Use backtracking when solving dependency constraints.

utils/pub/solver/greedy_solver.dart

 // 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.
@@ skipping 15 matching lines @@
 /// 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;
+library version_solver1;
 
 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';
+import 'dart:math' as math;
 
-/// 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();
-}
+import '../lock_file.dart';
+import '../log.dart' as log;
+import '../package.dart';
+import '../source.dart';
+import '../source_registry.dart';
+import '../version.dart';
+import 'version_solver.dart';
 
-class VersionSolver {
-  final SourceRegistry _sources;
-  final Package _root;
-  final LockFile lockFile;
-  final PubspecCache _pubspecs;
-  final Map<String, Dependency> _packages;
-  final Queue<WorkItem> _work;
+class GreedyVersionSolver extends VersionSolver {
+  final _packages = <String, DependencyNode>{};
+  final _work = new Queue<WorkItem>();
   int _numIterations = 0;
 
-  VersionSolver(SourceRegistry sources, this._root, this.lockFile)
-      : _sources = sources,
-        _pubspecs = new PubspecCache(sources),
-        _packages = <String, Dependency>{},
-        _work = new Queue<WorkItem>();
+  GreedyVersionSolver(SourceRegistry sources, Package root, LockFile lockFile,
+                      List<String> useLatest)
+      : super(sources, root, lockFile, useLatest);
 
-  /// 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) {
+  void forceLatestVersion(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() {
+  Future<SolveResult> runSolver() {
     // 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);
+    enqueue(new AddConstraint('(entrypoint)', new PackageRef.root(root)));
 
     Future processNextWorkItem(_) {
       while (true) {
         // Stop if we are done.
         if (_work.isEmpty) return new Future.immediate(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)) {
+        if (_numIterations > math.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) {
+  DependencyNode 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));
+    _packages.putIfAbsent(package, () => new DependencyNode(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) {
+  Future<Version> getBestVersion(DependencyNode dependency) {
+    return cache.getVersions(dependency.name,
+        dependency.source, dependency.description).then((versions) {
       var best = null;
-      for (var version in versions) {
+      for (var ref in versions) {
         if (dependency.useLatestVersion ||
-            dependency.constraint.allows(version)) {
-          if (best == null || version > best) best = version;
+            dependency.constraint.allows(ref.version)) {
+          if (best == null || ref.version > best) best = ref.version;
         }
       }
 
       // TODO(rnystrom): Better exception.
       if (best == null) {
         if (tryUnlockDepender(dependency)) return null;
         throw new NoVersionException(dependency.name, dependency.constraint,
-            dependency._refs);
+            dependency.toList());
       } 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]) {
+  bool tryUnlockDepender(DependencyNode 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;
+  SolveResult buildResults() {
+    var packages = _packages.values
+        .where((dep) => dep.isDependedOn)
+        .map(_dependencyToPackageId)
+        .toList();
 
-      // 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;
-      }
+    // The non-backtracking solver always only tries one solution.
+    return new SolveResult(packages, 1);
+  }
 
-      return new PackageId(dep.name, dep.source, dep.version, description);
-    })
-    .toList();
+  PackageId _dependencyToPackageId(DependencyNode 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);
   }
 }
 
 /// 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);
+  Future process(GreedyVersionSolver 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) {
+  Future process(GreedyVersionSolver 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),
@@ skipping 25 matching lines @@
   /// 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>>.immediate(
           <String, PackageRef>{});
     }
 
     var id = new PackageId(package, source, version, description);
-    return solver._pubspecs.load(id).then((pubspec) {
+    return solver.cache.getPubspec(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);
+  Future process(GreedyVersionSolver solver);
 
-  void undo(VersionSolver solver);
+  void undo(GreedyVersionSolver solver);
 
-  Future _processChange(VersionSolver solver, Dependency oldDependency,
-      Dependency newDependency) {
+  Future _processChange(GreedyVersionSolver solver,
+                        DependencyNode oldDependency,
+                        DependencyNode 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);
+      throw new DisjointConstraintException(name, newDependency.toList());
     }
 
     // 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) {
+    if (name == solver.root.name) {
       solver.enqueue(new ChangeVersion(
-          name, source, description, solver._root.version));
+          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(
@@ skipping 19 matching lines @@
   /// 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) {
+  Future process(GreedyVersionSolver 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) {
+  void undo(GreedyVersionSolver 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) {
+  Future process(GreedyVersionSolver 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) {
+  void undo(GreedyVersionSolver 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;
+  DependencyNode package;
 
   UnlockPackage(this.package);
 
-  Future process(VersionSolver solver) {
+  Future process(GreedyVersionSolver 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>.immediate(_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 {
+class DependencyNode {
   /// 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;
 
@@ skipping 37 matching lines @@
   /// 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)
+  DependencyNode(this.name)
       : _refs = <String, PackageRef>{};
 
-  Dependency._clone(Dependency other)
+  DependencyNode._clone(DependencyNode 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);
+  DependencyNode clone() => new DependencyNode._clone(this);
 
   /// 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(
@@ skipping 17 matching lines @@
       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 "
-          "${_dependencies[key].constraint}\n");
-    }
-
-    return buffer.toString();
+  /// Converts this to a list of [Dependency] objects like the error types
+  /// expect.
+  List<Dependency> toList() {
+    var result = <Dependency>[];
+    _refs.forEach((name, ref) {
+      result.add(new Dependency(name, ref));
+    });
+    return result;
   }
 }
-
-// TODO(rnystrom): Report the list of depending packages and their constraints.
-/// Exception thrown when the most recent version of [package] must be selected,
-/// but doesn't match the [VersionConstraint] imposed on the package.
-class CouldNotUpdateException implements Exception {
-  final String package;
-  final VersionConstraint constraint;
-  final Version best;
-
-  CouldNotUpdateException(this.package, this.constraint, this.best);
-
-  String toString() =>
-      "The latest version of '$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)}";
-  }
-}