Add trace logging to local engine path autodetection (#78482)

packages/flutter_tools/lib/src/runner/local_engine.dart

@@ -66,6 +66,7 @@ class LocalEngineLocator {
         Uri engineUri = packageConfig[kFlutterEnginePackageName]?.packageUriRoot;
         final String cachedPath = _fileSystem.path.join(_flutterRoot, 'bin', 'cache', 'pkg', kFlutterEnginePackageName, 'lib');
         if (engineUri != null && _fileSystem.identicalSync(cachedPath, engineUri.path)) {
+          _logger.printTrace('Local engine auto-detection sky_engine in $packagePath is the same version in bin/cache.');
           engineUri = null;
         }
         // If sky_engine is specified and the engineSourcePath not set, try to
@@ -92,7 +93,8 @@ class LocalEngineLocator {
             );
           }
         }
-      } on FileSystemException {
+      } on FileSystemException catch (e) {
+        _logger.printTrace('Local engine auto-detection file exception: $e');
         engineSourcePath = null;
       }
       // If engineSourcePath is still not set, try to determine it by flutter root.
@@ -109,7 +111,10 @@ class LocalEngineLocator {
     }
 
     if (engineSourcePath != null) {
+      _logger.printTrace('Local engine source at $engineSourcePath');
       return _findEngineBuildPath(localEngine, engineSourcePath);
+    } else if (localEngine != null) {
+      _logger.printTrace('Could not find engine source for $localEngine, skipping');
     }
     return null;
   }

packages/flutter_tools/test/general.shard/runner/local_engine_test.dart

@@ -36,10 +36,11 @@ void main() {
       .file('bin/cache/pkg/sky_engine/lib')
       .createSync(recursive: true);
 
+    final BufferLogger logger = BufferLogger.test();
     final LocalEngineLocator localEngineLocator = LocalEngineLocator(
       fileSystem: fileSystem,
       flutterRoot: '',
-      logger: BufferLogger.test(),
+      logger: logger,
       userMessages: UserMessages(),
       platform: FakePlatform(environment: <String, String>{}),
     );
@@ -51,6 +52,7 @@ void main() {
         targetEngine: '/arbitrary/engine/src/out/ios_debug',
       ),
     );
+    expect(logger.traceText, contains('Local engine source at /arbitrary/engine/src'));
 
     // Verify that this also works if the sky_engine path is a symlink to the engine root.
     fileSystem.link('/symlink').createSync(kArbitraryEngineRoot);
@@ -65,6 +67,7 @@ void main() {
         targetEngine: '/symlink/src/out/ios_debug',
       ),
     );
+    expect(logger.traceText, contains('Local engine source at /symlink/src'));
   });
 
   testWithoutContext('works if --local-engine is specified and --local-engine-src-path '
@@ -74,10 +77,11 @@ void main() {
     fileSystem.directory('$kArbitraryEngineRoot/src/out/ios_debug').createSync(recursive: true);
     fileSystem.directory('$kArbitraryEngineRoot/src/out/host_debug').createSync(recursive: true);
 
+    final BufferLogger logger = BufferLogger.test();
     final LocalEngineLocator localEngineLocator = LocalEngineLocator(
       fileSystem: fileSystem,
       flutterRoot: '',
-      logger: BufferLogger.test(),
+      logger: logger,
       userMessages: UserMessages(),
       platform: FakePlatform(environment: <String, String>{}),
     );
@@ -89,6 +93,7 @@ void main() {
         targetEngine: '/arbitrary/engine/src/out/ios_debug',
       ),
     );
+    expect(logger.traceText, contains('Local engine source at /arbitrary/engine/src'));
   });
 
   testWithoutContext('works if --local-engine is specified and --local-engine-src-path '
@@ -105,10 +110,11 @@ void main() {
       .file('bin/cache/pkg/sky_engine/lib')
       .createSync(recursive: true);
 
+    final BufferLogger logger = BufferLogger.test();
     final LocalEngineLocator localEngineLocator = LocalEngineLocator(
       fileSystem: fileSystem,
       flutterRoot: 'flutter/flutter',
-      logger: BufferLogger.test(),
+      logger: logger,
       userMessages: UserMessages(),
       platform: FakePlatform(environment: <String, String>{}),
     );
@@ -120,6 +126,7 @@ void main() {
         targetEngine: 'flutter/engine/src/out/ios_debug',
       ),
     );
+    expect(logger.traceText, contains('Local engine source at flutter/engine/src'));
   });
 }