Do not wait for connections after process has exited (#54497)

dev/devicelab/lib/framework/runner.dart

@@ -40,12 +40,20 @@ Future<Map<String, dynamic>> runTask(
 
   bool runnerFinished = false;
 
+  final Completer<Uri> uri = Completer<Uri>();
+
   runner.exitCode.whenComplete(() {
+    if (!uri.isCompleted) {
+      // The runner process exited prematurely.
+      uri.completeError(Exception(
+        'The task runner process exited before opening a VM service connection. '
+        'A common cause for this is when a task script does not actually create '
+        'a task.',
+      ));
+    }
     runnerFinished = true;
   });
 
-  final Completer<Uri> uri = Completer<Uri>();
-
   final StreamSubscription<String> stdoutSub = runner.stdout
       .transform<String>(const Utf8Decoder())
       .transform<String>(const LineSplitter())
@@ -68,7 +76,7 @@ Future<Map<String, dynamic>> runTask(
   });
 
   try {
-    final VMIsolateRef isolate = await _connectToRunnerIsolate(await uri.future);
+    final VMIsolateRef isolate = await _connectToRunnerIsolate(await uri.future, () => !runnerFinished);
     final Map<String, dynamic> taskResult = await isolate.invokeExtension('ext.cocoonRunTask') as Map<String, dynamic>;
     await runner.exitCode;
     return taskResult;
@@ -81,7 +89,7 @@ Future<Map<String, dynamic>> runTask(
   }
 }
 
-Future<VMIsolateRef> _connectToRunnerIsolate(Uri vmServiceUri) async {
+Future<VMIsolateRef> _connectToRunnerIsolate(Uri vmServiceUri, bool Function() keepTrying) async {
   final List<String> pathSegments = <String>[
     // Add authentication code.
     if (vmServiceUri.pathSegments.isNotEmpty) vmServiceUri.pathSegments[0],
@@ -91,7 +99,7 @@ Future<VMIsolateRef> _connectToRunnerIsolate(Uri vmServiceUri) async {
       pathSegments).toString();
   final Stopwatch stopwatch = Stopwatch()..start();
 
-  while (true) {
+  while (keepTrying()) {
     try {
       // Make sure VM server is up by successfully opening and closing a socket.
       await (await WebSocket.connect(url)).close();
@@ -110,6 +118,8 @@ Future<VMIsolateRef> _connectToRunnerIsolate(Uri vmServiceUri) async {
       await Future<void>.delayed(const Duration(milliseconds: 50));
     }
   }
+
+  throw Exception('Failed to connect to Dart VM service.');
 }
 
 Future<void> cleanupSystem() async {

dev/devicelab/test/run_test.dart

@@ -24,43 +24,43 @@ void main() {
       return scriptProcess;
     }
 
-    Future<void> expectScriptResult(List<String> testNames, int expectedExitCode) async {
+    Future<void> expectScriptResult(List<String> testNames, { bool expectSuccess }) async {
       final ProcessResult result = await runScript(testNames);
-      expect(result.exitCode, expectedExitCode,
+      expect(result.exitCode, expectSuccess ? 0 : isNot(equals(0)),
           reason: '[ stderr from test process ]\n\n${result.stderr}\n\n[ end of stderr ]'
           '\n\n[ stdout from test process ]\n\n${result.stdout}\n\n[ end of stdout ]');
     }
 
     test('exits with code 0 when succeeds', () async {
-      await expectScriptResult(<String>['smoke_test_success'], 0);
+      await expectScriptResult(<String>['smoke_test_success'], expectSuccess: true);
     });
 
     test('accepts file paths', () async {
-      await expectScriptResult(<String>['bin/tasks/smoke_test_success.dart'], 0);
+      await expectScriptResult(<String>['bin/tasks/smoke_test_success.dart'], expectSuccess: true);
     });
 
     test('rejects invalid file paths', () async {
-      await expectScriptResult(<String>['lib/framework/adb.dart'], 1);
+      await expectScriptResult(<String>['lib/framework/adb.dart'], expectSuccess: false);
     });
 
     test('exits with code 1 when task throws', () async {
-      await expectScriptResult(<String>['smoke_test_throws'], 1);
+      await expectScriptResult(<String>['smoke_test_throws'], expectSuccess: false);
     });
 
     test('exits with code 1 when fails', () async {
-      await expectScriptResult(<String>['smoke_test_failure'], 1);
+      await expectScriptResult(<String>['smoke_test_failure'], expectSuccess: false);
     });
 
     test('exits with code 1 when fails to connect', () async {
-      await expectScriptResult(<String>['smoke_test_setup_failure'], 1);
-    }, skip: true); // https://github.com/flutter/flutter/issues/53707
+      await expectScriptResult(<String>['smoke_test_setup_failure'], expectSuccess: false);
+    });
 
     test('exits with code 1 when results are mixed', () async {
       await expectScriptResult(<String>[
           'smoke_test_failure',
           'smoke_test_success',
         ],
-        1,
+        expectSuccess: false,
       );
     });
   });