[web] Encode AssetManifest.bin as JSON and use that on the web. (#131382)

This PR modifies the web build slightly to create an `AssetManifest.json`, that is a JSON(base64)-encoded version of the `AssetManifest.bin` file.

_(This should enable all browsers to download the file without any interference, and all servers to serve it with the correct headers.)_

It also modifies Flutter's `AssetManifest` class so it loads and uses said file `if (kIsWeb)`.

### Issues

* Fixes https://github.com/flutter/flutter/issues/124883

### Tests

* Unit tests added.
* Some tests that run on the Web needed to be informed of the new filename, but their behavior didn't have to change (binary contents are the same across all platforms).
* I've deployed a test app, so users affected by the BIN issue may take a look at the PR in action:
  * https://dit-tests.web.app

packages/flutter/lib/src/services/asset_bundle.dart

@@ -88,8 +88,8 @@ abstract class AssetBundle {
   Future<String> loadString(String key, { bool cache = true }) async {
     final ByteData data = await load(key);
     // 50 KB of data should take 2-3 ms to parse on a Moto G4, and about 400 μs
-    // on a Pixel 4.
-    if (data.lengthInBytes < 50 * 1024) {
+    // on a Pixel 4. On the web we can't bail to isolates, though...
+    if (data.lengthInBytes < 50 * 1024 || kIsWeb) {
       return utf8.decode(Uint8List.sublistView(data));
     }
     // For strings larger than 50 KB, run the computation in an isolate to

packages/flutter/lib/src/services/asset_manifest.dart

@@ -2,18 +2,22 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:convert';
+
 import 'package:flutter/foundation.dart';
 
 import 'asset_bundle.dart';
 import 'message_codecs.dart';
 
 // We use .bin as the extension since it is well-known to represent
-// data in some arbitrary binary format. Using a well-known extension here
-// is important for web, because some web servers will not serve files with
-// unrecognized file extensions by default.
-// See https://github.com/flutter/flutter/issues/128456.
+// data in some arbitrary binary format.
 const String _kAssetManifestFilename = 'AssetManifest.bin';
 
+// We use the same bin file for the web, but re-encoded as JSON(base64(bytes))
+// so it can be downloaded by even the dumbest of browsers.
+// See https://github.com/flutter/flutter/issues/128456
+const String _kAssetManifestWebFilename = 'AssetManifest.bin.json';
+
 /// Contains details about available assets and their variants.
 /// See [Resolution-aware image assets](https://docs.flutter.dev/ui/assets-and-images#resolution-aware)
 /// to learn about asset variants and how to declare them.
@@ -21,6 +25,22 @@ abstract class AssetManifest {
   /// Loads asset manifest data from an [AssetBundle] object and creates an
   /// [AssetManifest] object from that data.
   static Future<AssetManifest> loadFromAssetBundle(AssetBundle bundle) {
+    // The AssetManifest file contains binary data.
+    //
+    // On the web, the build process wraps this binary data in json+base64 so
+    // it can be transmitted over the network without special configuration
+    // (see #131382).
+    if (kIsWeb) {
+      // On the web, the AssetManifest is downloaded as a String, then
+      // json+base64-decoded to get to the binary data.
+      return bundle.loadStructuredData(_kAssetManifestWebFilename, (String jsonData) async {
+        // Decode the manifest JSON file to the underlying BIN, and convert to ByteData.
+        final ByteData message = ByteData.sublistView(base64.decode(json.decode(jsonData) as String));
+        // Now we can keep operating as usual.
+        return _AssetManifestBin.fromStandardMessageCodecMessage(message);
+      });
+    }
+    // On every other platform, the binary file contents are used directly.
     return bundle.loadStructuredBinaryData(_kAssetManifestFilename, _AssetManifestBin.fromStandardMessageCodecMessage);
   }
 

packages/flutter/test/painting/image_resolution_test.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:convert';
 import 'dart:ui' as ui;
 
 import 'package:flutter/foundation.dart';
@@ -22,6 +23,22 @@ class TestAssetBundle extends CachingAssetBundle {
       return const StandardMessageCodec().encodeMessage(_assetBundleMap)!;
     }
 
+    if (key == 'AssetManifest.bin.json') {
+      // Encode the manifest data that will be used by the app
+      final ByteData data = const StandardMessageCodec().encodeMessage(_assetBundleMap)!;
+      // Simulate the behavior of NetworkAssetBundle.load here, for web tests
+      return ByteData.sublistView(
+        utf8.encode(
+          json.encode(
+            base64.encode(
+              // Encode only the actual bytes of the buffer, and no more...
+              data.buffer.asUint8List(data.offsetInBytes, data.lengthInBytes)
+            )
+          )
+        )
+      );
+    }
+
     loadCallCount[key] = loadCallCount[key] ?? 0 + 1;
     if (key == 'one') {
       return ByteData(1)

packages/flutter/test/services/asset_bundle_test.dart

@@ -25,6 +25,22 @@ class TestAssetBundle extends CachingAssetBundle {
           .encodeMessage(<String, Object>{'one': <Object>[]})!;
     }
 
+    if (key == 'AssetManifest.bin.json') {
+      // Encode the manifest data that will be used by the app
+      final ByteData data = const StandardMessageCodec().encodeMessage(<String, Object> {'one': <Object>[]})!;
+      // Simulate the behavior of NetworkAssetBundle.load here, for web tests
+      return ByteData.sublistView(
+        utf8.encode(
+          json.encode(
+            base64.encode(
+              // Encode only the actual bytes of the buffer, and no more...
+              data.buffer.asUint8List(data.offsetInBytes, data.lengthInBytes)
+            )
+          )
+        )
+      );
+    }
+
     if (key == 'counter') {
       return ByteData.sublistView(utf8.encode(loadCallCount[key]!.toString()));
     }

packages/flutter/test/services/asset_manifest_test.dart

@@ -2,34 +2,52 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:convert';
+
 import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 class TestAssetBundle extends AssetBundle {
+  static const Map<String, List<Object>> _binManifestData = <String, List<Object>>{
+    'assets/foo.png': <Object>[
+      <String, Object>{
+        'asset': 'assets/foo.png',
+      },
+      <String, Object>{
+        'asset': 'assets/2x/foo.png',
+        'dpr': 2.0
+      },
+    ],
+    'assets/bar.png': <Object>[
+      <String, Object>{
+        'asset': 'assets/bar.png',
+      },
+    ],
+  };
+
   @override
   Future<ByteData> load(String key) async {
     if (key == 'AssetManifest.bin') {
-      final Map<String, List<Object>> binManifestData = <String, List<Object>>{
-        'assets/foo.png': <Object>[
-          <String, Object>{
-            'asset': 'assets/foo.png',
-          },
-          <String, Object>{
-            'asset': 'assets/2x/foo.png',
-            'dpr': 2.0
-          },
-        ],
-        'assets/bar.png': <Object>[
-          <String, Object>{
-            'asset': 'assets/bar.png',
-          },
-        ],
-      };
-
-      final ByteData data = const StandardMessageCodec().encodeMessage(binManifestData)!;
+      final ByteData data = const StandardMessageCodec().encodeMessage(_binManifestData)!;
       return data;
     }
 
+    if (key == 'AssetManifest.bin.json') {
+      // Encode the manifest data that will be used by the app
+      final ByteData data = const StandardMessageCodec().encodeMessage(_binManifestData)!;
+      // Simulate the behavior of NetworkAssetBundle.load here, for web tests
+      return ByteData.sublistView(
+        utf8.encode(
+          json.encode(
+            base64.encode(
+              // Encode only the actual bytes of the buffer, and no more...
+              data.buffer.asUint8List(data.offsetInBytes, data.lengthInBytes)
+            )
+          )
+        )
+      );
+    }
+
     throw ArgumentError('Unexpected key');
   }
 

packages/flutter_tools/lib/src/asset.dart

@@ -168,6 +168,7 @@ class ManifestAssetBundle implements AssetBundle {
   // We assume the main asset is designed for a device pixel ratio of 1.0.
   static const String _kAssetManifestJsonFilename = 'AssetManifest.json';
   static const String _kAssetManifestBinFilename = 'AssetManifest.bin';
+  static const String _kAssetManifestBinJsonFilename = 'AssetManifest.bin.json';
 
   static const String _kNoticeFile = 'NOTICES';
   // Comically, this can't be name with the more common .gz file extension
@@ -233,8 +234,6 @@ class ManifestAssetBundle implements AssetBundle {
     // device.
     _lastBuildTimestamp = DateTime.now();
     if (flutterManifest.isEmpty) {
-      entries[_kAssetManifestJsonFilename] = DevFSStringContent('{}');
-      entryKinds[_kAssetManifestJsonFilename] = AssetKind.regular;
       entries[_kAssetManifestJsonFilename] = DevFSStringContent('{}');
       entryKinds[_kAssetManifestJsonFilename] = AssetKind.regular;
       final ByteData emptyAssetManifest =
@@ -242,6 +241,11 @@ class ManifestAssetBundle implements AssetBundle {
       entries[_kAssetManifestBinFilename] =
         DevFSByteContent(emptyAssetManifest.buffer.asUint8List(0, emptyAssetManifest.lengthInBytes));
       entryKinds[_kAssetManifestBinFilename] = AssetKind.regular;
+      // Create .bin.json on web builds.
+      if (targetPlatform == TargetPlatform.web_javascript) {
+        entries[_kAssetManifestBinJsonFilename] = DevFSStringContent('""');
+        entryKinds[_kAssetManifestBinJsonFilename] = AssetKind.regular;
+      }
       return 0;
     }
 
@@ -437,8 +441,8 @@ class ManifestAssetBundle implements AssetBundle {
 
     final Map<String, List<String>> assetManifest =
       _createAssetManifest(assetVariants, deferredComponentsAssetVariants);
-    final DevFSStringContent assetManifestJson = DevFSStringContent(json.encode(assetManifest));
     final DevFSByteContent assetManifestBinary = _createAssetManifestBinary(assetManifest);
+    final DevFSStringContent assetManifestJson = DevFSStringContent(json.encode(assetManifest));
     final DevFSStringContent fontManifest = DevFSStringContent(json.encode(fonts));
     final LicenseResult licenseResult = _licenseCollector.obtainLicenses(packageConfig, additionalLicenseFiles);
     if (licenseResult.errorMessages.isNotEmpty) {
@@ -464,6 +468,13 @@ class ManifestAssetBundle implements AssetBundle {
 
     _setIfChanged(_kAssetManifestJsonFilename, assetManifestJson, AssetKind.regular);
     _setIfChanged(_kAssetManifestBinFilename, assetManifestBinary, AssetKind.regular);
+    // Create .bin.json on web builds.
+    if (targetPlatform == TargetPlatform.web_javascript) {
+      final DevFSStringContent assetManifestBinaryJson = DevFSStringContent(json.encode(
+        base64.encode(assetManifestBinary.bytes)
+      ));
+      _setIfChanged(_kAssetManifestBinJsonFilename, assetManifestBinaryJson, AssetKind.regular);
+    }
     _setIfChanged(kFontManifestJson, fontManifest, AssetKind.regular);
     _setLicenseIfChanged(licenseResult.combinedLicenses, targetPlatform);
     return 0;

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

@@ -325,6 +325,102 @@ flutter:
     });
   });
 
+  group('AssetBundle.build (web builds)', () {
+    late FileSystem testFileSystem;
+
+    setUp(() async {
+      testFileSystem = MemoryFileSystem(
+        style: globals.platform.isWindows
+          ? FileSystemStyle.windows
+          : FileSystemStyle.posix,
+      );
+      testFileSystem.currentDirectory = testFileSystem.systemTempDirectory.createTempSync('flutter_asset_bundle_test.');
+    });
+
+    testUsingContext('empty pubspec', () async {
+      globals.fs.file('pubspec.yaml')
+        ..createSync()
+        ..writeAsStringSync('');
+
+      final AssetBundle bundle = AssetBundleFactory.instance.createBundle();
+      await bundle.build(packagesPath: '.packages', targetPlatform: TargetPlatform.web_javascript);
+
+      expect(bundle.entries.keys,
+        unorderedEquals(<String>[
+          'AssetManifest.json',
+          'AssetManifest.bin',
+          'AssetManifest.bin.json',
+        ])
+      );
+      expect(
+        utf8.decode(await bundle.entries['AssetManifest.json']!.contentsAsBytes()),
+        '{}',
+      );
+      expect(
+        utf8.decode(await bundle.entries['AssetManifest.bin.json']!.contentsAsBytes()),
+        '""',
+      );
+    }, overrides: <Type, Generator>{
+      FileSystem: () => testFileSystem,
+      ProcessManager: () => FakeProcessManager.any(),
+    });
+
+    testUsingContext('pubspec contains an asset', () async {
+      globals.fs.file('.packages').createSync();
+      globals.fs.file('pubspec.yaml').writeAsStringSync(r'''
+name: test
+dependencies:
+  flutter:
+    sdk: flutter
+flutter:
+  assets:
+    - assets/bar/lizard.png
+''');
+      globals.fs.file(
+        globals.fs.path.joinAll(<String>['assets', 'bar', 'lizard.png'])
+      ).createSync(recursive: true);
+
+      final AssetBundle bundle = AssetBundleFactory.instance.createBundle();
+      await bundle.build(packagesPath: '.packages', targetPlatform: TargetPlatform.web_javascript);
+
+      expect(bundle.entries.keys,
+        unorderedEquals(<String>[
+          'AssetManifest.json',
+          'AssetManifest.bin',
+          'AssetManifest.bin.json',
+          'FontManifest.json',
+          'NOTICES', // not .Z
+          'assets/bar/lizard.png',
+        ])
+      );
+      final Map<Object?, Object?> manifestJson = json.decode(
+        utf8.decode(
+          await bundle.entries['AssetManifest.json']!.contentsAsBytes()
+        )
+      ) as Map<Object?, Object?>;
+      expect(manifestJson, isNotEmpty);
+      expect(manifestJson['assets/bar/lizard.png'], isNotNull);
+
+      final Uint8List manifestBinJsonBytes = base64.decode(
+        json.decode(
+          utf8.decode(
+            await bundle.entries['AssetManifest.bin.json']!.contentsAsBytes()
+          )
+        ) as String
+      );
+
+      final Uint8List manifestBinBytes = Uint8List.fromList(
+        await bundle.entries['AssetManifest.bin']!.contentsAsBytes()
+      );
+
+      expect(manifestBinJsonBytes, equals(manifestBinBytes),
+        reason: 'JSON-encoded binary content should be identical to BIN file.');
+    }, overrides: <Type, Generator>{
+      FileSystem: () => testFileSystem,
+      ProcessManager: () => FakeProcessManager.any(),
+    });
+  });
+
   testUsingContext('Failed directory delete shows message', () async {
     final FileExceptionHandler handler = FileExceptionHandler();
     final FileSystem fileSystem = MemoryFileSystem.test(opHandle: handler.opHandle);