Add option to delay file system writes

_test_common/lib/in_memory_writer.dart

@@ -38,4 +38,7 @@ class InMemoryRunnerAssetWriter extends InMemoryAssetWriter
     FakeWatcher.notifyWatchers(
         WatchEvent(ChangeType.REMOVE, p.absolute(id.package, id.path)));
   }
+
+  @override
+  void onBuildComplete() {}
 }

_test_common/lib/runner_asset_writer_spy.dart

@@ -20,4 +20,7 @@ class RunnerAssetWriterSpy extends AssetWriterSpy implements RunnerAssetWriter {
     _assetsDeleted.add(id);
     return _delegate.delete(id);
   }
+
+  @override
+  FutureOr<void> onBuildComplete() => _delegate.onBuildComplete();
 }

build_runner/CHANGELOG.md

@@ -1,4 +1,8 @@
-## 2.3.3-dev
+## 2.4.0-dev
+
+- Add `--delay-writes` option to delay all writes to the end of a build. As no
+  intermediate build state is visible on the file system, this may improve
+  performance of third-party tools like the analysis server.
 
 ## 2.3.2
 

build_runner/lib/src/daemon/daemon_builder.dart

@@ -220,6 +220,7 @@ class BuildRunnerDaemonBuilder implements DaemonBuilder {
       logSubscription,
       packageGraph: packageGraph,
       deleteFilesByDefault: daemonOptions.deleteFilesByDefault,
+      delayWrites: daemonOptions.delayAssetWrites,
       overrideBuildConfig: overrideBuildConfig,
       skipBuildScriptCheck: daemonOptions.skipBuildScriptCheck,
       enableLowResourcesMode: daemonOptions.enableLowResourcesMode,

build_runner/lib/src/entrypoint/base_command.dart

@@ -97,7 +97,12 @@ abstract class BuildRunnerCommand extends Command<int> {
               'If multiple filters are applied then outputs matching any filter '
               'will be built (they do not need to match all filters).')
       ..addMultiOption(enableExperimentOption,
-          help: 'A list of dart language experiments to enable.');
+          help: 'A list of dart language experiments to enable.')
+      ..addFlag(
+        delayWritesOption,
+        help: 'Delays all file system interactions until the end of a build, '
+            'potentially reducing load on tools like the analysis server.',
+      );
   }
 
   /// Must be called inside [run] so that [argResults] is non-null.

build_runner/lib/src/entrypoint/build.dart

@@ -47,6 +47,7 @@ class BuildCommand extends BuildRunnerCommand {
       trackPerformance: options.trackPerformance,
       skipBuildScriptCheck: options.skipBuildScriptCheck,
       logPerformanceDir: options.logPerformanceDir,
+      delayAssetWrites: options.delayAssetWrites,
     );
     if (result.status == BuildStatus.success) {
       return ExitCode.success.code;

build_runner/lib/src/entrypoint/options.dart

@@ -33,6 +33,7 @@ const skipBuildScriptCheckOption = 'skip-build-script-check';
 const symlinkOption = 'symlink';
 const usePollingWatcherOption = 'use-polling-watcher';
 const verboseOption = 'verbose';
+const delayWritesOption = 'delay-writes';
 
 enum BuildUpdatesOption { none, liveReload }
 
@@ -67,6 +68,9 @@ class SharedOptions {
   /// Enables performance tracking and the `/$perf` page.
   final bool trackPerformance;
 
+  /// Delay asset writes until the end of a build.
+  final bool delayAssetWrites;
+
   /// A directory to log performance information to.
   String? logPerformanceDir;
 
@@ -100,6 +104,7 @@ class SharedOptions {
     required this.isReleaseBuild,
     required this.logPerformanceDir,
     required this.enableExperiments,
+    required this.delayAssetWrites,
   });
 
   SharedOptions.fromParsedArgs(ArgResults argResults,
@@ -122,6 +127,7 @@ class SharedOptions {
           isReleaseBuild: argResults[releaseOption] as bool,
           logPerformanceDir: argResults[logPerformanceOption] as String?,
           enableExperiments: argResults[enableExperimentOption] as List<String>,
+          delayAssetWrites: argResults[delayWritesOption] as bool,
         );
 }
 
@@ -147,6 +153,7 @@ class DaemonOptions extends WatchOptions {
     required String? logPerformanceDir,
     required bool usePollingWatcher,
     required List<String> enableExperiments,
+    required super.delayAssetWrites,
   }) : super._(
           buildFilters: buildFilters,
           deleteFilesByDefault: deleteFilesByDefault,
@@ -202,6 +209,7 @@ class DaemonOptions extends WatchOptions {
       logPerformanceDir: argResults[logPerformanceOption] as String?,
       usePollingWatcher: argResults[usePollingWatcherOption] as bool,
       enableExperiments: argResults[enableExperimentOption] as List<String>,
+      delayAssetWrites: argResults[delayWritesOption] as bool,
     );
   }
 }
@@ -229,6 +237,7 @@ class WatchOptions extends SharedOptions {
     required bool isReleaseBuild,
     required String? logPerformanceDir,
     required List<String> enableExperiments,
+    required super.delayAssetWrites,
   }) : super._(
           buildFilters: buildFilters,
           deleteFilesByDefault: deleteFilesByDefault,
@@ -266,6 +275,7 @@ class WatchOptions extends SharedOptions {
           logPerformanceDir: argResults[logPerformanceOption] as String?,
           usePollingWatcher: argResults[usePollingWatcherOption] as bool,
           enableExperiments: argResults[enableExperimentOption] as List<String>,
+          delayAssetWrites: argResults[delayWritesOption] as bool,
         );
 }
 
@@ -295,6 +305,7 @@ class ServeOptions extends WatchOptions {
     required String? logPerformanceDir,
     required bool usePollingWatcher,
     required List<String> enableExperiments,
+    required super.delayAssetWrites,
   }) : super._(
           buildFilters: buildFilters,
           deleteFilesByDefault: deleteFilesByDefault,
@@ -381,6 +392,7 @@ class ServeOptions extends WatchOptions {
       logPerformanceDir: argResults[logPerformanceOption] as String?,
       usePollingWatcher: argResults[usePollingWatcherOption] as bool,
       enableExperiments: argResults[enableExperimentOption] as List<String>,
+      delayAssetWrites: argResults[delayWritesOption] as bool,
     );
   }
 }

build_runner/lib/src/generate/build.dart

@@ -12,9 +12,9 @@ import 'package:logging/logging.dart';
 import 'package:shelf/shelf.dart';
 import 'package:watcher/watcher.dart';
 
-import '../logging/std_io_logging.dart';
 import '../package_graph/build_config_overrides.dart';
 import '../server/server.dart';
+import 'environment.dart';
 import 'watch_impl.dart' as watch_impl;
 
 /// Runs all of the BuilderApplications in [builders] once.
@@ -68,7 +68,8 @@ Future<BuildResult> build(List<BuilderApplication> builders,
     bool? isReleaseBuild,
     Map<String, Map<String, dynamic>>? builderConfigOverrides,
     String? logPerformanceDir,
-    Set<BuildFilter>? buildFilters}) async {
+    Set<BuildFilter>? buildFilters,
+    bool? delayAssetWrites}) async {
   builderConfigOverrides ??= const {};
   buildDirs ??= <BuildDirectory>{};
   buildFilters ??= <BuildFilter>{};
@@ -80,20 +81,24 @@ Future<BuildResult> build(List<BuilderApplication> builders,
   skipBuildScriptCheck ??= false;
   trackPerformance ??= false;
   verbose ??= false;
-  var environment = OverrideableEnvironment(
-      IOEnvironment(
-        packageGraph,
-        assumeTty: assumeTty,
-        outputSymlinksOnly: outputSymlinksOnly,
-      ),
-      reader: reader,
-      writer: writer,
-      onLog: onLog ?? stdIOLogListener(assumeTty: assumeTty, verbose: verbose));
+
+  final environment = createEnvironment(
+    packageGraph: packageGraph,
+    assumeTty: assumeTty,
+    outputSymlinksOnly: outputSymlinksOnly,
+    reader: reader,
+    writer: writer,
+    delayAssetWrites: delayAssetWrites,
+    onLog: onLog,
+    verbose: verbose,
+  );
+
   var logSubscription =
       LogSubscription(environment, verbose: verbose, logLevel: logLevel);
   var options = await BuildOptions.create(
     logSubscription,
     deleteFilesByDefault: deleteFilesByDefault,
+    delayWrites: delayAssetWrites == true,
     packageGraph: packageGraph,
     skipBuildScriptCheck: skipBuildScriptCheck,
     overrideBuildConfig: await findBuildConfigOverrides(
@@ -165,7 +170,8 @@ Future<ServeHandler> watch(List<BuilderApplication> builders,
         bool? isReleaseBuild,
         Map<String, Map<String, dynamic>>? builderConfigOverrides,
         String? logPerformanceDir,
-        Set<BuildFilter>? buildFilters}) =>
+        Set<BuildFilter>? buildFilters,
+        bool? delayAssetWrites}) =>
     watch_impl.watch(
       builders,
       assumeTty: assumeTty,

build_runner/lib/src/generate/environment.dart

@@ -0,0 +1,36 @@
+import 'package:build_runner_core/build_runner_core.dart';
+import 'package:logging/logging.dart';
+
+import '../logging/std_io_logging.dart';
+
+BuildEnvironment createEnvironment({
+  required PackageGraph packageGraph,
+  required bool? assumeTty,
+  required bool outputSymlinksOnly,
+  required RunnerAssetReader? reader,
+  required RunnerAssetWriter? writer,
+  required bool? delayAssetWrites,
+  required void Function(LogRecord)? onLog,
+  required bool verbose,
+}) {
+  BuildEnvironment environment = IOEnvironment(
+    packageGraph,
+    assumeTty: assumeTty,
+    outputSymlinksOnly: outputSymlinksOnly,
+  );
+  reader ??= environment.reader;
+  writer ??= environment.writer;
+
+  if (delayAssetWrites == true) {
+    final delayed = writer = DelayedAssetWriter(writer);
+    reader = delayed.reader(reader, packageGraph.root.name);
+  }
+
+  environment = environment.change(
+    reader: reader,
+    writer: writer,
+    onLog: onLog ?? stdIOLogListener(assumeTty: assumeTty, verbose: verbose),
+  );
+
+  return environment;
+}

build_runner/lib/src/generate/watch_impl.dart

@@ -24,8 +24,8 @@ import 'package:logging/logging.dart';
 import 'package:stream_transform/stream_transform.dart';
 import 'package:watcher/watcher.dart';
 
-import '../logging/std_io_logging.dart';
 import '../server/server.dart';
+import 'environment.dart';
 import 'terminator.dart';
 
 final _logger = Logger('Watch');
@@ -55,6 +55,7 @@ Future<ServeHandler> watch(
   bool? isReleaseBuild,
   String? logPerformanceDir,
   Set<BuildFilter>? buildFilters,
+  bool? delayAssetWrites,
 }) async {
   builderConfigOverrides ??= const {};
   buildDirs ??= <BuildDirectory>{};
@@ -68,12 +69,16 @@ Future<ServeHandler> watch(
   trackPerformance ??= false;
   verbose ??= false;
 
-  var environment = OverrideableEnvironment(
-      IOEnvironment(packageGraph,
-          assumeTty: assumeTty, outputSymlinksOnly: outputSymlinksOnly),
-      reader: reader,
-      writer: writer,
-      onLog: onLog ?? stdIOLogListener(assumeTty: assumeTty, verbose: verbose));
+  final environment = createEnvironment(
+    packageGraph: packageGraph,
+    assumeTty: assumeTty,
+    outputSymlinksOnly: outputSymlinksOnly,
+    reader: reader,
+    writer: writer,
+    delayAssetWrites: delayAssetWrites,
+    onLog: onLog,
+    verbose: verbose,
+  );
   var logSubscription =
       LogSubscription(environment, verbose: verbose, logLevel: logLevel);
   overrideBuildConfig ??= await findBuildConfigOverrides(
@@ -82,6 +87,7 @@ Future<ServeHandler> watch(
   var options = await BuildOptions.create(
     logSubscription,
     deleteFilesByDefault: deleteFilesByDefault,
+    delayWrites: delayAssetWrites == true,
     packageGraph: packageGraph,
     overrideBuildConfig: overrideBuildConfig,
     debounceDelay: debounceDelay,

build_runner/lib/src/watcher/delete_writer.dart

@@ -29,4 +29,7 @@ class OnDeleteWriter implements RunnerAssetWriter {
   Future writeAsString(AssetId id, String contents,
           {Encoding encoding = utf8}) =>
       _writer.writeAsString(id, contents, encoding: encoding);
+
+  @override
+  FutureOr<void> onBuildComplete() => _writer.onBuildComplete();
 }

build_runner/pubspec.yaml

@@ -1,10 +1,10 @@
 name: build_runner
-version: 2.3.3-dev
+version: 2.4.0-dev
 description: A build system for Dart code generation and modular compilation.
 repository: https://github.com/dart-lang/build/tree/master/build_runner
 
 environment:
-  sdk: ">=2.14.0 <3.0.0"
+  sdk: ">=2.17.0 <3.0.0"
 
 platforms:
   linux:
@@ -19,7 +19,7 @@ dependencies:
   build_config: ">=1.1.0 <1.2.0"
   build_daemon: ^3.1.0
   build_resolvers: ^2.0.0
-  build_runner_core: ^7.2.0
+  build_runner_core: ^8.0.0-dev
   code_builder: ^4.2.0
   collection: ^1.15.0
   crypto: ^3.0.0
@@ -57,3 +57,7 @@ dev_dependencies:
   test_process: ^2.0.0
   _test_common:
     path: ../_test_common
+
+dependency_overrides:
+  build_runner_core:
+    path: ../build_runner_core/

build_runner/test/entrypoint/run_test.dart

@@ -105,6 +105,16 @@ main() {
       });
     }
 
+    void testBasicBuildCommandWithDelayedWrites(String command) {
+      test('is supported by the $command command', () async {
+        var args = ['build_runner', command, 'web', '--delayed-writes'];
+        expect(await runSingleBuild(command, args), ExitCode.success.code);
+        expectOutput('web/main.dart.js', exists: true);
+        expectOutput('test/hello_test.dart.browser_test.dart.js',
+            exists: false);
+      });
+    }
+
     void testBuildCommandWithOutput(String command) {
       test('works with -o and the $command command', () async {
         var outputDirName = 'foo';
@@ -126,6 +136,7 @@ main() {
 
     for (var command in ['build', 'serve', 'watch']) {
       testBasicBuildCommand(command);
+      testBasicBuildCommandWithDelayedWrites(command);
       testBuildCommandWithOutput(command);
     }
 

build_runner/test/server/serve_integration_test.dart

@@ -57,7 +57,7 @@ void main() {
       logLevel: Level.ALL,
       onLog: (record) => printOnFailure('[${record.level}] '
           '${record.loggerName}: ${record.message}'),
-      directoryWatcherFactory: (path) => FakeWatcher(path),
+      directoryWatcherFactory: FakeWatcher.new,
       terminateEventStream: terminateController.stream,
       skipBuildScriptCheck: true,
     );

build_runner_core/CHANGELOG.md

@@ -1,5 +1,8 @@
-## 7.2.8-dev
+## 8.0.0-dev
 
+- __Breaking__: Add `onBuildComplete` to `RunnerAssetWriter`.
+- Add `DelayedAssetWriter`, a writer implementation delaying all writes until
+ `onBuildComplete` is called.
 - Raise the minimum SDK constraint to 2.18.
 
 ## 7.2.7

build_runner_core/lib/src/asset/build_cache.dart

@@ -84,6 +84,9 @@ class BuildCacheWriter implements RunnerAssetWriter {
   @override
   Future delete(AssetId id) =>
       _delegate.delete(_cacheLocation(id, _assetGraph, _rootPackage));
+
+  @override
+  FutureOr<void> onBuildComplete() => _delegate.onBuildComplete();
 }
 
 AssetId _cacheLocation(AssetId id, AssetGraph assetGraph, String rootPackage) {

build_runner_core/lib/src/asset/file_based.dart

@@ -107,6 +107,11 @@ class FileBasedAssetWriter implements RunnerAssetWriter {
       }
     });
   }
+
+  @override
+  void onBuildComplete() {
+    // Nothing to do here
+  }
 }
 
 /// Returns the path to [id] for a given [packageGraph].