Fix running build once per output. (#4045)

Author: davidmorgan

build_runner_core/CHANGELOG.md

@@ -1,6 +1,7 @@
 ## 9.0.1-wip
 
 - Don't log stack traces for subclasses of `Exception`.
+- Bug fix: don't run builders with multiple outputs once per output.
 
 ## 9.0.0
 

build_runner_core/lib/src/generate/build.dart

@@ -310,7 +310,6 @@ class Build {
         // If `primaryInputs` is empty, the phase will only run lazily,
         // and might not run at all; so don't log it to start with.
         if (primaryInputs.isNotEmpty) {
-          primaryInputs.sort();
           primaryInputsByPhase[phase] = primaryInputs;
           primaryInputCountsByPhase[phase] = primaryInputs.length;
         }
@@ -382,7 +381,8 @@ class Build {
     String package,
     int phaseNumber,
   ) async {
-    var ids = <AssetId>[];
+    // Accumulate in a `Set` because inputs are found once per output.
+    var ids = <AssetId>{};
     var phase = buildPhases[phaseNumber];
     var packageNode = options.packageGraph[package]!;
 
@@ -406,7 +406,7 @@ class Build {
 
       ids.add(node.generatedNodeConfiguration!.primaryInput);
     }
-    return ids;
+    return ids.toList()..sort();
   }
 
   /// If [id] is a generated asset, ensures that it has been built.

build_runner_core/test/generate/build_test.dart

@@ -210,6 +210,25 @@ void main() {
         );
       });
 
+      test('runs once per input not once per output', () async {
+        var runs = 0;
+        final testBuilder = TestBuilder(
+          buildExtensions: {
+            '.txt': ['.txt.1', '.txt.2'],
+          },
+          extraWork: (_, _) {
+            ++runs;
+          },
+        );
+
+        await testPhases(
+          [applyToRoot(testBuilder)],
+          {'a|web/a.txt': ''},
+          outputs: {'a|web/a.txt.1': '', 'a|web/a.txt.2': ''},
+        );
+        expect(runs, 1);
+      });
+
       test('with a PostProcessBuilder', () async {
         await testPhases(
           [requiresPostProcessBuilderApplication, postCopyABuilderApplication],