Forbid multiple writes to the same asset (#3228)

Closes #3227.

build/CHANGELOG.md

@@ -1,3 +1,8 @@
+## 2.2.1
+
+- Fix a bug where builders were allowed to write to the same asset multiple
+  times.
+
 ## 2.2.0
 
 - Allow reusing the values held by `Resource`s when the resource has a `dispose`

build/lib/src/builder/build_step_impl.dart

@@ -47,6 +47,8 @@ class BuildStepImpl implements BuildStep {
   /// The result of any writes which are starting during this step.
   final _writeResults = <Future<Result<void>>>[];
 
+  final _writtenAssets = <AssetId>{};
+
   /// Used internally for reading files.
   final AssetReader _reader;
 
@@ -170,6 +172,14 @@ class BuildStepImpl implements BuildStep {
     if (!allowedOutputs.contains(id)) {
       throw UnexpectedOutputException(id, expected: allowedOutputs);
     }
+
+    if (!_writtenAssets.add(id)) {
+      throw InvalidOutputException(
+        id,
+        'This build step already wrote to `$id`. Duplicate writes to the same '
+        'asset are forbidden.',
+      );
+    }
   }
 
   @override

build/pubspec.yaml

@@ -1,5 +1,5 @@
 name: build
-version: 2.2.0
+version: 2.2.1
 description: A package for authoring build_runner compatible code generators.
 repository: https://github.com/dart-lang/build/tree/master/build
 

build/test/builder/build_step_impl_test.dart

@@ -55,6 +55,19 @@ void main() {
       var actual = await buildStep.fetchResource(intResource);
       expect(actual, expected);
     });
+
+    test('does not allow multiple writes to the same output', () async {
+      final id = outputs.first;
+      await buildStep.writeAsString(id, 'foo');
+
+      final expectedException = isA<InvalidOutputException>()
+          .having((e) => e.assetId, 'assetId', id)
+          .having((e) => e.message, 'message', contains('already wrote to'));
+
+      expect(
+          () => buildStep.writeAsString(id, 'bar'), throwsA(expectedException));
+      expect(() => buildStep.writeAsBytes(id, []), throwsA(expectedException));
+    });
   });
 
   group('with in memory file system', () {