For #18836: prevent StartupActivityLog from growing infinitely.

We do this is as a separate commit over the original implementation
because it's simpler to implement the class without this optimization.

app/src/main/java/org/mozilla/fenix/perf/StartupActivityLog.kt

@@ -23,6 +23,10 @@ private val logger = Logger("StartupActivityLog")
  * A record of the [Activity] created, started, and stopped events as well as [Application]
  * foreground and background events. See [log] for the log. This class is expected to be
  * registered in [Application.onCreate] by calling [registerInAppOnCreate].
+ *
+ * To prevent this list from growing infinitely, we clear the list when the application is stopped.
+ * This is acceptable from the current requirements: we never need to inspect more than the current
+ * start up.
  */
 class StartupActivityLog {
 
@@ -66,8 +70,9 @@ class StartupActivityLog {
         }
 
         override fun onStop(owner: LifecycleOwner) {
-            _log.add(LogEntry.AppStopped)
             logEntries()
+            _log.clear() // Optimization: see class kdoc for details.
+            _log.add(LogEntry.AppStopped)
         }
     }
 

app/src/test/java/org/mozilla/fenix/perf/StartupActivityLogTest.kt

@@ -50,15 +50,20 @@ class StartupActivityLogTest {
     @Test // we test start and stop individually due to the clear-on-stop behavior.
     fun `WHEN app observer start is called THEN it is added directly to the log`() {
         assertTrue(log.log.isEmpty())
-        val expected = mutableListOf<LogEntry>()
 
         appObserver.onStart(mockk())
-        expected.add(LogEntry.AppStarted)
-        assertEquals(expected, log.log)
+        assertEquals(listOf(LogEntry.AppStarted), log.log)
+
+        appObserver.onStart(mockk())
+        assertEquals(listOf(LogEntry.AppStarted, LogEntry.AppStarted), log.log)
+    }
+
+    @Test // we test start and stop individually due to the clear-on-stop behavior.
+    fun `WHEN app observer stop is called THEN it is added directly to the log`() {
+        assertTrue(log.log.isEmpty())
 
         appObserver.onStop(mockk())
-        expected.add(LogEntry.AppStopped)
-        assertEquals(expected, log.log)
+        assertEquals(listOf(LogEntry.AppStopped), log.log)
     }
 
     @Test
@@ -81,6 +86,19 @@ class StartupActivityLogTest {
         assertEquals(expected, log.log)
     }
 
+    @Test
+    fun `WHEN app STOPPED is called THEN the log is emptied expect for the stop event`() {
+        assertTrue(log.log.isEmpty())
+
+        activityCallbacks.onActivityCreated(mockk(), null)
+        activityCallbacks.onActivityStarted(mockk())
+        appObserver.onStart(mockk())
+        assertEquals(3, log.log.size)
+
+        appObserver.onStop(mockk())
+        assertEquals(listOf(LogEntry.AppStopped), log.log)
+    }
+
     @Test
     fun `GIVEN debug log level WHEN logEntries is called THEN there is no logcat call`() {
         log.logEntries(logger, Priority.DEBUG)