ensure test isolate is paused before collecting coverage (#35188)

packages/flutter_tools/lib/src/test/coverage_collector.dart

@@ -47,16 +47,16 @@ class CoverageCollector extends TestWatcher {
   /// has been run to completion so that all coverage data has been recorded.
   ///
   /// The returned [Future] completes when the coverage is collected.
-  Future<void> collectCoverageIsolate(Uri observatoryUri) async {
+  Future<void> collectCoverageIsolate(Uri observatoryUri, String debugName) async {
     assert(observatoryUri != null);
-    printTrace('collecting coverage data from $observatoryUri...');
+    print('collecting coverage data from $observatoryUri...');
     final Map<String, dynamic> data = await collect(observatoryUri, (String libraryName) {
       // If we have a specified coverage directory or could not find the package name, then
       // accept all libraries.
       return (coverageDirectory != null)
           || (flutterProject == null)
           || libraryName.contains(flutterProject.manifest.appName);
-    });
+    }, waitPaused: true, debugName: debugName);
     if (data == null) {
       throw Exception('Failed to collect coverage.');
     }
@@ -185,13 +185,39 @@ class CoverageCollector extends TestWatcher {
 }
 
 Future<VMService> _defaultConnect(Uri serviceUri) {
-  return VMService.connect(serviceUri, compression: CompressionOptions.compressionOff);
+  return VMService.connect(
+      serviceUri, compression: CompressionOptions.compressionOff);
 }
 
-Future<Map<String, dynamic>> collect(Uri serviceUri, bool Function(String) libraryPredicate,
-    [Future<VMService> Function(Uri) connector = _defaultConnect]) async {
+Future<Map<String, dynamic>> collect(Uri serviceUri, bool Function(String) libraryPredicate, {
+  bool waitPaused = false,
+  String debugName,
+  Future<VMService> Function(Uri) connector = _defaultConnect,
+}) async {
   final VMService vmService = await connector(serviceUri);
   await vmService.getVM();
+  final Isolate isolate = vmService.vm.isolates.firstWhere((Isolate isolate) => isolate.name == debugName);
+  if (!waitPaused) {
+    return _getAllCoverage(vmService, libraryPredicate);
+  }
+  const int kPollAttempts = 20;
+  int i = 0;
+  while (i < kPollAttempts) {
+    await isolate.load();
+    if (isolate.pauseEvent?.kind == ServiceEvent.kPauseStart) {
+      break;
+    }
+    await Future<void>.delayed(const Duration(milliseconds: 50));
+    i += 1;
+  }
+  if (i == kPollAttempts) {
+    print('Isolate $debugName was never paused, refusing to collect coverage');
+    return const <String, dynamic>{
+      'type': 'CodeCoverage',
+      'coverage': <Object>[]
+    };
+  }
+  print('isolate is paused, collecting coverage...');
   return _getAllCoverage(vmService, libraryPredicate);
 }
 

packages/flutter_tools/test/coverage_collector_test.dart

@@ -23,7 +23,7 @@ void main() {
       .thenAnswer((Invocation invocation) async {
         return <String, Object>{'type': 'Sentinel', 'kind': 'Collected', 'valueAsString': '<collected>'};
       });
-    final Map<String, Object> result = await collect(null, (String predicate) => true, (Uri uri) async {
+    final Map<String, Object> result = await collect(null, (String predicate) => true, connector: (Uri uri) async {
       return mockVMService;
     });
 

packages/flutter_tools/tool/tool_coverage.dart

@@ -77,8 +77,10 @@ class VMPlatform extends PlatformPlugin {
     final dynamic channel = IsolateChannel<Object>.connectReceive(receivePort)
         .transformStream(StreamTransformer<Object, Object>.fromHandlers(handleDone: (EventSink<Object> sink) async {
       try {
+        // Pause the isolate so it is ready for coverage collection.
+        isolate.pause();
         // this will throw if collection fails.
-        await coverageCollector.collectCoverageIsolate(info.serverUri);
+        await coverageCollector.collectCoverageIsolate(info.serverUri, path);
       } finally {
         isolate.kill(priority: Isolate.immediate);
         isolate = null;
@@ -115,6 +117,7 @@ class VMPlatform extends PlatformPlugin {
     return await Isolate.spawnUri(p.toUri(testPath), <String>[], message,
       packageConfig: p.toUri('.packages'),
       checked: true,
+      debugName: path,
     );
   }