIfAbsent(package, () => new Dependency(package));
-    return _packages[package];
-  }
-
-  /// Sets the best selected version of [package] to [version].
-  void setVersion(String package, Version version) {
-    _packages[package].version = version;
-  }
-
-  /// Returns the most recent version of [dependency] that satisfies all of its
-  /// version constraints.
-  Future<Version> getBestVersion(Dependency dependency) {
-    return dependency.getVersions().then((versions) {
-      var best = null;
-      for (var version in versions) {
-        if (dependency.useLatestVersion ||
-            dependency.constraint.allows(version)) {
-          if (best == null || version > best) best = version;
-        }
-      }
-
-      // TODO(rnystrom): Better exception.
-      if (best == null) {
-        if (tryUnlockDepender(dependency)) return null;
-        throw new NoVersionException(dependency.name, dependency.constraint,
-            dependency._refs);
-      } else if (!dependency.constraint.allows(best)) {
-        if (tryUnlockDepender(dependency)) return null;
-        throw new CouldNotUpdateException(
-            dependency.name, dependency.constraint, best);
-      }
-
-      return best;
-    });
-  }
-
-  /// Looks for a package that depends (transitively) on [dependency] and has
-  /// its version locked in the lockfile. If one is found, enqueues an
-  /// [UnlockPackage] work item for it and returns true. Otherwise, returns
-  /// false.
-  ///
-  /// This does a breadth-first search; immediate dependers will be unlocked
-  /// first, followed by transitive dependers.
-  bool tryUnlockDepender(Dependency dependency, [Set<String> seen]) {
-    if (seen == null) seen = new Set();
-    // Avoid an infinite loop if there are circular dependencies.
-    if (seen.contains(dependency.name)) return false;
-    seen.add(dependency.name);
-
-    for (var dependerName in dependency.dependers) {
-      var depender = getDependency(dependerName);
-      var locked = lockFile.packages[dependerName];
-      if (locked != null && depender.version == locked.version &&
-          depender.source.name == locked.source.name) {
-        enqueue(new UnlockPackage(depender));
-        return true;
-      }
-    }
-
-    return dependency.dependers.map(getDependency).any((subdependency) =>
-        tryUnlockDepender(subdependency, seen));
-  }
-
-  List<PackageId> buildResults() {
-    return _packages.values.where((dep) => dep.isDependedOn).map((dep) {
-      var description = dep.description;
-
-      // If the lockfile contains a fully-resolved description for the package,
-      // use that. This allows e.g. Git to ensure that the same commit is used.
-      var lockedPackage = lockFile.packages[dep.name];
-      if (lockedPackage != null && lockedPackage.version == dep.version &&
-          lockedPackage.source.name == dep.source.name &&
-          dep.source.descriptionsEqual(
-              description, lockedPackage.description)) {
-        description = lockedPackage.description;
-      }
-
-      return new PackageId(dep.name, dep.source, dep.version, description);
-    })
-    .toList();
-  }
-}
-
-/// The constraint solver works by iteratively processing a queue of work items.
-/// Each item is a single atomic change to the dependency graph. Handling them
-/// in a queue lets us handle asynchrony (resolving versions requires
-/// information from servers) as well as avoid deeply nested recursion.
-abstract class WorkItem {
-  /// Processes this work item. Returns a future that completes when the work is
-  /// done. If `null` is returned, that means the work has completed
-  /// synchronously and the next item can be started immediately.
-  Future process(VersionSolver solver);
-}
-
-/// The best selected version for a package has changed to [version]. If the
-/// previous version of the package is `null`, that means the package is being
-/// added to the graph. If [version] is `null`, it is being removed.
-class ChangeVersion implements WorkItem {
-  /// The name of the package whose version is being changed.
-  final String package;
-
-  /// The source of the package whose version is changing.
-  final Source source;
-
-  /// The description identifying the package whose version is changing.
-  final description;
-
-  /// The new selected version.
-  final Version version;
-
-  ChangeVersion(this.package, this.source, this.description, this.version);
-
-  Future process(VersionSolver solver) {
-    log.fine("Changing $package to version $version.");
-
-    var dependency = solver.getDependency(package);
-    var oldVersion = dependency.version;
-    solver.setVersion(package, version);
-
-    // The dependencies between the old and new version may be different. Walk
-    // them both and update any constraints that differ between the two.
-    return Future.wait([
-        getDependencyRefs(solver, oldVersion),
-        getDependencyRefs(solver, version)]).then((list) {
-      var oldDependencyRefs = list[0];
-      var newDependencyRefs = list[1];
-
-      for (var oldRef in oldDependencyRefs.values) {
-        if (newDependencyRefs.containsKey(oldRef.name)) {
-          // The dependency is in both versions of this package, but its
-          // constraint may have changed.
-          var newRef = newDependencyRefs.remove(oldRef.name);
-          solver.enqueue(new AddConstraint(package, newRef));
-        } else {
-          // The dependency is not in the new version of the package, so just
-          // remove its constraint.
-          solver.enqueue(new RemoveConstraint(package, oldRef.name));
-        }
-      }
-
-      // Everything that's left is a depdendency that's only in the new
-      // version of the package.
-      for (var newRef in newDependencyRefs.values) {
-        solver.enqueue(new AddConstraint(package, newRef));
-      }
-    });
-  }
-
-  /// Get the dependencies at [version] of the package being changed.
-  Future<Map<String, PackageRef>> getDependencyRefs(VersionSolver solver,
-      Version version) {
-    // If there is no version, it means no package, so no dependencies.
-    if (version == null) {
-      return new Future<Map<String, PackageRef>>.value(<String, PackageRef>{});
-    }
-
-    var id = new PackageId(package, source, version, description);
-    return solver._pubspecs.load(id).then((pubspec) {
-      var dependencies = <String, PackageRef>{};
-      for (var dependency in pubspec.dependencies) {
-        dependencies[dependency.name] = dependency;
-      }
-
-      // Include dev dependencies only from the root package.
-      if (id.isRoot) {
-        for (var dependency in pubspec.devDependencies) {
-          dependencies[dependency.name] = dependency;
-        }
-      }
-
-      return dependencies;
-    });
-  }
-}
-
-/// A constraint that a depending package places on a dependent package has
-/// changed.
-///
-/// This is an abstract class that contains logic for updating the dependency
-/// graph once a dependency has changed. Changing the dependency is the
-/// responsibility of subclasses.
-abstract class ChangeConstraint implements WorkItem {
-  Future process(VersionSolver solver);
-
-  void undo(VersionSolver solver);
-
-  Future _processChange(VersionSolver solver, Dependency oldDependency,
-      Dependency newDependency) {
-    var name = newDependency.name;
-    var source = oldDependency.source != null ?
-      oldDependency.source : newDependency.source;
-    var description = oldDependency.description != null ?
-      oldDependency.description : newDependency.description;
-    var oldConstraint = oldDependency.constraint;
-    var newConstraint = newDependency.constraint;
-
-    // If the package is over-constrained, i.e. the packages depending have
-    // disjoint constraints, then try unlocking a depender that's locked by the
-    // lockfile. If there are no remaining locked dependencies, throw an error.
-    if (newConstraint != null && newConstraint.isEmpty) {
-      if (solver.tryUnlockDepender(newDependency)) {
-        undo(solver);
-        return null;
-      }
-
-      throw new DisjointConstraintException(name, newDependency._refs);
-    }
-
-    // If this constraint change didn't cause the overall constraint on the
-    // package to change, then we don't need to do any further work.
-    if (oldConstraint == newConstraint) return null;
-
-    // If the dependency has been cut free from the graph, just remove it.
-    if (!newDependency.isDependedOn) {
-      solver.enqueue(new ChangeVersion(name, source, description, null));
-      return null;
-    }
-
-    // If the dependency is on the root package, then we don't need to do
-    // anything since it's already at the best version.
-    if (name == solver._root.name) {
-      solver.enqueue(new ChangeVersion(
-          name, source, description, solver._root.version));
-      return null;
-    }
-
-    // If the dependency is on a package in the lockfile, use the lockfile's
-    // version for that package if it's valid given the other constraints.
-    var lockedPackage = solver.lockFile.packages[name];
-    if (lockedPackage != null && newDependency.source == lockedPackage.source) {
-      var lockedVersion = lockedPackage.version;
-      if (newConstraint.allows(lockedVersion)) {
-        solver.enqueue(
-            new ChangeVersion(name, source, description, lockedVersion));
-        return null;
-      }
-    }
-
-    // The constraint has changed, so see what the best version of the package
-    // that meets the new constraint is.
-    return solver.getBestVersion(newDependency).then((best) {
-      if (best == null) {
-        undo(solver);
-      } else if (newDependency.version != best) {
-        solver.enqueue(new ChangeVersion(name, source, description, best));
-      }
-    });
-  }
-}
-
-/// The constraint given by [ref] is being placed by [depender].
-class AddConstraint extends ChangeConstraint {
-  /// The package that has the dependency.
-  final String depender;
-
-  /// The package being depended on and the constraints being placed on it. The
-  /// source, version, and description in this ref are all considered
-  /// constraints on the dependent package.
-  final PackageRef ref;
-
-  AddConstraint(this.depender, this.ref);
-
-  Future process(VersionSolver solver) {
-    log.fine("Adding $depender's constraint $ref.");
-
-    var dependency = solver.getDependency(ref.name);
-    var oldDependency = dependency.clone();
-    dependency.placeConstraint(depender, ref);
-    return _processChange(solver, oldDependency, dependency);
-  }
-
-  void undo(VersionSolver solver) {
-    solver.getDependency(ref.name).removeConstraint(depender);
-  }
-}
-
-/// [depender] is no longer placing a constraint on [dependent].
-class RemoveConstraint extends ChangeConstraint {
-  /// The package that was placing a constraint on [dependent].
-  String depender;
-
-  /// The package that was being depended on.
-  String dependent;
-
-  /// The constraint that was removed.
-  PackageRef _removed;
-
-  RemoveConstraint(this.depender, this.dependent);
-
-  Future process(VersionSolver solver) {
-    log.fine("Removing $depender's constraint ($_removed) on $dependent.");
-
-    var dependency = solver.getDependency(dependent);
-    var oldDependency = dependency.clone();
-    _removed = dependency.removeConstraint(depender);
-    return _processChange(solver, oldDependency, dependency);
-  }
-
-  void undo(VersionSolver solver) {
-    solver.getDependency(dependent).placeConstraint(depender, _removed);
-  }
-}
-
-/// [package]'s version is no longer constrained by the lockfile.
-class UnlockPackage implements WorkItem {
-  /// The package being unlocked.
-  Dependency package;
-
-  UnlockPackage(this.package);
-
-  Future process(VersionSolver solver) {
-    log.fine("Unlocking ${package.name}.");
-
-    solver.lockFile.packages.remove(package.name);
-    return solver.getBestVersion(package).then((best) {
-      if (best == null) return null;
-      solver.enqueue(new ChangeVersion(
-          package.name, package.source, package.description, best));
-    });
-  }
-}
-
-// TODO(rnystrom): Instead of always pulling from the source (which will mean
-// hitting a server), we should consider caching pubspecs of uninstalled
-// packages in the system cache.
-/// Maintains a cache of previously-loaded pubspecs. Used to avoid requesting
-/// the same pubspec from the server repeatedly.
-class PubspecCache {
-  final SourceRegistry _sources;
-  final Map<PackageId, Pubspec> _pubspecs;
-
-  PubspecCache(this._sources)
-      : _pubspecs = new Map<PackageId, Pubspec>();
-
-  /// Caches [pubspec] as the [Pubspec] for the package identified by [id].
-  void cache(PackageId id, Pubspec pubspec) {
-    _pubspecs[id] = pubspec;
-  }
-
-  /// Loads the pubspec for the package identified by [id].
-  Future<Pubspec> load(PackageId id) {
-    // Complete immediately if it's already cached.
-    if (_pubspecs.containsKey(id)) {
-      return new Future<Pubspec>.value(_pubspecs[id]);
-    }
-
-    return id.describe().then((pubspec) {
-      // Cache it.
-      _pubspecs[id] = pubspec;
-      return pubspec;
-    });
-  }
-}
-
-/// Describes one [Package] in the [DependencyGraph] and keeps track of which
-/// packages depend on it and what constraints they place on it.
-class Dependency {
-  /// The name of the this dependency's package.
-  final String name;
-
-  /// The [PackageRefs] that represent constraints that depending packages have
-  /// placed on this one.
-  final Map<String, PackageRef> _refs;
-
-  /// The currently-selected best version for this dependency.
-  Version version;
-
-  /// Whether this dependency should always select the latest version.
-  bool useLatestVersion = false;
-
-  /// Gets whether or not any other packages are currently depending on this
-  /// one. If `false`, then it means this package is not part of the dependency
-  /// graph and should be omitted.
-  bool get isDependedOn => !_refs.isEmpty;
-
-  /// The names of all the packages that depend on this dependency.
-  Iterable<String> get dependers => _refs.keys;
-
-  /// Gets the overall constraint that all packages are placing on this one.
-  /// If no packages have a constraint on this one (which can happen when this
-  /// package is in the process of being added to the graph), returns `null`.
-  VersionConstraint get constraint {
-    if (_refs.isEmpty) return null;
-    return new VersionConstraint.intersection(
-        _refs.values.map((ref) => ref.constraint));
-  }
-
-  /// The source of this dependency's package.
-  Source get source {
-     var canonical = _canonicalRef();
-     if (canonical == null) return null;
-     return canonical.source;
-  }
-
-  /// The description of this dependency's package.
-  get description {
-     var canonical = _canonicalRef();
-     if (canonical == null) return null;
-     return canonical.description;
-  }
-
-  /// Return the PackageRef that has the canonical source and description for
-  /// this package. If any dependency is on the root package, that will be used;
-  /// otherwise, it will be the source and description that all dependencies
-  /// agree upon.
-  PackageRef _canonicalRef() {
-    if (_refs.isEmpty) return null;
-    var refs = _refs.values;
-    for (var ref in refs) {
-      if (ref.isRoot) return ref;
-    }
-    return refs.first;
-  }
-
-  Dependency(this.name)
-      : _refs = <String, PackageRef>{};
-
-  Dependency._clone(Dependency other)
-      : name = other.name,
-        version = other.version,
-        _refs = new Map<String, PackageRef>.from(other._refs);
-
-  /// Creates a copy of this dependency.
-  Dependency clone() => new Dependency._clone(this);
-
-  /// Return a list of available versions for this dependency.
-  Future<List<Version>> getVersions() => source.getVersions(name, description);
-
-  /// Places [ref] as a constraint from [package] onto this.
-  void placeConstraint(String package, PackageRef ref) {
-    var requiredDepender = _requiredDepender();
-    if (requiredDepender != null) {
-      var required = _refs[requiredDepender];
-      if (required.source.name != ref.source.name) {
-        throw new SourceMismatchException(name,
-            requiredDepender, required.source, package, ref.source);
-      } else if (!required.source.descriptionsEqual(
-                     required.description, ref.description)) {
-        throw new DescriptionMismatchException(name,
-            requiredDepender, required.description, package, ref.description);
-      }
-    }
-
-    _refs[package] = ref;
-  }
-
-  /// Returns the name of a package whose constraint source and description
-  /// all other constraints must match. Returns null if there are no
-  /// requirements on new constraints.
-  String _requiredDepender() {
-    if (_refs.isEmpty) return null;
-
-    var dependers = _refs.keys.toList();
-    if (dependers.length == 1) {
-      var depender = dependers[0];
-      if (_refs[depender].isRoot) return null;
-      return depender;
-    }
-
-    return dependers[1];
-  }
-
-  /// Removes the constraint from [package] onto this.
-  PackageRef removeConstraint(String package) => _refs.remove(package);
-}
-
-/// Exception thrown when the [VersionConstraint] used to match a package is
-/// valid (i.e. non-empty), but there are no released versions of the package
-/// that fit that constraint.
-class NoVersionException implements Exception {
-  final String package;
-  final VersionConstraint constraint;
-  final Map<String, PackageRef> _dependencies;
-
-  NoVersionException(this.package, this.constraint, this._dependencies);
-
-  String toString() {
-    var buffer = new StringBuffer();
-    buffer.write("Package '$package' has no versions that match $constraint "
-        "derived from:\n");
-
-    var keys = new List.from(_dependencies.keys);
-    keys.sort();
-
-    for (var key in keys) {
-      buffer.write("- '$key' depends on version "
-          "${_dependencies[key].constraint}\n");
-    }
-
-    return buffer.toString();
-  }
-}
-
-// TODO(rnystrom): Report the list of depending packages and their constraints.
-/// Exception thrown when the most recent version of [package] must be selected,
-/// but doesn't match the [VersionConstraint] imposed on the package.
-class CouldNotUpdateException implements Exception {
-  final String package;
-  final VersionConstraint constraint;
-  final Version best;
-
-  CouldNotUpdateException(this.package, this.constraint, this.best);
-
-  String toString() =>
-      "The latest version of '$package', $best, does not match $constraint.";
-}
-
-/// Exception thrown when the [VersionConstraint] used to match a package is
-/// the empty set: in other words, multiple packages depend on it and have
-/// conflicting constraints that have no overlap.
-class DisjointConstraintException implements Exception {
-  final String package;
-  final Map<String, PackageRef> _dependencies;
-
-  DisjointConstraintException(this.package, this._dependencies);
-
-  String toString() {
-    var buffer = new StringBuffer();
-    buffer.write("Incompatible version constraints on '$package':\n");
-
-    var keys = new List.from(_dependencies.keys);
-    keys.sort();
-
-    for (var key in keys) {
-      buffer.write("- '$key' depends on version "
-          "${_dependencies[key].constraint}\n");
-    }
-
-    return buffer.toString();
-  }
-}
-
-/// Exception thrown when the [VersionSolver] fails to find a solution after a
-/// certain number of iterations.
-class CouldNotSolveException implements Exception {
-  CouldNotSolveException();
-
-  String toString() =>
-      "Could not find a solution that met all version constraints.";
-}
-
-/// Exception thrown when two packages with the same name but different sources
-/// are depended upon.
-class SourceMismatchException implements Exception {
-  final String package;
-  final String depender1;
-  final Source source1;
-  final String depender2;
-  final Source source2;
-
-  SourceMismatchException(this.package, this.depender1, this.source1,
-      this.depender2, this.source2);
-
-  String toString() {
-    return "Incompatible dependencies on '$package':\n"
-        "- '$depender1' depends on it from source '$source1'\n"
-        "- '$depender2' depends on it from source '$source2'";
-  }
-}
-
-/// Exception thrown when two packages with the same name and source but
-/// different descriptions are depended upon.
-class DescriptionMismatchException implements Exception {
-  final String package;
-  final String depender1;
-  final description1;
-  final String depender2;
-  final description2;
-
-  DescriptionMismatchException(this.package, this.depender1, this.description1,
-      this.depender2, this.description2);
-
-  String toString() {
-    // TODO(nweiz): Dump descriptions to YAML when that's supported.
-    return "Incompatible dependencies on '$package':\n"
-        "- '$depender1' depends on it with description "
-        "${json.stringify(description1)}\n"
-        "- '$depender2' depends on it with description "
-        "${json.stringify(description2)}";
-  }
-}