extension_discovery: add findPackageConfig() (#590)

pkgs/extension_discovery/CHANGELOG.md

@@ -2,6 +2,8 @@
 
 - Require Dart 3.4.
 - Update to the latest version of `package:dart_flutter_team_lints`.
+- Add `findPackageConfig()` allowing for automatic discovery of the
+  packageConfig based on a package directory.
 
 ## 2.0.0
 

pkgs/extension_discovery/lib/extension_discovery.dart

@@ -14,7 +14,7 @@ import 'src/package_config.dart';
 import 'src/registry.dart';
 import 'src/yaml_config_format.dart';
 
-export 'src/package_config.dart' show PackageConfigException;
+export 'src/package_config.dart' show PackageConfigException, findPackageConfig;
 
 /// Information about an extension for target package.
 final class Extension {
@@ -101,7 +101,7 @@ final class Extension {
 /// ### Caching results
 ///
 /// When [useCache] is `true` then the detected extensions will be cached
-/// in `.dart_tool/extension_discovery/<targetPackage>.yaml`.
+/// in `.dart_tool/extension_discovery/<targetPackage>.json`.
 /// This function will compare modification timestamps of
 /// `.dart_tool/package_config.json` with the cache file, before reusing cached
 /// results.

pkgs/extension_discovery/lib/src/package_config.dart

@@ -8,6 +8,36 @@ import 'expect_json.dart';
 typedef PackageConfigEntry = ({String name, Uri rootUri, Uri packageUri});
 typedef PackageConfig = List<PackageConfigEntry>;
 
+/// Searches in [packageDir] and all directories above for a
+/// `.dart_tool/package_config.json` file. Returns the Uri of that file if
+/// found.
+///
+/// Returns `null` if no file was found.
+Uri? findPackageConfig(Uri packageDir) {
+  if (!packageDir.isScheme('file')) {
+    throw ArgumentError(
+      'Expected [packageDir] to be a file URI, got $packageDir',
+    );
+  }
+  if (!packageDir.path.endsWith('/')) {
+    packageDir = packageDir.replace(path: '${packageDir.path}/');
+  }
+  while (true) {
+    final packageConfigCandidate =
+        packageDir.resolve('.dart_tool/package_config.json');
+    try {
+      if (File.fromUri(packageConfigCandidate).existsSync()) {
+        return packageConfigCandidate;
+      }
+    } on IOException {
+      return null; // if we get a permission error, etc, we return null
+    }
+    final next = packageDir.resolve('..');
+    if (next == packageDir) return null;
+    packageDir = next;
+  }
+}
+
 /// Load list of packages and associated URIs from the
 /// `.dart_tool/package_config.json` file in [packageConfigFile].
 Future<PackageConfig> loadPackageConfig(

pkgs/extension_discovery/test/package_config_test.dart

@@ -159,4 +159,34 @@ void main() {
       throwsA(isA<PackageConfigException>()),
     );
   });
+
+  test('`findPackageConfig()`', () async {
+    await d.dir('workspace', [
+      d.dir('.dart_tool', [
+        d.file('package_config.json'),
+      ]),
+      d.dir('myapp', [])
+    ]).create();
+
+    expect(
+      findPackageConfig(d.fileUri('workspace')),
+      d.fileUri('workspace/.dart_tool/package_config.json'),
+    );
+    expect(
+      findPackageConfig(d.fileUri('workspace/')),
+      d.fileUri('workspace/.dart_tool/package_config.json'),
+    );
+    expect(
+      findPackageConfig(d.fileUri('workspace/myapp')),
+      d.fileUri('workspace/.dart_tool/package_config.json'),
+    );
+    expect(
+      findPackageConfig(d.fileUri('.')),
+      isNull,
+    );
+    expect(
+      findPackageConfig(d.fileUri('foo')),
+      isNull,
+    );
+  });
 }

pkgs/extension_discovery/test/test_descriptor.dart

@@ -53,4 +53,4 @@ Future<String> dart([
 }
 
 Future<String> dartPubGet(String folder) async =>
-    await dart('pub', 'get', '-C', d.path('myapp'));
+    await dart('pub', 'get', '-C', folder);