Use backtracking when solving dependency constraints.

utils/tests/pub/version_solver_test.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.
 
 library pub_update_test;
 
 import 'dart:async';
 import 'dart:io';
 
 import '../../../pkg/unittest/lib/unittest.dart';
@@ skipping 34 matching lines @@
     if (x is! DescriptionMismatchException) return false;
 
     // Make sure the error string mentions the conflicting dependers.
     if (!x.toString().contains(package1)) return false;
     if (!x.toString().contains(package2)) return false;
 
     return true;
   }, "is a DescriptionMismatchException");
 }
 
-final couldNotSolve = predicate((x) => x is CouldNotSolveException,
-    "is a CouldNotSolveException");
+// If no solution can be found, the solver just reports the last failure that
+// happened during propagation. Since we don't specify the order that solutions
+// are tried, this just validates that *some* failure occurred, but not which.
+final couldNotSolve = predicate((x) => x is SolverFailure,
+    "is a SolverFailure");
 
 Matcher sourceMismatch(String package1, String package2) {
   return predicate((x) {
     if (x is! SourceMismatchException) return false;
 
     // Make sure the error string mentions the conflicting dependers.
     if (!x.toString().contains(package1)) return false;
     if (!x.toString().contains(package2)) return false;
 
     return true;
@@ skipping 131 matching lines @@
     'bar 1.0.2': {},
     'baz 1.0.0': {}
   }, lockfile: {
     'baz': '1.0.0'
   }, result: {
     'myapp from root': '0.0.0',
     'foo': '1.0.2',
     'bar': '1.0.2'
   });
 
+  testResolve('unlocks dependencies if necessary to ensure that a new '
+      'dependency is satisfied', {
+    'myapp 0.0.0': {
+      'foo': 'any',
+      'newdep': 'any'
+    },
+    'foo 1.0.0': { 'bar': '<2.0.0' },
+    'bar 1.0.0': { 'baz': '<2.0.0' },
+    'baz 1.0.0': { 'qux': '<2.0.0' },
+    'qux 1.0.0': {},
+    'foo 2.0.0': { 'bar': '<3.0.0' },
+    'bar 2.0.0': { 'baz': '<3.0.0' },
+    'baz 2.0.0': { 'qux': '<3.0.0' },
+    'qux 2.0.0': {},
+    'newdep 2.0.0': { 'baz': '>=1.5.0' }
+  }, lockfile: {
+    'foo': '1.0.0',
+    'bar': '1.0.0',
+    'baz': '1.0.0',
+    'qux': '1.0.0'
+  }, result: {
+    'myapp from root': '0.0.0',
+    'foo': '2.0.0',
+    'bar': '2.0.0',
+    'baz': '2.0.0',
+    'qux': '1.0.0',
+    'newdep': '2.0.0'
+  });
+
   testResolve('circular dependency', {
     'myapp 1.0.0': {
       'foo': '1.0.0'
     },
     'foo 1.0.0': {
       'bar': '1.0.0'
     },
     'bar 1.0.0': {
       'foo': '1.0.0'
     }
@@ skipping 40 matching lines @@
     }
   }, error: sourceMismatch('foo', 'bar'));
 
   testResolve('dependency back onto root package with wrong version', {
     'myapp 1.0.0': {
       'foo': '1.0.0'
     },
     'foo 1.0.0': {
       'myapp': '<1.0.0'
     }
-  }, error: disjointConstraint(['foo']));
+  }, error: couldNotSolve);
 
   testResolve('no version that matches requirement', {
     'myapp 0.0.0': {
       'foo': '>=1.0.0 <2.0.0'
     },
     'foo 2.0.0': {},
     'foo 2.1.3': {}
   }, error: noVersion(['myapp']));
 
   testResolve('no version that matches combined constraint', {
@@ skipping 49 matching lines @@
     'foo 1.0.0': {
       'shared': '1.0.0'
     },
     'bar 1.0.0': {
       'shared from mock2': '1.0.0'
     },
     'shared 1.0.0': {},
     'shared 1.0.0 from mock2': {}
   }, error: sourceMismatch('foo', 'bar'));
 
-  testResolve('unstable dependency graph', {
+  testResolve('circular dependency on older version', {
     'myapp 0.0.0': {
       'a': '>=1.0.0'
     },
     'a 1.0.0': {},
     'a 2.0.0': {
       'b': '1.0.0'
     },
     'b 1.0.0': {
       'a': '1.0.0'
     }
+  }, result: {
+    'myapp from root': '0.0.0',
+    'a': '1.0.0'
+  }, requiresBacktracking: true);
+
+  testResolve('unsolvable', {
+    'myapp 0.0.0': {
+      'a': 'any',
+      'b': 'any'
+    },
+    'a 1.0.0': {
+      'b': '1.0.0'
+    },
+    'a 2.0.0': {
+      'b': '2.0.0'
+    },
+    'b 1.0.0': {
+      'a': '2.0.0'
+    },
+    'b 2.0.0': {
+      'a': '1.0.0'
+    }
   }, error: couldNotSolve);
 
   group('dev dependencies', () {
     testResolve("includes root package's dev dependencies", {
       'myapp 1.0.0': {
         '(dev) foo': '1.0.0',
         '(dev) bar': '1.0.0'
       },
       'foo 1.0.0': {},
       'bar 1.0.0': {}
@@ skipping 23 matching lines @@
       },
       'foo 1.0.0': {
         '(dev) bar': '1.0.0'
       },
       'bar 1.0.0': {}
     }, result: {
       'myapp from root': '1.0.0',
       'foo': '1.0.0'
     });
   });
+
+  // Only one version of baz, so foo and bar will have to downgrade until they
+  // reach it.
+  testResolve('simple backtracking', {
+    'myapp 0.0.0': {'foo': 'any'},
+    'foo 1.0.0': {'bar': '1.0.0'},
+    'foo 2.0.0': {'bar': '2.0.0'},
+    'foo 3.0.0': {'bar': '3.0.0'},
+    'bar 1.0.0': {'baz': 'any'},
+    'bar 2.0.0': {'baz': '2.0.0'},
+    'bar 3.0.0': {'baz': '3.0.0'},
+    'baz 1.0.0': {}
+  }, result: {
+    'myapp from root': '0.0.0',
+    'foo': '1.0.0',
+    'bar': '1.0.0',
+    'baz': '1.0.0'
+  }, requiresBacktracking: true);
+
+  // This sets up a hundred versions of foo and bar, 0.0.0 through 9.9.0. Each
+  // version of foo depends on a baz with the same major version. Each version
+  // of bar depends on a baz with the same minor version. There is only one
+  // version of baz, 0.0.0, so only older versions of foo and bar will
+  // satisfy it.
+  var map = {
+    'myapp 0.0.0': {
+      'foo': 'any',
+      'bar': 'any'
+    },
+    'baz 0.0.0': {}
+  };
+
+  for (var i = 0; i < 10; i++) {
+    for (var j = 0; j < 10; j++) {
+      map['foo $i.$j.0'] = {'baz': '$i.0.0'};
+      map['bar $i.$j.0'] = {'baz': '0.$j.0'};
+    }
+  }
+
+  testResolve('complex backtrack', map, result: {
+    'myapp from root': '0.0.0',
+    'foo': '0.9.0',
+    'bar': '9.0.0',
+    'baz': '0.0.0'
+  }, requiresBacktracking: true);
 }
 
 // TODO(rnystrom): More stuff to test:
 // - Depending on a non-existent package.
-// - Test that only a certain number requests are sent to the mock source so we
-//   can keep track of server traffic.
 
-testResolve(description, packages, {lockfile, result, Matcher error}) {
+testResolve(description, packages,
+            {lockfile, result, Matcher error, bool requiresBacktracking}) {
+  _testResolveWithSolver(description, packages,
+      solver: 1, lockfile: lockfile, result: result, error: error,
+      requiresBacktracking: requiresBacktracking);
+  _testResolveWithSolver(description, packages,
+      solver: 2, lockfile: lockfile, result: result, error: error,
+      requiresBacktracking: requiresBacktracking);
+}
+
+_testResolveWithSolver(description, packages, {int solver, lockfile,
+                       result, Matcher error, bool requiresBacktracking}) {
+  if (requiresBacktracking == null) requiresBacktracking = false;
+
+  if (solver == 1) {
+    description = 'greedy solver $description';
+
+    // The greedy solver should fail any graph that does require backtracking.
+    if (requiresBacktracking) {
+      result = null;
+      error = couldNotSolve;
+    }
+  } else {
+    description = 'backtracking solver $description';
+  }
+
   test(description, () {
     var cache = new SystemCache('.');
     source1 = new MockSource('mock1');
     source2 = new MockSource('mock2');
     cache.register(source1);
     cache.register(source2);
     cache.sources.setDefault(source1.name);
 
     // Build the test package graph.
     var root;
     packages.forEach((nameVersion, dependencies) {
       var parsed = parseSource(nameVersion, (isDev, nameVersion, source) {
         var parts = nameVersion.split(' ');
         var name = parts[0];
         var version = parts[1];
 
-        var package = source1.mockPackage(name, version, dependencies);
+        var package = mockPackage(name, version, dependencies);
         if (name == 'myapp') {
           // Don't add the root package to the server, so we can verify that Pub
           // doesn't try to look up information about the local package on the
           // remote server.
           root = package;
         } else {
-          source.addPackage(package);
+          source.addPackage(name, package);
         }
       });
     });
 
     // Clean up the expectation.
     if (result != null) {
       var newResult = {};
       result.forEach((name, version) {
         parseSource(name, (isDev, name, source) {
           version = new Version.parse(version);
           newResult[name] = new PackageId(name, source, version, name);
         });
       });
       result = newResult;
     }
 
     var realLockFile = new LockFile.empty();
     if (lockfile != null) {
       lockfile.forEach((name, version) {
         version = new Version.parse(version);
         realLockFile.packages[name] =
           new PackageId(name, source1, version, name);
       });
     }
 
     // Resolve the versions.
-    var future = resolveVersions(cache.sources, root, realLockFile);
+    var future = resolveVersions(cache.sources, root, solver: solver,
+        lockFile: realLockFile);
 
     if (result != null) {
       expect(future, completion(predicate((actualResult) {
         for (var actualId in actualResult) {
           if (!result.containsKey(actualId.name)) return false;
           var expectedId = result.remove(actualId.name);
           if (actualId != expectedId) return false;
         }
         return result.isEmpty;
       }, 'packages to match $result')));
     } else if (error != null) {
       expect(future, throwsA(error));
     }
   });
 }
 
 /// A source used for testing. This both creates mock package objects and acts
 /// as a source for them.
 ///
 /// In order to support testing packages that have the same name but different
 /// descriptions, a package's name is calculated by taking the description
 /// string and stripping off any trailing hyphen followed by non-hyphen
 /// characters.
 class MockSource extends Source {
-  final Map<String, Map<Version, Package>> _packages;
+  final _packages = <String, Map<Version, Package>>{};
+
+  /// Keeps track of which package version lists have been requested. Ensures
+  /// that a source is only hit once for a given package and that pub
+  /// internally caches the results.
+  final _requestedVersions = new Set<String>();
+
+  /// Keeps track of which package pubspecs have been requested. Ensures that a
+  /// source is only hit once for a given package and that pub internally
+  /// caches the results.
+  final _requestedPubspecs = new Map<String, Set<Version>>();
 
   final String name;
   bool get shouldCache => true;
 
-  MockSource(this.name)
-      : _packages = <String, Map<Version, Package>>{};
+  MockSource(this.name);
 
   Future<List<Version>> getVersions(String name, String description) {
-    return defer(() => _packages[description].keys.toList());
+    return defer(() {
+      // Make sure the solver doesn't request the same thing twice.
+      if (_requestedVersions.contains(description)) {
+        throw 'Version list for $description was already requested.';
+      }
+
+      _requestedVersions.add(description);
+
+      if (!_packages.containsKey(description)){
+        throw 'MockSource does not have a package matching "$description".';
+      }
+      return _packages[description].keys.toList();
+    });
   }
 
   Future<Pubspec> describe(PackageId id) {
     return defer(() {
-      return _packages[id.name][id.version].pubspec;
+      // Make sure the solver doesn't request the same thing twice.
+      if (_requestedPubspecs.containsKey(id.description) &&
+          _requestedPubspecs[id.description].contains(id.version)) {
+        throw 'Pubspec for $id was already requested.';
+      }
+
+      _requestedPubspecs.putIfAbsent(id.description, () => new Set<Version>());
+      _requestedPubspecs[id.description].add(id.version);
+
+      return _packages[id.description][id.version].pubspec;
     });
   }
 
   Future<bool> install(PackageId id, String path) {
     throw 'no';
   }
 
-  Package mockPackage(String description, String version,
-      Map dependencyStrings) {
-    // Build the pubspec dependencies.
-    var dependencies = <PackageRef>[];
-    var devDependencies = <PackageRef>[];
-
-    dependencyStrings.forEach((name, constraint) {
-      parseSource(name, (isDev, name, source) {
-        var packageName = name.replaceFirst(new RegExp(r"-[^-]+$"), "");
-        var ref = new PackageRef(packageName, source,
-            new VersionConstraint.parse(constraint), name);
-
-        if (isDev) {
-          devDependencies.add(ref);
-        } else {
-          dependencies.add(ref);
-        }
-      });
-    });
-
-    var pubspec = new Pubspec(
-        description, new Version.parse(version), dependencies, devDependencies,
-        new PubspecEnvironment());
-    return new Package.inMemory(pubspec);
-  }
-
-  void addPackage(Package package) {
-    _packages.putIfAbsent(package.name, () => new Map<Version, Package>());
-    _packages[package.name][package.version] = package;
+  void addPackage(String description, Package package) {
+    _packages.putIfAbsent(description, () => new Map<Version, Package>());
+    _packages[description][package.version] = package;
   }
 }
 
+Package mockPackage(String description, String version,
+                    Map dependencyStrings) {
+  // Build the pubspec dependencies.
+  var dependencies = <PackageRef>[];
+  var devDependencies = <PackageRef>[];
+
+  dependencyStrings.forEach((name, constraint) {
+    parseSource(name, (isDev, name, source) {
+      var packageName = name.replaceFirst(new RegExp(r"-[^-]+$"), "");
+      var ref = new PackageRef(packageName, source,
+          new VersionConstraint.parse(constraint), name);
+
+      if (isDev) {
+        devDependencies.add(ref);
+      } else {
+        dependencies.add(ref);
+      }
+    });
+  });
+
+  var name = description.replaceFirst(new RegExp(r"-[^-]+$"), "");
+  var pubspec = new Pubspec(
+      name, new Version.parse(version), dependencies, devDependencies,
+      new PubspecEnvironment());
+  return new Package.inMemory(pubspec);
+}
+
 void parseSource(String description,
     callback(bool isDev, String name, Source source)) {
   var isDev = false;
 
   if (description.startsWith("(dev) ")) {
     description = description.substring("(dev) ".length);
     isDev = true;
   }
 
   var name = description;
   var source = source1;
 
   var sourceNames = {
     'mock1': source1,
     'mock2': source2,
     'root': null
   };
 
   var match = new RegExp(r"(.*) from (.*)").firstMatch(description);
   if (match != null) {
     name = match[1];
     source = sourceNames[match[2]];
   }
 
   callback(isDev, name, source);
 }