Fixing DDC + FES not properly reading entrypointAssetId when re-serving an app. (#4273)

Read from our .web.entrypoint.json file instead, which would have been generated in a previous compile.

Author: Markzipan

build_modules/lib/src/frontend_server_resources.dart

@@ -13,7 +13,7 @@ class FrontendServerState {
   ///
   /// This must be set before any asset builders run when compiling with DDC and
   /// hot reload.
-  late AssetId entrypointAssetId;
+  AssetId? entrypointAssetId;
 }
 
 /// A shared [Resource] for a [FrontendServerState].

build_web_compilers/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 4.4.2
+
+- Fixes DDC + FES not reading the entrypoint when serving a prebuilt app.
+
 ## 4.4.1
 
 - Read `multiRootScheme` from `build_modules`.

build_web_compilers/lib/src/ddc_frontend_server_builder.dart

@@ -66,8 +66,31 @@ class DdcFrontendServerBuilder implements Builder {
     final frontendServerState = await buildStep.fetchResource(
       frontendServerStateResource,
     );
+
+    if (frontendServerState.entrypointAssetId == null) {
+      // Reuse the recorded entrypoint asset if it was generated in a previous
+      // build.
+      final webEntrypointAsset = AssetId(
+        buildStep.inputId.package,
+        'web/.web.entrypoint.json',
+      );
+      if (await buildStep.canRead(webEntrypointAsset)) {
+        final contents =
+            json.decode(await buildStep.readAsString(webEntrypointAsset))
+                as Map<String, dynamic>;
+        frontendServerState.entrypointAssetId = AssetId.parse(
+          contents['entrypoint'] as String,
+        );
+      } else {
+        log.severe(
+          'Unable to read entrypoint when building ${buildStep.inputId}.',
+        );
+        return;
+      }
+    }
+
     final scratchSpace = await buildStep.fetchResource(scratchSpaceResource);
-    final webEntrypointAsset = frontendServerState.entrypointAssetId;
+    final webEntrypointAsset = frontendServerState.entrypointAssetId!;
     await buildStep.trackStage(
       'EnsureAssets',
       () => scratchSpace.ensureAssets([
@@ -127,8 +150,8 @@ class DdcFrontendServerBuilder implements Builder {
     final metadataId = ddcEntrypointId.changeExtension(metadataExtension);
     final file = scratchSpace.fileFor(metadataId);
     final content = await file.readAsString();
-    final json = jsonDecode(content) as Map<String, Object?>;
-    fixMetadataSources(json, scratchSpace.tempDir.uri);
-    await buildStep.writeAsString(metadataId, jsonEncode(json));
+    final metadataJson = jsonDecode(content) as Map<String, Object?>;
+    fixMetadataSources(metadataJson, scratchSpace.tempDir.uri);
+    await buildStep.writeAsString(metadataId, jsonEncode(metadataJson));
   }
 }

build_web_compilers/lib/src/web_entrypoint_marker_builder.dart

@@ -30,6 +30,10 @@ class WebEntrypointMarkerBuilder implements Builder {
     final frontendServerState = await buildStep.fetchResource(
       frontendServerStateResource,
     );
+    final webEntrypointAsset = AssetId(
+      buildStep.inputId.package,
+      'web/.web.entrypoint.json',
+    );
     final webAssets = await buildStep.findAssets(Glob('web/**')).toList();
     final webEntrypointJson = <String, dynamic>{};
 
@@ -43,14 +47,14 @@ class WebEntrypointMarkerBuilder implements Builder {
           // We must save the main entrypoint as the recompilation target for
           // the Frontend Server before any JS files are emitted.
           frontendServerState.entrypointAssetId = asset;
-          webEntrypointJson['entrypoint'] = asset.uri.toString();
+          webEntrypointJson['entrypoint'] = asset.toString();
           break;
         }
       }
     }
 
     await buildStep.writeAsString(
-      AssetId(buildStep.inputId.package, 'web/.web.entrypoint.json'),
+      webEntrypointAsset,
       jsonEncode(webEntrypointJson),
     );
   }

build_web_compilers/pubspec.yaml

@@ -1,5 +1,5 @@
 name: build_web_compilers
-version: 4.4.1
+version: 4.4.2
 description: Builder implementations wrapping the dart2js and DDC compilers.
 repository: https://github.com/dart-lang/build/tree/master/build_web_compilers
 resolution: workspace