fix: handle unresolvable symbolic links (#280)

Fixes #269

packages/melos/lib/src/common/utils.dart

@@ -407,7 +407,7 @@ extension DirectoryUtils on Directory {
           }
         } else if (entity is Link) {
           if (!followLinks ||
-              visitedLinks.contains(await entity.resolveSymbolicLinks())) {
+              visitedLinks.contains(await entity.tryResolveSymbolicLinks())) {
             yield entity;
             break;
           }
@@ -440,6 +440,21 @@ extension DirectoryUtils on Directory {
   }
 }
 
+extension FileSystemEntityUtils on FileSystemEntity {
+  /// Tries to resolve the path of this [FileSystemEntity] through
+  /// [resolveSymbolicLinks] and returns `null` if the path cannot be resolved.
+  ///
+  /// For example, a path cannot be resolved when it is a link to a non-existing
+  /// file.
+  Future<String?> tryResolveSymbolicLinks() async {
+    try {
+      return await resolveSymbolicLinks();
+    } on FileSystemException {
+      return null;
+    }
+  }
+}
+
 extension StreamUtils<T> on Stream<T> {
   /// Runs [convert] for each event in this stream and emits the result, while
   /// ensuring that no more events than specified by [parallelism] are being

packages/melos/test/workspace_test.dart

@@ -20,6 +20,7 @@ import 'package:melos/src/common/glob.dart';
 import 'package:melos/src/package.dart';
 import 'package:melos/src/workspace.dart';
 import 'package:melos/src/workspace_configs.dart';
+import 'package:path/path.dart' as p;
 import 'package:pubspec/pubspec.dart';
 import 'package:test/test.dart';
 
@@ -135,6 +136,19 @@ The packages that caused the problem are:
       }),
     );
 
+    test('load workspace config when workspace contains broken symlink',
+        () async {
+      final workspaceDir = createTemporaryWorkspaceDirectory();
+
+      final link = Link(p.join(workspaceDir.path, 'link'));
+      await link.create(p.join(workspaceDir.path, 'does-not-exist'));
+
+      await MelosWorkspace.fromConfig(
+        await MelosWorkspaceConfig.fromDirectory(workspaceDir),
+        logger: TestLogger(),
+      );
+    });
+
     group('package filtering', () {
       group('--include-dependencies', () {
         test(