Add option to delay file system writes

build_runner/lib/src/daemon/daemon_builder.dart

@@ -12,6 +12,7 @@ import 'package:build_daemon/data/build_status.dart';
 import 'package:build_daemon/data/build_target.dart' hide OutputLocation;
 import 'package:build_daemon/data/server_log.dart';
 import 'package:build_runner/src/entrypoint/options.dart';
+import 'package:build_runner/src/generate/environment.dart';
 import 'package:build_runner/src/package_graph/build_config_overrides.dart';
 import 'package:build_runner/src/watcher/asset_change.dart';
 import 'package:build_runner/src/watcher/change_filter.dart';
@@ -199,13 +200,17 @@ class BuildRunnerDaemonBuilder implements DaemonBuilder {
     var expectedDeletes = <AssetId>{};
     var outputStreamController = StreamController<ServerLog>();
 
-    var environment = OverrideableEnvironment(
-        IOEnvironment(packageGraph,
-            outputSymlinksOnly: daemonOptions.outputSymlinksOnly),
-        onLog: (record) {
-      outputStreamController.add(ServerLog.fromLogRecord(record));
-    });
-
+    var environment = createDefaultEnvironment(
+      packageGraph: packageGraph,
+      assumeTty: null,
+      outputSymlinksOnly: daemonOptions.outputSymlinksOnly,
+      reader: null,
+      writer: null,
+      delayAssetWrites: daemonOptions.delayAssetWrites,
+      onLog: (record) =>
+          outputStreamController.add(ServerLog.fromLogRecord(record)),
+      verbose: true,
+    );
     var daemonEnvironment = OverrideableEnvironment(environment,
         writer: OnDeleteWriter(environment.writer, expectedDeletes.add));
 
@@ -220,7 +225,6 @@ 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

@@ -102,6 +102,7 @@ abstract class BuildRunnerCommand extends Command<int> {
         delayWritesOption,
         help: 'Delays all file system interactions until the end of a build, '
             'potentially reducing load on tools like the analysis server.',
+        defaultsTo: null,
       );
   }
 

build_runner/lib/src/entrypoint/options.dart

@@ -104,8 +104,11 @@ class SharedOptions {
     required this.isReleaseBuild,
     required this.logPerformanceDir,
     required this.enableExperiments,
-    required this.delayAssetWrites,
-  });
+    bool? delayAssetWrites,
+  }) :
+        // Delayed asset writes should be enabled by default if we're not in the
+        // low-resources mode.
+        delayAssetWrites = delayAssetWrites ?? !enableLowResourcesMode;
 
   SharedOptions.fromParsedArgs(ArgResults argResults,
       Iterable<String> positionalArgs, String rootPackage, Command command)
@@ -127,7 +130,7 @@ class SharedOptions {
           isReleaseBuild: argResults[releaseOption] as bool,
           logPerformanceDir: argResults[logPerformanceOption] as String?,
           enableExperiments: argResults[enableExperimentOption] as List<String>,
-          delayAssetWrites: argResults[delayWritesOption] as bool,
+          delayAssetWrites: argResults[delayWritesOption] as bool?,
         );
 }
 
@@ -209,7 +212,7 @@ 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,
+      delayAssetWrites: argResults[delayWritesOption] as bool?,
     );
   }
 }
@@ -275,7 +278,7 @@ 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,
+          delayAssetWrites: argResults[delayWritesOption] as bool?,
         );
 }
 
@@ -392,7 +395,7 @@ 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,
+      delayAssetWrites: argResults[delayWritesOption] as bool?,
     );
   }
 }

build_runner/lib/src/generate/build.dart

@@ -82,7 +82,7 @@ Future<BuildResult> build(List<BuilderApplication> builders,
   trackPerformance ??= false;
   verbose ??= false;
 
-  final environment = createEnvironment(
+  final environment = createDefaultEnvironment(
     packageGraph: packageGraph,
     assumeTty: assumeTty,
     outputSymlinksOnly: outputSymlinksOnly,
@@ -98,7 +98,6 @@ Future<BuildResult> build(List<BuilderApplication> builders,
   var options = await BuildOptions.create(
     logSubscription,
     deleteFilesByDefault: deleteFilesByDefault,
-    delayWrites: delayAssetWrites == true,
     packageGraph: packageGraph,
     skipBuildScriptCheck: skipBuildScriptCheck,
     overrideBuildConfig: await findBuildConfigOverrides(

build_runner/lib/src/generate/environment.dart

@@ -3,7 +3,7 @@ import 'package:logging/logging.dart';
 
 import '../logging/std_io_logging.dart';
 
-BuildEnvironment createEnvironment({
+BuildEnvironment createDefaultEnvironment({
   required PackageGraph packageGraph,
   required bool? assumeTty,
   required bool outputSymlinksOnly,

build_runner/lib/src/generate/watch_impl.dart

@@ -69,7 +69,7 @@ Future<ServeHandler> watch(
   trackPerformance ??= false;
   verbose ??= false;
 
-  final environment = createEnvironment(
+  final environment = createDefaultEnvironment(
     packageGraph: packageGraph,
     assumeTty: assumeTty,
     outputSymlinksOnly: outputSymlinksOnly,
@@ -87,7 +87,6 @@ Future<ServeHandler> watch(
   var options = await BuildOptions.create(
     logSubscription,
     deleteFilesByDefault: deleteFilesByDefault,
-    delayWrites: delayAssetWrites == true,
     packageGraph: packageGraph,
     overrideBuildConfig: overrideBuildConfig,
     debounceDelay: debounceDelay,

build_runner/test/entrypoint/run_test.dart

@@ -105,16 +105,6 @@ 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';
@@ -136,7 +126,6 @@ main() {
 
     for (var command in ['build', 'serve', 'watch']) {
       testBasicBuildCommand(command);
-      testBasicBuildCommandWithDelayedWrites(command);
       testBuildCommandWithOutput(command);
     }
 

build_runner_core/lib/src/asset/writer.dart

@@ -7,6 +7,7 @@ import 'dart:convert';
 
 import 'package:build/build.dart';
 import 'package:glob/glob.dart';
+import 'package:meta/meta.dart';
 
 import 'reader.dart';
 
@@ -19,7 +20,8 @@ abstract class RunnerAssetWriter implements AssetWriter {
   /// Potentially runs pending work that needs to happen just before the build
   /// completes, after all other interactions with this [AssetWriter] have
   /// already been completed.
-  FutureOr<void> onBuildComplete() {}
+  @mustCallSuper
+  FutureOr<void> onBuildComplete();
 }
 
 /// A [RunnerAssetWriter] implementation that performs all writes at once in
@@ -30,8 +32,12 @@ abstract class RunnerAssetWriter implements AssetWriter {
 class DelayedAssetWriter implements RunnerAssetWriter {
   final RunnerAssetWriter _delegate;
 
+  /// A map of asset ids to content that has been written with the [AssetWriter]
+  /// API but not yet been flushed to the underlying delegate (in [onBuildComplete]).
+  ///
+  /// The values are non-null if the contents of the file have been overwritten,
+  /// or null if the asset has been deleted.
   final Map<AssetId, List<int>?> overlay = {};
-  final List<Future Function()> _work = [];
 
   DelayedAssetWriter(this._delegate);
 
@@ -44,22 +50,26 @@ class DelayedAssetWriter implements RunnerAssetWriter {
   @override
   Future delete(AssetId id) async {
     overlay[id] = null;
-    _work.add(() => _delegate.delete(id));
   }
 
   @override
   Future<void> onBuildComplete() async {
-    final todos = _work.toList();
-    _work.clear();
+    await Future.wait(overlay.entries.map((entry) {
+      final value = entry.value;
+      if (value == null) {
+        return _delegate.delete(entry.key);
+      } else {
+        return _delegate.writeAsBytes(entry.key, value);
+      }
+    }));
 
-    await Future.wait([for (final unit in todos) unit()]);
     await _delegate.onBuildComplete();
+    overlay.clear();
   }
 
   @override
   Future<void> writeAsBytes(AssetId id, List<int> bytes) async {
     overlay[id] = bytes;
-    _work.add(() => _delegate.writeAsBytes(id, bytes));
   }
 
   @override

build_runner_core/lib/src/generate/build_definition.dart

@@ -543,15 +543,8 @@ class _Loader {
     var done = false;
     while (!done) {
       try {
-        int choice;
-
-        if (_options.delayWrites) {
-          choice = await _environment.prompt('Overwrite these files?',
-              ['Overwrite', 'Cancel build', 'List conflicts']);
-        } else {
-          choice = await _environment.prompt('Delete these files?',
-              ['Delete', 'Cancel build', 'List conflicts']);
-        }
+        var choice = await _environment.prompt('Delete these files?',
+            ['Delete', 'Cancel build', 'List conflicts']);
 
         switch (choice) {
           case 0:

build_runner_core/lib/src/generate/build_impl.dart

@@ -250,7 +250,13 @@ class _SingleBuild {
     try {
       await _environment.writer.onBuildComplete();
     } catch (e, s) {
-      _logger.severe('Assets could not be written', e, s);
+      _logger.severe(
+        'Could not complete writes for this build.\n'
+        'The current state of the file system may not reflect the expected '
+        'build outputs.',
+        e,
+        s,
+      );
       result = BuildResult(BuildStatus.failure, result.outputs,
           performance: result.performance);
     }

build_runner_core/lib/src/generate/options.dart

@@ -134,7 +134,6 @@ class BuildFilter {
 /// Manages setting up consistent defaults for all options and build modes.
 class BuildOptions {
   final bool deleteFilesByDefault;
-  final bool delayWrites;
   final bool enableLowResourcesMode;
   final StreamSubscription logListener;
 
@@ -155,7 +154,6 @@ class BuildOptions {
   BuildOptions._({
     required this.debounceDelay,
     required this.deleteFilesByDefault,
-    required this.delayWrites,
     required this.enableLowResourcesMode,
     required this.logListener,
     required this.packageGraph,
@@ -174,7 +172,6 @@ class BuildOptions {
     LogSubscription logSubscription, {
     Duration debounceDelay = const Duration(milliseconds: 250),
     bool deleteFilesByDefault = false,
-    bool delayWrites = false,
     bool enableLowResourcesMode = false,
     required PackageGraph packageGraph,
     Map<String, BuildConfig> overrideBuildConfig = const {},
@@ -216,7 +213,6 @@ feature, you may need to run `dart run build_runner clean` and then rebuild.
     return BuildOptions._(
       debounceDelay: debounceDelay,
       deleteFilesByDefault: deleteFilesByDefault,
-      delayWrites: delayWrites,
       enableLowResourcesMode: enableLowResourcesMode,
       logListener: logSubscription.logListener,
       packageGraph: packageGraph,

build_runner_core/test/asset/writer_test.dart

@@ -4,7 +4,9 @@ import 'package:_test_common/common.dart';
 import 'package:build/build.dart';
 import 'package:build_runner_core/build_runner_core.dart';
 import 'package:glob/glob.dart';
+import 'package:path/path.dart' as p;
 import 'package:test/test.dart';
+import 'package:watcher/watcher.dart';
 
 void main() {
   late InMemoryRunnerAssetReader underlyingReader;
@@ -36,7 +38,7 @@ void main() {
       expect(underlyingWriter.assets, {a: decodedMatches('a')});
     });
 
-    test('does not write twice', () async {
+    test('does not write data that has already been written', () async {
       final writer = DelayedAssetWriter(underlyingWriter);
       final asset = makeAssetId('a|lib/a.dart');
 
@@ -49,6 +51,25 @@ void main() {
       expect(underlyingWriter.assets, isEmpty);
     });
 
+    test('avoids duplicate writes on the same asset', () async {
+      final writer = DelayedAssetWriter(underlyingWriter);
+      final asset = makeAssetId('a|lib/a.dart');
+
+      expect(
+          FakeWatcher(p.absolute('a')).events,
+          emitsInOrder([
+            isA<WatchEvent>().having(
+                (e) => e.path, 'path', p.absolute('a', 'lib', 'a.dart')),
+            isA<WatchEvent>().having(
+                (e) => e.path, 'path', p.absolute('a', 'lib', 'a.dart.copy')),
+          ]));
+
+      await writer.writeAsString(asset, 'old content');
+      await writer.writeAsString(asset, 'new content');
+      await writer.writeAsString(asset.addExtension('.copy'), 'new content');
+      await writer.onBuildComplete();
+    });
+
     group('has consistent reader', () {
       test('reading unmodified sources', () async {
         final asset = makeAssetId('a|lib/a.dart');
@@ -89,6 +110,22 @@ void main() {
         expect(await reader.readAsBytes(asset), decodedMatches('contents'));
       });
 
+      test('does not use overlay after flush', () async {
+        final asset = makeAssetId('a|lib/a.dart');
+        underlyingWriter.assets[asset] = [1, 2, 3];
+
+        final writer = DelayedAssetWriter(underlyingWriter);
+        final reader = writer.reader(underlyingReader, rootPackage);
+
+        await writer.writeAsString(asset, 'contents');
+        await writer.onBuildComplete();
+        // "contents" should have been written, but there may be another tool
+        // overwriting the file afterwards.
+        underlyingWriter.assets[asset] = [4, 5, 6];
+
+        expect(await reader.readAsBytes(asset), [4, 5, 6]);
+      });
+
       test('finding right assets', () async {
         final a = makeAssetId('a|lib/a.dart');
         final b = makeAssetId('a|lib/b.dart');