Allow multi-root web builds (#34896)

* fix multiroot builds

* remove shout

* add basic test

* Update web_compilation_delegate.dart

* Address review feedback

* Update multiroot_asset_reader_test.dart

* Update multiroot_asset_reader_test.dart

* remove unpassable test

* fix for windows

* facepalm

packages/flutter_tools/lib/src/build_runner/web_compilation_delegate.dart

@@ -4,6 +4,7 @@
 
 // ignore_for_file: implementation_imports
 import 'dart:async';
+import 'dart:io' as io; // ignore: dart_io_import
 
 import 'package:build/build.dart';
 import 'package:build_runner_core/build_runner_core.dart' as core;
@@ -11,6 +12,7 @@ import 'package:build_runner_core/src/asset_graph/graph.dart';
 import 'package:build_runner_core/src/asset_graph/node.dart';
 import 'package:build_runner_core/src/generate/build_impl.dart';
 import 'package:build_runner_core/src/generate/options.dart';
+import 'package:glob/glob.dart';
 import 'package:logging/logging.dart';
 import 'package:meta/meta.dart';
 import 'package:path/path.dart' as path;
@@ -21,6 +23,7 @@ import '../base/file_system.dart';
 import '../base/logger.dart';
 import '../base/platform.dart';
 import '../compile.dart';
+import '../convert.dart';
 import '../dart/package_map.dart';
 import '../globals.dart';
 import '../web/compile.dart';
@@ -42,6 +45,10 @@ class BuildRunnerWebCompilationProxy extends WebCompilationProxy {
   }) async {
     // Create the .dart_tool directory if it doesn't exist.
     projectDirectory.childDirectory('.dart_tool').createSync();
+    final Directory generatedDirectory = projectDirectory
+      .childDirectory('.dart_tool')
+      .childDirectory('build')
+      .childDirectory('generated');
 
     // Override the generated output directory so this does not conflict with
     // other build_runner output.
@@ -56,7 +63,7 @@ class BuildRunnerWebCompilationProxy extends WebCompilationProxy {
       } else {
         printTrace(record.message);
       }
-    });
+    }, reader: MultirootFileBasedAssetReader(_packageGraph, generatedDirectory));
     final LogSubscription logSubscription = LogSubscription(
       buildEnvironment,
       verbose: false,
@@ -197,3 +204,85 @@ class BuildRunnerWebCompilationProxy extends WebCompilationProxy {
     }
   }
 }
+
+/// Handles mapping a single root file scheme to a multiroot scheme.
+///
+/// This allows one build_runner build to read the output from a previous
+/// isolated build.
+class MultirootFileBasedAssetReader extends core.FileBasedAssetReader {
+  MultirootFileBasedAssetReader(
+    core.PackageGraph packageGraph,
+    this.generatedDirectory,
+  ) : super(packageGraph);
+
+  final Directory generatedDirectory;
+
+  @override
+  Future<bool> canRead(AssetId id) {
+    if (packageGraph[id.package] == packageGraph.root && _missingSource(id)) {
+      return _generatedFile(id).exists();
+    }
+    return super.canRead(id);
+  }
+
+  @override
+  Future<List<int>> readAsBytes(AssetId id) {
+    if (packageGraph[id.package] == packageGraph.root && _missingSource(id)) {
+      return _generatedFile(id).readAsBytes();
+    }
+    return super.readAsBytes(id);
+  }
+
+  @override
+  Future<String> readAsString(AssetId id, {Encoding encoding}) {
+    if (packageGraph[id.package] == packageGraph.root && _missingSource(id)) {
+      return _generatedFile(id).readAsString();
+    }
+    return super.readAsString(id, encoding: encoding);
+  }
+
+  @override
+  Stream<AssetId> findAssets(Glob glob, {String package}) async* {
+    if (package == null || packageGraph.root.name == package) {
+      await for (io.FileSystemEntity entity in glob.list(followLinks: true, root: packageGraph.root.path)) {
+        if (entity is io.File && _isNotHidden(entity)) {
+          yield _fileToAssetId(entity, packageGraph.root);
+        }
+      }
+      final String generatedRoot = fs.path.join(
+        generatedDirectory.path, packageGraph.root.name
+      );
+      if (!fs.isDirectorySync(generatedRoot)) {
+        return;
+      }
+      await for (io.FileSystemEntity entity in glob.list(followLinks: true, root: generatedRoot)) {
+        if (entity is io.File && _isNotHidden(entity)) {
+          yield _fileToAssetId(entity, packageGraph.root, generatedRoot);
+        }
+      }
+      return;
+    }
+    yield* super.findAssets(glob, package: package);
+  }
+
+  bool _isNotHidden(io.FileSystemEntity entity) {
+    return !path.basename(entity.path).startsWith('._');
+  }
+
+  bool _missingSource(AssetId id) {
+    return !fs.file(path.joinAll(<String>[packageGraph.root.path, ...id.pathSegments])).existsSync();
+  }
+
+  File _generatedFile(AssetId id) {
+    return fs.file(
+      path.joinAll(<String>[generatedDirectory.path, packageGraph.root.name, ...id.pathSegments])
+    );
+  }
+
+  /// Creates an [AssetId] for [file], which is a part of [packageNode].
+  AssetId _fileToAssetId(io.File file, core.PackageNode packageNode, [String root]) {
+    final String filePath = path.normalize(file.absolute.path);
+    final String relativePath = path.relative(filePath, from: root ?? packageNode.path);
+    return AssetId(packageNode.name, relativePath);
+  }
+}

packages/flutter_tools/test/build_runner/multiroot_asset_reader_test.dart

+// Copyright 2019 The Chromium Authors. All rights reserved.
+// 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:build/build.dart';
+import 'package:build_runner_core/build_runner_core.dart';
+import 'package:flutter_tools/src/base/file_system.dart';
+import 'package:flutter_tools/src/build_runner/web_compilation_delegate.dart';
+
+import '../src/common.dart';
+import '../src/testbed.dart';
+
+void main() {
+  group('MultirootFileBasedAssetReader', () {
+    Testbed testbed;
+    FakePackageGraph packageGraph;
+
+    setUp(() {
+      testbed = Testbed(setup: () {
+        final PackageNode root = PackageNode('foobar', fs.currentDirectory.path, DependencyType.path);
+        packageGraph = FakePackageGraph(root, <String, PackageNode>{'foobar': root});
+        fs.file(fs.path.join('lib', 'main.dart'))
+          ..createSync(recursive: true)
+          ..writeAsStringSync('main');
+        fs.file(fs.path.join('.dart_tool', 'build', 'generated', 'foobar', 'lib', 'bar.dart'))
+          ..createSync(recursive: true)
+          ..writeAsStringSync('bar');
+        fs.file('pubspec.yaml')
+          ..createSync()
+          ..writeAsStringSync('name: foobar');
+      });
+    });
+
+    test('Can find assets from the generated directory', () => testbed.run(() async {
+      final MultirootFileBasedAssetReader reader = MultirootFileBasedAssetReader(
+        packageGraph,
+        fs.directory(fs.path.join('.dart_tool', 'build', 'generated'))
+      );
+
+      // Note: we can't read from the regular directory because the default
+      // asset reader uses the regular file system.
+      expect(await reader.canRead(AssetId('foobar', 'lib/bar.dart')), true);
+      expect(await reader.readAsString(AssetId('foobar', 'lib/bar.dart')), 'bar');
+      expect(await reader.readAsBytes(AssetId('foobar', 'lib/bar.dart')), utf8.encode('bar'));
+    }));
+  });
+}
+
+class FakePackageGraph implements PackageGraph {
+  FakePackageGraph(this.root, this.allPackages);
+
+  @override
+  final Map<String, PackageNode> allPackages;
+
+  @override
+  final PackageNode root;
+
+  @override
+  PackageNode operator [](String packageName) => allPackages[packageName];
+}

packages/flutter_tools/test/src/testbed.dart

@@ -5,6 +5,7 @@
 import 'dart:async';
 
 import 'package:file/memory.dart';
+import 'package:flutter_tools/src/base/platform.dart';
 import 'package:flutter_tools/src/base/context.dart';
 import 'package:flutter_tools/src/base/file_system.dart';
 import 'package:flutter_tools/src/base/logger.dart';
@@ -22,7 +23,8 @@ export 'package:flutter_tools/src/base/context.dart' show Generator;
 //      [BufferLogger], [MemoryFileSystem].
 //    - More TBD.
 final Map<Type, Generator> _testbedDefaults = <Type, Generator>{
-  FileSystem: () => MemoryFileSystem(), // Keeps tests fast by avoid actual file system.
+  // Keeps tests fast by avoid actual file system.
+  FileSystem: () => MemoryFileSystem(style: platform.isWindows ? FileSystemStyle.windows : FileSystemStyle.posix),
   Logger: () => BufferLogger(), // Allows reading logs and prevents stdout.
   OutputPreferences: () => OutputPreferences(showColor: false), // configures BufferLogger to avoid color codes.
   Usage: () => NoOpUsage(), // prevent addition of analytics from burdening test mocks