Implement GC for FileState(s).

Also fix for invoking watch().

R=ahe@google.com, paulberry@google.com, sigmund@google.com
BUG=

Review-Url: https://codereview.chromium.org/2929363002 .

pkg/front_end/lib/src/incremental/file_state.dart

@@ -54,6 +54,10 @@ class FileState {
   Set<FileState> _directReferencedFiles = new Set<FileState>();
   List<FileState> _directReferencedLibraries = <FileState>[];
 
+  /// This flag is set to `true` during the mark phase of garbage collection
+  /// and set back to `false` for survived instances.
+  bool _gcMarked = false;
+
   FileState._(this._fsState, this.uri, this.fileUri);
 
   /// The MD5 signature of the file API as a byte array.
@@ -309,6 +313,35 @@ class FileSystemState {
   /// The `file:` URI of all files currently tracked by this instance.
   Iterable<Uri> get fileUris => _fileUriToFile.keys;
 
+  /// Perform mark and sweep garbage collection of [FileState]s.
+  void gc(Uri entryPoint) {
+    void mark(FileState file) {
+      if (!file._gcMarked) {
+        file._gcMarked = true;
+        file._directReferencedFiles.forEach(mark);
+      }
+    }
+
+    var file = _uriToFile[entryPoint];
+    if (file == null) return;
+
+    mark(file);
+
+    var urisToRemove = new Set<Uri>();
+    var fileUrisToRemove = new Set<Uri>();
+    for (var file in _uriToFile.values) {
+      if (file._gcMarked) {
+        file._gcMarked = false;
+      } else {
+        urisToRemove.add(file.uri);
+        fileUrisToRemove.add(file.fileUri);
+      }
+    }
+
+    urisToRemove.forEach(_uriToFile.remove);
+    fileUrisToRemove.forEach(_fileUriToFile.remove);
+  }
+
   /// Return the [FileState] for the given [absoluteUri], or `null` if the
   /// [absoluteUri] cannot be resolved into a file URI.
   ///

pkg/front_end/lib/src/incremental_kernel_generator_impl.dart

@@ -79,8 +79,16 @@ class IncrementalKernelGeneratorImpl implements IncrementalKernelGenerator {
       : _logger = _options.logger,
         _byteStore = _options.byteStore {
     _computeSalt();
+
+    Future<Null> onFileAdded(Uri uri) {
+      if (watch != null) {
+        return watch(uri, true);
+      }
+      return new Future.value();
+    }
+
     _fsState = new FileSystemState(
-        _options.fileSystem, _uriTranslator, _salt, (uri) => watch(uri, true));
+        _options.fileSystem, _uriTranslator, _salt, onFileAdded);
   }
 
   @override
@@ -304,6 +312,9 @@ class IncrementalKernelGeneratorImpl implements IncrementalKernelGenerator {
           await file.refresh();
         }
       }
+
+      // The file graph might have changed, perform GC.
+      _fsState.gc(_entryPoint);
     });
   }
 

pkg/front_end/test/src/incremental/file_state_test.dart

@@ -141,6 +141,51 @@ baz() => 44;
 ''');
   }
 
+  test_gc() async {
+    var a = writeFile('/a.dart', '');
+    var b = writeFile('/b.dart', '');
+    var c = writeFile('/c.dart', 'import "a.dart";');
+    var d = writeFile('/d.dart', 'import "b.dart";');
+    var e = writeFile(
+        '/e.dart',
+        r'''
+import "c.dart";
+import "d.dart";
+''');
+
+    var eFile = await fsState.getFile(e);
+
+    // The root and four files.
+    expect(fsState.fileUris, contains(e));
+    expect(fsState.fileUris, contains(a));
+    expect(fsState.fileUris, contains(b));
+    expect(fsState.fileUris, contains(c));
+    expect(fsState.fileUris, contains(d));
+
+    // No changes after GC.
+    fsState.gc(e);
+    expect(fsState.fileUris, contains(e));
+    expect(fsState.fileUris, contains(a));
+    expect(fsState.fileUris, contains(b));
+    expect(fsState.fileUris, contains(c));
+    expect(fsState.fileUris, contains(d));
+
+    // Update e.dart so that it does not reference c.dart anymore.
+    // Then GC removes both c.dart and a.dart it references.
+    writeFile(
+        '/e.dart',
+        r'''
+import "d.dart";
+''');
+    await eFile.refresh();
+    fsState.gc(e);
+    expect(fsState.fileUris, contains(e));
+    expect(fsState.fileUris, isNot(contains(a)));
+    expect(fsState.fileUris, contains(b));
+    expect(fsState.fileUris, isNot(contains(c)));
+    expect(fsState.fileUris, contains(d));
+  }
+
   test_getFile() async {
     var a = writeFile('/a.dart', '');
     var b = writeFile('/b.dart', '');