Fix race condition in readJsonResults (#100243)

dev/devicelab/lib/framework/devices.dart

@@ -152,6 +152,14 @@ abstract class Device {
   /// with some prefix.
   Stream<String> get logcat;
 
+  /// Clears the device logs.
+  ///
+  /// This is important because benchmarks tests rely on the logs produced by
+  /// the flutter run command.
+  ///
+  /// On Android, those logs may contain logs from previous test.
+  Future<void> clearLogs();
+
   /// Whether this device supports calls to [startLoggingToSink]
   /// and [stopLoggingToSink].
   bool get canStreamLogs => false;
@@ -667,6 +675,11 @@ class AndroidDevice extends Device {
     }
   }
 
+  @override
+  Future<void> clearLogs() {
+    return adb(<String>['logcat', '-c']);
+  }
+
   @override
   Stream<String> get logcat {
     final Completer<void> stdoutDone = Completer<void>();
@@ -677,7 +690,7 @@ class AndroidDevice extends Device {
     late final StreamController<String> stream;
     stream = StreamController<String>(
       onListen: () async {
-        await adb(<String>['logcat', '-c']);
+        await clearLogs();
         final Process process = await startProcess(
           adbPath,
           // Make logcat less chatty by filtering down to just ActivityManager
@@ -971,6 +984,9 @@ class IosDevice extends Device {
     throw UnimplementedError();
   }
 
+  @override
+  Future<void> clearLogs() async {}
+
   @override
   Future<void> stop(String packageName) async {}
 
@@ -1007,6 +1023,9 @@ class WindowsDevice extends Device {
   @override
   Stream<String> get logcat => const Stream<String>.empty();
 
+  @override
+  Future<void> clearLogs() async {}
+
   @override
   Future<void> reboot() async { }
 
@@ -1074,6 +1093,9 @@ class FuchsiaDevice extends Device {
     throw UnimplementedError();
   }
 
+  @override
+  Future<void> clearLogs() async {}
+
   @override
   Future<void> reboot() async {
     // Unsupported.
@@ -1142,6 +1164,9 @@ class FakeDevice extends Device {
     throw UnimplementedError();
   }
 
+  @override
+  Future<void> clearLogs() async {}
+
   @override
   Future<void> stop(String packageName) async {}
 

dev/devicelab/lib/microbenchmarks.dart

@@ -31,14 +31,14 @@ Future<Map<String, double>> readJsonResults(Process process) {
       .transform<String>(const Utf8Decoder())
       .transform<String>(const LineSplitter())
       .listen((String line) async {
-    print(line);
+    print('[STDOUT] $line');
 
     if (line.contains(jsonStart)) {
       jsonStarted = true;
       return;
     }
 
-    if (line.contains(jsonEnd)) {
+    if (jsonStarted && line.contains(jsonEnd)) {
       final String jsonOutput = jsonBuf.toString();
 
       // If we end up here and have already parsed the results, it suggests that

dev/devicelab/lib/tasks/microbenchmarks.dart

@@ -19,6 +19,7 @@ TaskFunction createMicrobenchmarkTask() {
   return () async {
     final Device device = await devices.workingDevice;
     await device.unlock();
+    await device.clearLogs();
 
     Future<Map<String, double>> _runMicrobench(String benchmarkPath) async {
       Future<Map<String, double>> _run() async {