log full timeline when transition test fails; print stack chain in task errors (#6772)

dev/devicelab/lib/framework/framework.dart

@@ -9,6 +9,7 @@ import 'dart:io';
 import 'dart:isolate';
 
 import 'package:logging/logging.dart';
+import 'package:stack_trace/stack_trace.dart';
 
 import 'utils.dart';
 
@@ -115,16 +116,24 @@ class _TaskRunner {
     _keepAlivePort?.close();
   }
 
-  Future<TaskResult> _performTask() async {
-    try {
-      return await task();
-    } catch (taskError, taskErrorStack) {
+  Future<TaskResult> _performTask() {
+    Completer<TaskResult> completer = new Completer<TaskResult>();
+    Chain.capture(() async {
+      completer.complete(await task());
+    }, onError: (dynamic taskError, Chain taskErrorStack) {
       String message = 'Task failed: $taskError';
-      if (taskErrorStack != null) {
-        message += '\n\n$taskErrorStack';
-      }
-      return new TaskResult.failure(message);
-    }
+      stderr
+        ..writeln(message)
+        ..writeln('\nStack trace:')
+        ..writeln(taskErrorStack.terse);
+      // IMPORTANT: We're completing the future _successfully_ but with a value
+      // that indicates a task failure. This is intentional. At this point we
+      // are catching errors coming from arbitrary (and untrustworthy) task
+      // code. Our goal is to convert the failure into a readable message.
+      // Propagating it further is not useful.
+      completer.complete(new TaskResult.failure(message));
+    });
+    return completer.future;
   }
 }
 

examples/flutter_gallery/test_driver/transitions_perf_test.dart

@@ -59,6 +59,8 @@ final List<String> demoTitles = <String>[
   'Typography'
 ];
 
+final FileSystem _fs = new LocalFileSystem();
+
 Future<Null> saveDurationsHistogram(List<Map<String, dynamic>> events) async {
   final Map<String, List<int>> durations = new Map<String, List<int>>();
   Map<String, dynamic> startEvent;
@@ -87,9 +89,8 @@ Future<Null> saveDurationsHistogram(List<Map<String, dynamic>> events) async {
 
   // Save the durations Map to a file.
   final String destinationDirectory = 'build';
-  final FileSystem fs = new LocalFileSystem();
-  await fs.directory(destinationDirectory).create(recursive: true);
-  final File file = fs.file(path.join(destinationDirectory, 'transition_durations.timeline.json'));
+  await _fs.directory(destinationDirectory).create(recursive: true);
+  final File file = _fs.file(path.join(destinationDirectory, 'transition_durations.timeline.json'));
   await file.writeAsString(new JsonEncoder.withIndent('  ').convert(durations));
 }
 
@@ -136,10 +137,16 @@ void main() {
       // Save the duration (in microseconds) of the first timeline Frame event
       // that follows a 'Start Transition' event. The Gallery app adds a
       // 'Start Transition' event when a demo is launched (see GalleryItem).
-      saveDurationsHistogram(timeline.json['traceEvents']);
-
       TimelineSummary summary = new TimelineSummary.summarize(timeline);
-      summary.writeSummaryToFile('transitions');
+      summary.writeSummaryToFile('transitions', pretty: true);
+      try {
+        saveDurationsHistogram(timeline.json['traceEvents']);
+      } catch(_) {
+        summary.writeTimelineToFile('transitions', pretty: true);
+        print('ERROR: failed to extract transition events. Here is the full timeline:\n');
+        print(await _fs.file('build/transitions.timeline.json').readAsString());
+        rethrow;
+      }
 
     }, timeout: new Timeout(new Duration(minutes: 5)));
   });