Avoid network requests while resolving versions when possible.

sdk/lib/_internal/pub/lib/src/solver/dependency_queue.dart

Index: sdk/lib/_internal/pub/lib/src/solver/dependency_queue.dart
diff --git a/sdk/lib/_internal/pub/lib/src/solver/dependency_queue.dart b/sdk/lib/_internal/pub/lib/src/solver/dependency_queue.dart
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+// Copyright (c) 2013, 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.
+
+library pub.solver.dependency_queue;
+
+import 'dart:async';
+import 'dart:collection' show Queue;
+
+import '../log.dart' as log;
+import '../package.dart';
+import 'backtracking_solver.dart';
+
+/// A queue of one package's dependencies, ordered by how the solver should
+/// traverse them.
+///
+/// It prefers locked versions so that they stay locked if possible. Then it
+/// prefers a currently selected package so that it only has to consider a
+/// single version.
+///
+/// After that, it orders the remaining packages by the number of versions they
+/// have so that packages with fewer versions are solved first. (If two
+/// packages have the same number of versions, they are sorted alphabetically
+/// just to be deterministic.)
+///
+/// Critically, this queue will *not* sort the dependencies by number of
+/// versions until actually needed. This ensures we don't do any network
+/// requests until we actually need to. In particular, it means that solving
+/// a package graph with an already up-to-date lockfile will do no network
+/// requests.
+class DependencyQueue {
+  final BacktrackingSolver _solver;
+
+  /// The dependencies for packages that have already been selected.
+  final Queue<PackageDep> _presorted;
+
+  /// The dependencies on the remaining packages.
+  ///
+  /// This is lazily sorted right before the first item is requested.
+  final List<PackageDep> _remaining;
+
+  bool _isSorted = false;
+
+  /// Gets whether there are any dependencies left to iterate over.
+  bool get isEmpty => _presorted.isEmpty && _remaining.isEmpty;
+
+  factory DependencyQueue(BacktrackingSolver solver,
+      Iterable<PackageDep> deps) {
+    // Separate out the presorted ones.
+    var presorted = <PackageDep>[];
+    var remaining = <PackageDep>[];
+
+    for (var dep in deps) {
+      // Selected or locked packages come first.
+      if (solver.getSelected(dep.name) != null ||
+          solver.getLocked(dep.name) != null) {
+        presorted.add(dep);
+      } else {
+        remaining.add(dep);
+      }
+    }
+
+    return new DependencyQueue._(solver, new Queue<PackageDep>.from(presorted),
+        remaining);
+  }
+
+  DependencyQueue._(this._solver, this._presorted, this._remaining);
+
+  /// Emits the next dependency in priority order.
+  ///
+  /// It is an error to call this if [isEmpty] returns `true`. Note that this
+  /// function is *not* re-entrant. You should only advance after the previous
+  /// advance has completed.
+  Future<PackageDep> advance() {
+    // Emit the sorted ones first.
+    if (_presorted.isNotEmpty) {
+      return new Future.value(_presorted.removeFirst());
+    }
+
+    // Sort the remaining packages when we need the first one.
+    if (!_isSorted) return _sort().then((_) => _remaining.removeAt(0));
+
+    return new Future.value(_remaining.removeAt(0));
+  }
+
+  /// Sorts the unselected packages by number of versions and name.
+  Future _sort() {
+    return Future.wait(_remaining.map(_getNumVersions)).then((versions) {
+      // Map deps to the number of versions they have.
+      var versionMap = new Map.fromIterables(_remaining, versions);
+
+      // Sort in best-first order to minimize backtracking.
+      _remaining.sort((a, b) {
+        // Traverse into packages with fewer versions since they will lead to
+        // less backtracking.
+        if (versionMap[a] != versionMap[b]) {
+          return versionMap[a].compareTo(versionMap[b]);
+        }
+
+        // Otherwise, just sort by name so that it's deterministic.
+        return a.name.compareTo(b.name);
+      });
+
+      _isSorted = true;
+    });
+  }
+
+  /// Given a dependency, returns a future that completes to the number of
+  /// versions available for it.
+  Future<int> _getNumVersions(PackageDep dep) {
+    // There is only ever one version of the root package.
+    if (dep.isRoot) {
+      return new Future.value(1);
+    }
+
+    return _solver.cache.getVersions(dep.toRef()).then((versions) {
+      return versions.length;
+    }).catchError((error, trace) {
+      // If it fails for any reason, just treat that as no versions. This
+      // will sort this reference higher so that we can traverse into it
+      // and report the error more properly.
+      log.solver("Could not get versions for $dep:\n$error\n\n$trace");
+      return 0;
+    });
+  }
+}