Hacking.

Author: davidmorgan

build_runner/bin/src/commands/generate_build_script.dart

@@ -33,7 +33,10 @@ class GenerateBuildScript extends Command<int> {
     var buildScript = await generateBuildScript();
     File(scriptLocation)
       ..createSync(recursive: true)
-      ..writeAsStringSync(buildScript);
+      ..writeAsStringSync(buildScript.script);
+    File(scriptDepsPath)
+      ..createSync(recursive: true)
+      ..writeAsStringSync(buildScript.dependencyPaths.join('\n'));
     print(p.absolute(scriptLocation));
     return 0;
   }

build_runner/lib/src/build_script_generate/bootstrap.dart

@@ -26,31 +26,29 @@ import 'build_script_generate.dart';
 /// Returns the exit code from running the build script.
 ///
 /// If an exit code of 75 is returned, this function should be re-ran.
+///
+/// Pass [script] to override the default build script for testing.
 Future<int> generateAndRun(
   List<String> args, {
   List<String>? experiments,
   Logger? logger,
-  Future<String> Function() generateBuildScript = generateBuildScript,
   void Function(Object error, StackTrace stackTrace) handleUncaughtError =
       _defaultHandleUncaughtError,
+  GeneratedScript? script,
 }) {
   return buildLog.runWithLoggerDisplay(
     logger,
-    () => _generateAndRun(
-      args,
-      experiments,
-      generateBuildScript,
-      handleUncaughtError,
-    ),
+    () =>
+        _generateAndRun(args, experiments, handleUncaughtError, script: script),
   );
 }
 
 Future<int> _generateAndRun(
   List<String> args,
   List<String>? experiments,
-  Future<String> Function() generateBuildScript,
-  void Function(Object error, StackTrace stackTrace) handleUncaughtError,
-) async {
+  void Function(Object error, StackTrace stackTrace) handleUncaughtError, {
+  GeneratedScript? script,
+}) async {
   experiments ??= [];
   ReceivePort? exitPort;
   ReceivePort? errorPort;
@@ -72,15 +70,15 @@ Future<int> _generateAndRun(
       if (buildScript.existsSync()) {
         oldContents = buildScript.readAsStringSync();
       }
-      var newContents = await generateBuildScript();
-      if (newContents == oldContents) {
-        buildLog.debug('same contents');
-      }
+      var newContents = script ?? await generateBuildScript();
       // Only trigger a build script update if necessary.
-      if (newContents != oldContents) {
+      if (newContents.script != oldContents) {
         buildScript
           ..createSync(recursive: true)
-          ..writeAsStringSync(newContents);
+          ..writeAsStringSync(newContents.script);
+        File(scriptDepsPath)
+          ..createSync(recursive: true)
+          ..writeAsStringSync(newContents.dependencyPaths.join('\n'));
         // Delete the kernel file so it will be rebuilt.
         final kernelFile = File(scriptKernelLocation);
         if (kernelFile.existsSync()) {

build_runner/lib/src/build_script_generate/build_script_generate.dart

@@ -17,6 +17,7 @@ import '../package_graph/build_config_overrides.dart';
 import 'builder_ordering.dart';
 
 const scriptLocation = '$entryPointDir/build.dart';
+const scriptDepsPath = '$entryPointDir/build.dart.deps';
 const scriptKernelLocation = '$scriptLocation$scriptKernelSuffix';
 const scriptKernelSuffix = '.dill';
 const scriptKernelCachedLocation =
@@ -25,7 +26,7 @@ const scriptKernelCachedSuffix = '.cached';
 
 final _lastShortFormatDartVersion = Version(3, 6, 0);
 
-Future<String> generateBuildScript() async {
+Future<GeneratedScript> generateBuildScript() async {
   buildLog.doing('Generating the build script.');
   final info = await findBuildScriptOptions();
   final builders = info.builderApplications;
@@ -55,14 +56,15 @@ Future<String> generateBuildScript() async {
     // the host<->isolate relationship changed in a breaking way, for example
     // if command line args or messages passed via sendports have changed
     // in a breaking way.
-    return DartFormatter(languageVersion: _lastShortFormatDartVersion).format(
-      '''
+    final script = DartFormatter(
+      languageVersion: _lastShortFormatDartVersion,
+    ).format('''
 // @dart=${_lastShortFormatDartVersion.major}.${_lastShortFormatDartVersion.minor}
 // ignore_for_file: directives_ordering
 // build_runner >=2.4.16
 ${library.accept(emitter)}
-''',
-    );
+''');
+    return GeneratedScript(script: script, dependencyPaths: info.inputs);
   } on FormatterException {
     buildLog.error(
       'Generated build script could not be parsed. '
@@ -195,13 +197,20 @@ Future<BuildScriptInfo> findBuildScriptOptions({
       _applyPostProcessBuilder(builder),
   ];
 
-  return BuildScriptInfo(applications);
+  final inputs = <String>[];
+  for (final package in packageGraph.allPackages.values) {
+    inputs.add('${package.path}/build.yaml');
+  }
+  inputs.sort();
+
+  return BuildScriptInfo(inputs, applications);
 }
 
 class BuildScriptInfo {
+  final List<String> inputs;
   final Iterable<Expression> builderApplications;
 
-  BuildScriptInfo(this.builderApplications);
+  BuildScriptInfo(this.inputs, this.builderApplications);
 }
 
 /// A method forwarding to `run`.
@@ -409,3 +418,10 @@ extension ConvertToExpression on Object? {
     }
   }
 }
+
+class GeneratedScript {
+  String script;
+  List<String> dependencyPaths;
+
+  GeneratedScript({required this.script, required this.dependencyPaths});
+}

build_runner/lib/src/compiler/compiler.dart

@@ -0,0 +1,73 @@
+// Copyright (c) 2025, 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.
+
+import 'dart:async';
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:convert/convert.dart';
+import 'package:crypto/crypto.dart';
+
+void main(List<String> arguments) async {
+  await Compiler().compile(arguments[0]);
+}
+
+class Compiler {
+  Future<void> compile(String path) async {
+    final depfilePath = '$path.deps';
+    final digestFilePath = '$path.digest';
+
+    final depfile = File(depfilePath);
+    final digestFile = File(digestFilePath);
+
+    if (depfile.existsSync() && digestFile.existsSync()) {
+      final expectedDigest = digestFile.readAsStringSync();
+      final actualDigest = computeDigest(parseDepfile(depfilePath));
+      if (expectedDigest == actualDigest) {
+        print('Input digests matched, nothing to do.');
+        return;
+      } else {
+        print(
+          'Input digests changed from $expectedDigest to $actualDigest, rebuild.',
+        );
+      }
+    }
+
+    final result = await Process.run('dart', [
+      'compile',
+      'kernel',
+      path,
+      '--depfile',
+      depfilePath,
+    ]);
+    if (result.exitCode != 0) {
+      print('Compile failed: ${result.stdout} ${result.stderr}');
+      exit(1);
+    }
+    final deps = parseDepfile(depfilePath);
+    final digest = computeDigest(deps);
+    digestFile.writeAsStringSync(digest);
+
+    //unawaited(dill.then((result) => print(result.stdout)));
+  }
+
+  List<String> parseDepfile(String depfilePath) {
+    // TODO(davidmorgan): handle spaces, they seem to be backslash escaped.
+    final result =
+        File(depfilePath).readAsStringSync().split(' ').skip(1).toList();
+    // File ends in a newline.
+    result.last = result.last.substring(0, result.last.length - 1);
+    return result;
+  }
+
+  String computeDigest(Iterable<String> deps) {
+    final digestSink = AccumulatorSink<Digest>();
+    final result = md5.startChunkedConversion(digestSink);
+    for (final dep in deps) {
+      result.add(File(dep).readAsBytesSync());
+    }
+    result.close();
+    return base64.encode(digestSink.events.first.bytes);
+  }
+}

build_runner/lib/src/compiler/compiler.exe

@@ -20,6 +20,7 @@ dependencies:
   build_daemon: ^4.0.0
   build_runner_core: '9.2.1-wip'
   code_builder: ^4.2.0
+  convert: ^3.1.2
   crypto: ^3.0.0
   dart_style: '>=2.3.7 <4.0.0'
   frontend_server_client: ">=3.0.0 <5.0.0"

build_runner/pubspec.yaml

@@ -28,8 +28,7 @@ void main() {
     test('writes dill', () async {
       await generateAndRun(
         [],
-        generateBuildScript:
-            () async => '''
+        script: '''
 import 'dart:isolate';
 import 'package:build_runner/src/build_script_generate/build_process_state.dart';
 
@@ -55,18 +54,17 @@ void main(_, [SendPort? sendPort]) async {
 ''';
 
       buildProcessState.isolateExitCode = 6;
-      await generateAndRun([], generateBuildScript: () async => script);
+      await generateAndRun([], script: script);
       expect(buildProcessState.isolateExitCode, 7);
-      await generateAndRun([], generateBuildScript: () async => script);
+      await generateAndRun([], script: script);
       expect(buildProcessState.isolateExitCode, 8);
     });
 
     test('rewrites dill if script changed', () async {
       expect(
         await generateAndRun(
           [],
-          generateBuildScript:
-              () async => '''
+          script: '''
 import 'dart:isolate';
 import 'package:build_runner/src/build_script_generate/build_process_state.dart';
 
@@ -84,8 +82,7 @@ void main(_, [SendPort? sendPort]) async {
       expect(
         await generateAndRun(
           [],
-          generateBuildScript:
-              () async => '''
+          script: '''
 import 'dart:isolate';
 import 'package:build_runner/src/build_script_generate/build_process_state.dart';
 
@@ -109,8 +106,7 @@ void main(_, [SendPort? sendPort]) async {
         expect(
           await generateAndRun(
             [],
-            generateBuildScript:
-                () async => '''
+            script: '''
 import 'dart:isolate';
 import 'package:build_runner/src/build_script_generate/build_process_state.dart';
 
@@ -132,8 +128,7 @@ void main(_, [SendPort? sendPort]) async {
         expect(
           await generateAndRun(
             [],
-            generateBuildScript:
-                () async => '''
+            script: '''
 import 'dart:isolate';
 import 'package:build_runner/src/build_script_generate/build_process_state.dart';
 
@@ -156,17 +151,15 @@ void main(_, [SendPort? sendPort]) async {
       await expectLater(
         generateAndRun(
           [],
-          generateBuildScript: () async {
-            return '''
+          script: '''
 import 'dart:isolate';
 import 'package:build_runner/src/build_script_generate/build_process_state.dart';
 
 void main(_, [SendPort? sendPort]) async {
   await buildProcessState.receive(sendPort);
   throw 'expected error';
 }
-''';
-          },
+''',
           handleUncaughtError: (err, trace) {
             error = err;
             stackTrace = trace;

build_runner/test/build_script_generate/bootstrap_test.dart

@@ -17,8 +17,7 @@ void main() {
     final exitCode = await generateAndRun(
       [],
       experiments: ['records'],
-      generateBuildScript: () async {
-        return '''
+      script: '''
               // @dart=3.0
               import 'dart:io';
               import 'dart:isolate';
@@ -30,8 +29,7 @@ void main() {
                 buildProcessState.isolateExitCode = (x.\$2);
                 buildProcessState.send(sendPort);
               }
-              ''';
-      },
+              ''',
       logger: logger,
     );
     expect(exitCode, 2);