Fix asset transformation in the presence of resolution-aware asset variants (#151932)

For the necessary background knowledge, see the flutter.dev content on [Resolution-aware image assets](https://docs.flutter.dev/ui/assets/assets-and-images#resolution-aware) and [Conditional bundling of assets based on app flavor](https://docs.flutter.dev/ui/assets/assets-and-images#conditional-bundling-of-assets-based-on-app-flavor) if you don't have a basic understanding of these features.

Fixes https://github.com/flutter/flutter/issues/151813 by using unique temporary directories, per asset file, for transformations. Currently, only a single directory is used and the name of the temporary files was based only on the basename of files. This means that `assets/image.png` and `assets/2x/image.png` would share an output path (`<temp dir path>/image.png`), causing a race. If this quick and rough explanation is a bit confusing, the original issue—#151813—provides a full repro and correct identification of the exact cause of the failure that can occur in the asset transformation process.

packages/flutter_tools/lib/src/build_system/targets/assets.dart

@@ -154,7 +154,8 @@ Future<Depfile> copyAssets(
                 );
                 doCopy = false;
                 if (failure != null) {
-                  throwToolExit(failure.message);
+                  throwToolExit('User-defined transformation of asset "${entry.key}" failed.\n'
+                      '${failure.message}');
                 }
               }
             case AssetKind.font:

packages/flutter_tools/lib/src/build_system/tools/asset_transformer.dart

@@ -55,16 +55,21 @@ final class AssetTransformer {
     required Logger logger,
   }) async {
 
-    String getTempFilePath(int transformStep) {
+    final Directory tempDirectory = _fileSystem.systemTempDirectory.createTempSync();
+
+    int transformStep = 0;
+    File nextTempFile() {
       final String basename = _fileSystem.path.basename(asset.path);
       final String ext = _fileSystem.path.extension(asset.path);
-      return '$basename-transformOutput$transformStep$ext';
+
+      final File result = tempDirectory.childFile('$basename-transformOutput$transformStep$ext');
+      transformStep++;
+      return result;
     }
 
-    File tempInputFile = _fileSystem.systemTempDirectory.childFile(getTempFilePath(0));
+    File tempInputFile = nextTempFile();
     await asset.copy(tempInputFile.path);
-    File tempOutputFile = _fileSystem.systemTempDirectory.childFile(getTempFilePath(1));
-    ErrorHandlingFileSystem.deleteIfExists(tempOutputFile);
+    File tempOutputFile = nextTempFile();
 
     final Stopwatch stopwatch = Stopwatch()..start();
     try {
@@ -78,10 +83,7 @@ final class AssetTransformer {
         );
 
         if (transformerFailure != null) {
-          return AssetTransformationFailure(
-            'User-defined transformation of asset "${asset.path}" failed.\n'
-            '${transformerFailure.message}',
-          );
+          return AssetTransformationFailure(transformerFailure.message);
         }
 
         ErrorHandlingFileSystem.deleteIfExists(tempInputFile);
@@ -90,15 +92,13 @@ final class AssetTransformer {
           await tempOutputFile.copy(outputPath);
         } else {
           tempInputFile = tempOutputFile;
-          tempOutputFile = _fileSystem.systemTempDirectory.childFile(getTempFilePath(i+2));
-          ErrorHandlingFileSystem.deleteIfExists(tempOutputFile);
+          tempOutputFile = nextTempFile();
         }
       }
 
       logger.printTrace("Finished transforming asset at path '${asset.path}' (${stopwatch.elapsedMilliseconds}ms)");
     } finally {
-      ErrorHandlingFileSystem.deleteIfExists(tempInputFile);
-      ErrorHandlingFileSystem.deleteIfExists(tempOutputFile);
+      ErrorHandlingFileSystem.deleteIfExists(tempDirectory, recursive: true);
     }
 
     return null;
@@ -124,6 +124,11 @@ final class AssetTransformer {
       ...transformerArguments,
     ];
 
+    // Delete the output file if it already exists for whatever reason.
+    // With this, we can check for the existence of the file after transformation
+    // to make sure the transformer produced an output file.
+    ErrorHandlingFileSystem.deleteIfExists(output);
+
     logger.printTrace("Transforming asset using command '${command.join(' ')}'");
     final ProcessResult result = await _processManager.run(
       command,

packages/flutter_tools/lib/src/bundle_builder.dart

@@ -213,7 +213,8 @@ Future<void> writeBundle(
             );
             doCopy = false;
             if (failure != null) {
-              throwToolExit(failure.message);
+              throwToolExit('User-defined transformation of asset "${entry.key}" failed.\n'
+                  '${failure.message}');
             }
             case AssetKind.font:
               break;

packages/flutter_tools/test/general.shard/build_system/targets/asset_transformer_test.dart

@@ -31,8 +31,8 @@ void main() {
           artifacts.getArtifactPath(Artifact.engineDartBinary),
           'run',
           'my_copy_transformer',
-          '--input=/.tmp_rand0/asset.txt-transformOutput0.txt',
-          '--output=/.tmp_rand0/asset.txt-transformOutput1.txt',
+          '--input=/.tmp_rand0/rand0/asset.txt-transformOutput0.txt',
+          '--output=/.tmp_rand0/rand0/asset.txt-transformOutput1.txt',
           '-f',
           '--my_option',
           'my_option_value',
@@ -95,8 +95,8 @@ void main() {
           dartBinaryPath,
           'run',
           'my_copy_transformer',
-          '--input=/.tmp_rand0/asset.txt-transformOutput0.txt',
-          '--output=/.tmp_rand0/asset.txt-transformOutput1.txt',
+          '--input=/.tmp_rand0/rand0/asset.txt-transformOutput0.txt',
+          '--output=/.tmp_rand0/rand0/asset.txt-transformOutput1.txt',
         ],
         onRun: (List<String> args) {
           final ArgResults parsedArgs = (ArgParser()
@@ -136,10 +136,9 @@ void main() {
     expect(failure, isNotNull);
     expect(failure!.message,
 '''
-User-defined transformation of asset "asset.txt" failed.
 Transformer process terminated with non-zero exit code: 1
 Transformer package: my_copy_transformer
-Full command: $dartBinaryPath run my_copy_transformer --input=/.tmp_rand0/asset.txt-transformOutput0.txt --output=/.tmp_rand0/asset.txt-transformOutput1.txt
+Full command: $dartBinaryPath run my_copy_transformer --input=/.tmp_rand0/rand0/asset.txt-transformOutput0.txt --output=/.tmp_rand0/rand0/asset.txt-transformOutput1.txt
 stdout:
 Beginning transformation
 stderr:
@@ -162,8 +161,8 @@ Something went wrong''');
           dartBinaryPath,
           'run',
           'my_transformer',
-          '--input=/.tmp_rand0/asset.txt-transformOutput0.txt',
-          '--output=/.tmp_rand0/asset.txt-transformOutput1.txt',
+          '--input=/.tmp_rand0/rand0/asset.txt-transformOutput0.txt',
+          '--output=/.tmp_rand0/rand0/asset.txt-transformOutput1.txt',
         ],
         onRun: (_) {
           // Do nothing.
@@ -196,11 +195,10 @@ Something went wrong''');
     expect(failure, isNotNull);
     expect(failure!.message,
 '''
-User-defined transformation of asset "asset.txt" failed.
 Asset transformer my_transformer did not produce an output file.
-Input file provided to transformer: "/.tmp_rand0/asset.txt-transformOutput0.txt"
-Expected output file at: "/.tmp_rand0/asset.txt-transformOutput1.txt"
-Full command: $dartBinaryPath run my_transformer --input=/.tmp_rand0/asset.txt-transformOutput0.txt --output=/.tmp_rand0/asset.txt-transformOutput1.txt
+Input file provided to transformer: "/.tmp_rand0/rand0/asset.txt-transformOutput0.txt"
+Expected output file at: "/.tmp_rand0/rand0/asset.txt-transformOutput1.txt"
+Full command: $dartBinaryPath run my_transformer --input=/.tmp_rand0/rand0/asset.txt-transformOutput0.txt --output=/.tmp_rand0/rand0/asset.txt-transformOutput1.txt
 stdout:
 
 stderr:
@@ -225,8 +223,8 @@ Transformation failed, but I forgot to exit with a non-zero code.'''
           dartBinaryPath,
           'run',
           'my_lowercase_transformer',
-          '--input=/.tmp_rand0/asset.txt-transformOutput0.txt',
-          '--output=/.tmp_rand0/asset.txt-transformOutput1.txt',
+          '--input=/.tmp_rand0/rand0/asset.txt-transformOutput0.txt',
+          '--output=/.tmp_rand0/rand0/asset.txt-transformOutput1.txt',
         ],
         onRun: (List<String> args) {
           final ArgResults parsedArgs = (ArgParser()
@@ -245,8 +243,8 @@ Transformation failed, but I forgot to exit with a non-zero code.'''
           dartBinaryPath,
           'run',
           'my_distance_from_ascii_a_transformer',
-          '--input=/.tmp_rand0/asset.txt-transformOutput1.txt',
-          '--output=/.tmp_rand0/asset.txt-transformOutput2.txt',
+          '--input=/.tmp_rand0/rand0/asset.txt-transformOutput1.txt',
+          '--output=/.tmp_rand0/rand0/asset.txt-transformOutput2.txt',
         ],
         onRun: (List<String> args) {
           final ArgResults parsedArgs = (ArgParser()
@@ -314,8 +312,8 @@ Transformation failed, but I forgot to exit with a non-zero code.'''
           dartBinaryPath,
           'run',
           'my_lowercase_transformer',
-          '--input=/.tmp_rand0/asset.txt-transformOutput0.txt',
-          '--output=/.tmp_rand0/asset.txt-transformOutput1.txt',
+          '--input=/.tmp_rand0/rand0/asset.txt-transformOutput0.txt',
+          '--output=/.tmp_rand0/rand0/asset.txt-transformOutput1.txt',
         ],
         onRun: (List<String> args) {
           final ArgResults parsedArgs = (ArgParser()
@@ -334,8 +332,8 @@ Transformation failed, but I forgot to exit with a non-zero code.'''
           dartBinaryPath,
           'run',
           'my_distance_from_ascii_a_transformer',
-          '--input=/.tmp_rand0/asset.txt-transformOutput1.txt',
-          '--output=/.tmp_rand0/asset.txt-transformOutput2.txt',
+          '--input=/.tmp_rand0/rand0/asset.txt-transformOutput1.txt',
+          '--output=/.tmp_rand0/rand0/asset.txt-transformOutput2.txt',
         ],
         onRun: (List<String> args) {
           // Do nothing.
@@ -374,11 +372,10 @@ Transformation failed, but I forgot to exit with a non-zero code.'''
     expect(failure, isNotNull);
     expect(failure!.message,
 '''
-User-defined transformation of asset "asset.txt" failed.
 Asset transformer my_distance_from_ascii_a_transformer did not produce an output file.
-Input file provided to transformer: "/.tmp_rand0/asset.txt-transformOutput1.txt"
-Expected output file at: "/.tmp_rand0/asset.txt-transformOutput2.txt"
-Full command: Artifact.engineDartBinary run my_distance_from_ascii_a_transformer --input=/.tmp_rand0/asset.txt-transformOutput1.txt --output=/.tmp_rand0/asset.txt-transformOutput2.txt
+Input file provided to transformer: "/.tmp_rand0/rand0/asset.txt-transformOutput1.txt"
+Expected output file at: "/.tmp_rand0/rand0/asset.txt-transformOutput2.txt"
+Full command: Artifact.engineDartBinary run my_distance_from_ascii_a_transformer --input=/.tmp_rand0/rand0/asset.txt-transformOutput1.txt --output=/.tmp_rand0/rand0/asset.txt-transformOutput2.txt
 stdout:
 
 stderr:

packages/flutter_tools/test/general.shard/build_system/targets/assets_test.dart

@@ -252,6 +252,7 @@ flutter:
     ),
   });
 
+
   testUsingContext('exits tool if an asset transformation fails', () async {
     Cache.flutterRoot = Cache.defaultFlutterRoot(
       platform: globals.platform,
@@ -289,7 +290,7 @@ flutter:
 
     await expectToolExitLater(
       const CopyAssets().build(environment),
-      startsWith('User-defined transformation of asset "/input.txt" failed.\n'),
+      startsWith('User-defined transformation of asset "input.txt" failed.\n'),
     );
     expect(globals.processManager, hasNoRemainingExpectations);
   }, overrides: <Type, Generator> {
@@ -316,6 +317,90 @@ flutter:
     ),
   });
 
+  testUsingContext('asset transformation, per each asset, uses unique paths for temporary files', () async {
+    final List<String> inputFilePaths = <String>[];
+    final List<String> outputFilePaths = <String>[];
+
+    final FakeCommand transformerCommand = FakeCommand(
+      command: <Pattern>[
+        Artifacts.test().getArtifactPath(Artifact.engineDartBinary),
+        'run',
+        'my_capitalizer_transformer',
+        RegExp('--input=.*'),
+        RegExp('--output=.*'),
+      ],
+      onRun: (List<String> args) {
+        final ArgResults parsedArgs = (ArgParser()
+              ..addOption('input')
+              ..addOption('output'))
+            .parse(args);
+
+        final String input = parsedArgs['input'] as String;
+        final String output = parsedArgs['output'] as String;
+
+        inputFilePaths.add(input);
+        outputFilePaths.add(output);
+
+        fileSystem.file(output)
+          ..createSync()
+          ..writeAsStringSync('foo');
+      },
+    );
+
+    Cache.flutterRoot = Cache.defaultFlutterRoot(
+      platform: globals.platform,
+      fileSystem: fileSystem,
+      userMessages: UserMessages(),
+    );
+
+    final Environment environment = Environment.test(
+      fileSystem.currentDirectory,
+      processManager: FakeProcessManager.list(
+        <FakeCommand>[
+          transformerCommand,
+          transformerCommand,
+        ],
+      ),
+      artifacts: Artifacts.test(),
+      fileSystem: fileSystem,
+      logger: logger,
+      platform: globals.platform,
+      defines: <String, String>{
+        kBuildMode: BuildMode.debug.cliName,
+      },
+    );
+
+    await fileSystem.file('.packages').create();
+
+    fileSystem.file('pubspec.yaml')
+      ..createSync()
+      ..writeAsStringSync('''
+  name: example
+  flutter:
+    assets:
+      - path: input.txt
+        transformers:
+          - package: my_capitalizer_transformer
+  ''');
+
+    fileSystem.file('input.txt')
+      ..createSync(recursive: true)
+      ..writeAsStringSync('abc');
+
+    fileSystem.directory('2x').childFile('input.txt')
+      ..createSync(recursive: true)
+      ..writeAsStringSync('def');
+
+    await const CopyAssets().build(environment);
+
+    expect(inputFilePaths.toSet(), hasLength(inputFilePaths.length));
+    expect(outputFilePaths.toSet(), hasLength(outputFilePaths.length));
+  }, overrides: <Type, Generator>{
+    Logger: () => logger,
+    FileSystem: () => fileSystem,
+    Platform: () => FakePlatform(),
+    ProcessManager: () => FakeProcessManager.empty(),
+  });
 
   testUsingContext('Throws exception if pubspec contains missing files', () async {
     fileSystem.file('pubspec.yaml')

packages/flutter_tools/test/general.shard/bundle_builder_test.dart

@@ -71,8 +71,8 @@ void main() {
               artifacts.getArtifactPath(Artifact.engineDartBinary),
               'run',
               'increment',
-              '--input=/.tmp_rand0/my-asset.txt-transformOutput0.txt',
-              '--output=/.tmp_rand0/my-asset.txt-transformOutput1.txt'
+              '--input=/.tmp_rand0/rand0/my-asset.txt-transformOutput0.txt',
+              '--output=/.tmp_rand0/rand0/my-asset.txt-transformOutput1.txt'
             ],
             onRun: (List<String> command) {
               final ArgResults argParseResults = (ArgParser()

packages/flutter_tools/test/general.shard/devfs_test.dart

@@ -733,8 +733,8 @@ void main() {
               artifacts.getArtifactPath(Artifact.engineDartBinary),
               'run',
               'increment',
-              '--input=/.tmp_rand0/retransformerInput-asset.txt-transformOutput0.txt',
-              '--output=/.tmp_rand0/retransformerInput-asset.txt-transformOutput1.txt',
+              '--input=/.tmp_rand0/rand0/retransformerInput-asset.txt-transformOutput0.txt',
+              '--output=/.tmp_rand0/rand0/retransformerInput-asset.txt-transformOutput1.txt',
             ],
             onRun: (List<String> command) {
               final ArgResults argParseResults = (ArgParser()
@@ -831,8 +831,8 @@ void main() {
               artifacts.getArtifactPath(Artifact.engineDartBinary),
               'run',
               'increment',
-              '--input=/.tmp_rand0/retransformerInput-asset.txt-transformOutput0.txt',
-              '--output=/.tmp_rand0/retransformerInput-asset.txt-transformOutput1.txt',
+              '--input=/.tmp_rand0/rand0/retransformerInput-asset.txt-transformOutput0.txt',
+              '--output=/.tmp_rand0/rand0/retransformerInput-asset.txt-transformOutput1.txt',
             ],
             exitCode: 1,
           ),
@@ -895,10 +895,9 @@ void main() {
       expect(devFSWriter.entries, isNull, reason: 'DevFS should not have written anything since the update failed.');
       expect(
         logger.errorText,
-        'User-defined transformation of asset "/.tmp_rand0/retransformerInput-asset.txt" failed.\n'
         'Transformer process terminated with non-zero exit code: 1\n'
         'Transformer package: increment\n'
-        'Full command: Artifact.engineDartBinary run increment --input=/.tmp_rand0/retransformerInput-asset.txt-transformOutput0.txt --output=/.tmp_rand0/retransformerInput-asset.txt-transformOutput1.txt\n'
+        'Full command: Artifact.engineDartBinary run increment --input=/.tmp_rand0/rand0/retransformerInput-asset.txt-transformOutput0.txt --output=/.tmp_rand0/rand0/retransformerInput-asset.txt-transformOutput1.txt\n'
         'stdout:\n'
         '\n'
         'stderr:\n'