add test for dangling fragment behavior

Author: leotianlizhan

firebase-perf/src/main/java/com/google/firebase/perf/application/FrameMetricsRecorder.java

@@ -89,7 +89,7 @@ public Optional<PerfFrameMetrics> stop() {
       return Optional.absent();
     }
     if (!fragmentSnapshotMap.isEmpty()) {
-      // Instrumentation will be stopped and return activity data. Dangling fragments will fail.
+      // Instrumentation stops and still return activity data, but invalidate all dangling fragments
       logger.debug(
           "Sub-recordings are still ongoing! Sub-recordings should be stopped first before stopping Activity screen trace.");
       fragmentSnapshotMap.clear();

firebase-perf/src/test/java/com/google/firebase/perf/application/FrameMetricsRecorderTest.java

@@ -199,6 +199,27 @@ public void startAndStopSubTrace_whenTwoSubTracesOverlap_returnsCorrectResults()
     assertThat(subTrace2.get().getFrozenFrames()).isEqualTo(5 - 2);
   }
 
+  /**
+   * This case happens when AppStateMonitor calls stop() before all Fragment traces are stopped by
+   * stopFragment(fragment), leaving some dangling fragment traces that are invalid. Even if
+   * activity recording starts again later, then stopFragment(fragment) is called, it should not
+   * return a result which is incorrect.
+   */
+  @Test
+  public void
+      startAndStopSubTrace_notContainedWithinActivityRecordingStartAndStop_returnsEmptyResult() {
+    Fragment fragment = new Fragment();
+    recorder.start();
+    stubFrameMetricsAggregatorData(fma, frameTimesDefault);
+    recorder.startFragment(fragment);
+    stubFrameMetricsAggregatorData(fma, frameTimes1);
+    recorder.stop();
+    recorder.start();
+    stubFrameMetricsAggregatorData(fma, frameTimes2);
+    Optional<PerfFrameMetrics> result = recorder.stopFragment(fragment); // invalid dangling trace
+    assertThat(result.isAvailable()).isFalse();
+  }
+
   private static Activity createFakeActivity(boolean isHardwareAccelerated) {
     ActivityController<Activity> fakeActivityController = Robolectric.buildActivity(Activity.class);