Fix package root resolution after build_runner 2.14.0 AOT changes (#750)

## What does this change?

This fixes package root resolution in `flutter_gen_runner` after the
`build_runner 2.14.0` change that runs `build` / `watch` through AOT by
default.

Previously, `FlutterGenBuilder` relied on `Isolate.packageConfigSync!`
as the only source for package roots. With the newer AOT build-script
flow, the runtime isolate package config can be null or no longer
represent the active build graph, which may crash the builder or fail to
resolve the target package root, especially on Windows.

The package root is now resolved in this order:

1. Use `BuildStep.packageConfig` as the primary source for the active
build graph.
2. Fall back to the runtime isolate package config when available.
3. Fall back to discovering the package from
`.dart_tool/package_config.json` or a matching `pubspec.yaml` under the
current working directory.

This also removes the eager static runtime package config load, avoiding
the null assertion crash from `Isolate.packageConfigSync!`.

Fixes #749 🎯

Author: Carapacik

packages/runner/lib/flutter_gen_runner.dart

@@ -1,5 +1,5 @@
 import 'dart:convert';
-import 'dart:io' show File;
+import 'dart:io' show Directory, File;
 import 'dart:isolate';
 
 import 'package:build/build.dart';
@@ -41,17 +41,6 @@ class FlutterGenBuilder extends Builder {
 
   final BuilderOptions _options;
 
-  /// We resolve package roots from the runtime package configuration of the
-  /// build script isolate.
-  ///
-  /// `buildStep.packageConfig` is package-aware, but the URIs exposed there are
-  /// asset-style URIs. The legacy generator code still needs a real file-system
-  /// root so it can reuse the existing `dart:io` based generation pipeline.
-  /// Loading the isolate package config once gives us stable `file:` package
-  /// roots for all packages in the build graph, including workspace members.
-  static final Future<PackageConfig> _runtimePackageConfig =
-      loadPackageConfigUri(Isolate.packageConfigSync!);
-
   @override
   Future<void> build(BuildStep buildStep) async {
     // Resolve the package being built from the current BuildStep instead of the
@@ -90,16 +79,16 @@ class FlutterGenBuilder extends Builder {
     );
 
     // `Config` still carries a `File pubspecFile` because the lower-level core
-    // generator APIs remain file-system based. We construct a package-local file
-    // path here after resolving the correct package root above.
+    // generator APIs remain file-system based. We construct a package-local
+    // file path here after resolving the correct package root above.
     final pubspecFile = File(join(packageRoot, 'pubspec.yaml'));
     final config = loadPubspecConfigFromInputOrNull(
       ConfigLoadInput(
         pubspecFile: pubspecFile,
         pubspecContent: pubspecContent,
-        // BuilderOptions are now the supported way to pass build.yaml options in
-        // the build_runner path. This keeps config target-local and workspace
-        // aware without relying on process cwd.
+        // BuilderOptions are now the supported way to pass build.yaml options
+        // in the build_runner path. This keeps config target-local and
+        // workspace aware without relying on process cwd.
         buildOptions: _options.config,
         pubspecLockContent: pubspecLockContent,
         analysisOptionsContent: analysisOptionsContent,
@@ -143,7 +132,7 @@ class FlutterGenBuilder extends Builder {
       writer: (contents, path) {
         outputs.add(
           FlutterGenManifestOutput(
-            path: relative(path, from: packageRoot),
+            path: _packageRelativePath(path, from: packageRoot),
             contents: contents,
           ),
         );
@@ -214,14 +203,125 @@ class FlutterGenBuilder extends Builder {
   }
 
   Future<String> _packageRoot(BuildStep buildStep) async {
-    final packageConfig = await _runtimePackageConfig;
-    final package = packageConfig[buildStep.inputId.package];
-    if (package == null) {
-      throw StateError(
-        'Unable to resolve package root for ${buildStep.inputId.package}.',
+    final packageName = buildStep.inputId.package;
+
+    // Prefer build_runner's package config because it describes the active
+    // build graph, even when the build script runs in an AOT isolate.
+    final buildStepPackageConfig = await buildStep.packageConfig;
+    final buildStepPackage = buildStepPackageConfig[packageName];
+    final buildStepRoot = _tryFilePath(buildStepPackage?.root);
+    if (buildStepRoot != null) {
+      return normalize(buildStepRoot);
+    }
+
+    // Fall back to the runtime isolate config when it still exposes a usable
+    // `file:` root for the target package.
+    final runtimePackageConfig = await _loadRuntimePackageConfig();
+    final runtimePackage = runtimePackageConfig?[packageName];
+    final runtimeRoot = _tryFilePath(runtimePackage?.root);
+    if (runtimeRoot != null) {
+      return normalize(runtimeRoot);
+    }
+
+    // Last resort for AOT/Windows edge cases: discover the package from the
+    // current working directory and nearby package config files.
+    final discoveredRoot = await _findPackageRootFromCwd(packageName);
+    if (discoveredRoot != null) {
+      return normalize(discoveredRoot);
+    }
+
+    throw StateError(
+      'Unable to resolve package root for $packageName. '
+      'buildStep.packageConfig, runtime package config, and current working '
+      'directory discovery did not provide a usable file: root.',
+    );
+  }
+
+  Future<PackageConfig?> _loadRuntimePackageConfig() async {
+    final packageConfigUri = Isolate.packageConfigSync;
+    if (packageConfigUri == null) {
+      return null;
+    }
+    return loadPackageConfigUri(packageConfigUri);
+  }
+
+  String? _tryFilePath(Uri? uri) {
+    if (uri == null || uri.scheme != 'file') {
+      return null;
+    }
+
+    try {
+      return uri.toFilePath();
+    } on UnsupportedError {
+      return null;
+    }
+  }
+
+  String _packageRelativePath(String path, {required String from}) {
+    return split(relative(path, from: from)).join('/');
+  }
+
+  Future<String?> _findPackageRootFromCwd(String packageName) async {
+    var dir = Directory.current.absolute;
+
+    while (true) {
+      final packageConfigRoot = await _findPackageRootFromPackageConfig(
+        packageName,
+        File(join(dir.path, '.dart_tool', 'package_config.json')),
       );
+      if (packageConfigRoot != null) {
+        return packageConfigRoot;
+      }
+
+      final pubspecRoot = _findPackageRootFromPubspec(
+        packageName,
+        File(join(dir.path, 'pubspec.yaml')),
+      );
+      if (pubspecRoot != null) {
+        return pubspecRoot;
+      }
+
+      final parent = dir.parent;
+      if (parent.path == dir.path) {
+        return null;
+      }
+      dir = parent;
     }
-    return normalize(package.root.toFilePath());
+  }
+
+  Future<String?> _findPackageRootFromPackageConfig(
+    String packageName,
+    File packageConfigFile,
+  ) async {
+    if (!packageConfigFile.existsSync()) {
+      return null;
+    }
+
+    try {
+      final packageConfig = await loadPackageConfigUri(packageConfigFile.uri);
+      return _tryFilePath(packageConfig[packageName]?.root);
+    } on Object {
+      return null;
+    }
+  }
+
+  String? _findPackageRootFromPubspec(
+    String packageName,
+    File pubspecFile,
+  ) {
+    if (!pubspecFile.existsSync()) {
+      return null;
+    }
+
+    try {
+      final pubspec = loadYaml(pubspecFile.readAsStringSync());
+      if (pubspec is YamlMap && pubspec['name'] == packageName) {
+        return pubspecFile.parent.path;
+      }
+    } on Object {
+      return null;
+    }
+    return null;
   }
 
   /// Reads an optional package-local asset if it exists.
@@ -292,8 +392,8 @@ class FlutterGenPostProcessBuilder extends PostProcessBuilder {
     final nextOutputs = manifest.outputs.map((output) => output.path).toSet();
 
     // Explicit stale cleanup is required because these source outputs are not
-    // regular declared outputs of the original builder. build_runner manages the
-    // manifest lifecycle, but FlutterGen owns the lifecycle of the final
+    // regular declared outputs of the original builder. build_runner manages
+    // the manifest lifecycle, but FlutterGen owns the lifecycle of the final
     // materialized files.
     for (final output in previousOutputs.difference(nextOutputs)) {
       final absolutePath = normalize(join(manifest.packageRoot, output));
@@ -309,8 +409,8 @@ class FlutterGenPostProcessBuilder extends PostProcessBuilder {
     // Materialize the exact output set described by the manifest.
     //
     // These files are intentionally managed outside build_runner's declared
-    // output model because their paths are configuration-dependent. Writing them
-    // directly avoids `InvalidOutputException` when the same files already
+    // output model because their paths are configuration-dependent. Writing
+    // them directly avoids `InvalidOutputException` when the same files already
     // exist, for example after a previous `fluttergen` command run or a stale
     // checked-out generated file.
     for (final output in manifest.outputs) {
@@ -344,7 +444,11 @@ class FlutterGenPostProcessBuilder extends PostProcessBuilder {
     if (paths is! List) {
       return <String>{};
     }
-    return paths.whereType<String>().toSet();
+    return paths.whereType<String>().map(_packageRelativePath).toSet();
+  }
+
+  String _packageRelativePath(String path) {
+    return split(path).join('/');
   }
 
   /// Guards cleanup against deleting files outside the active package.
@@ -356,10 +460,11 @@ class FlutterGenPostProcessBuilder extends PostProcessBuilder {
   }
 }
 
-/// Self-contained description of the desired FlutterGen outputs for one package.
+/// Self-contained description of the desired FlutterGen outputs for one
+/// package.
 ///
-/// The manifest is designed to be replayable by the post-process builder with no
-/// additional context: package root, package name, and final rendered file
+/// The manifest is designed to be replayable by the post-process builder with
+/// no additional context: package root, package name, and final rendered file
 /// contents are all embedded here.
 class FlutterGenManifest {
   const FlutterGenManifest({

packages/runner/pubspec.yaml

@@ -11,7 +11,7 @@ environment:
 
 dependencies:
   flutter_gen_core: 5.14.0
-  build: '>=2.0.0 <5.0.0'
+  build: ^4.0.0
   build_config: ^1.1.2
   collection: ^1.17.0
   crypto: ^3.0.0

packages/runner/test/workspace_build_test.dart

@@ -125,7 +125,9 @@ void main() {
         'lib/alt_gen/fonts.gen.dart',
       ]),
     );
-  });
+  },
+    timeout: const Timeout(Duration(minutes: 5)),
+  );
 
   test('applies package build.yaml options in workspace mode', () async {
     final workspaceDir = await _createWorkspaceFixture();
@@ -184,7 +186,9 @@ targets:
       File(p.join(appDir.path, 'lib', 'gen', 'assets.gen.dart')).existsSync(),
       isFalse,
     );
-  });
+  },
+    timeout: const Timeout(Duration(minutes: 5)),
+  );
 
   test('overwrites existing generated files in workspace mode', () async {
     final workspaceDir = await _createWorkspaceFixture();
@@ -224,7 +228,9 @@ targets:
       generatedFile.readAsStringSync(),
       isNot(contains('// stale contents')),
     );
-  });
+  },
+    timeout: const Timeout(Duration(minutes: 5)),
+  );
 }
 
 Future<Directory> _createWorkspaceFixture() async {