Bug 1910449 - Use daemon-thread with MPS timer instead of user-thread

Use a daemon-thread instead of the default user-thread in the `java.util.Timer` used to schedule pings within the MetricsPingScheduler. User-threads can prevent the application from terminating gracefully while daemon-threads do not.

glean-core/android/src/main/java/mozilla/telemetry/glean/Dispatchers.kt

@@ -6,7 +6,6 @@ package mozilla.telemetry.glean
 
 import androidx.annotation.VisibleForTesting
 import kotlinx.coroutines.CoroutineScope
-import kotlinx.coroutines.DelicateCoroutinesApi
 import kotlinx.coroutines.Job
 import kotlinx.coroutines.SupervisorJob
 import kotlinx.coroutines.launch

glean-core/android/src/main/java/mozilla/telemetry/glean/Glean.kt

@@ -83,8 +83,7 @@ internal class OnGleanEventsImpl(val glean: GleanInternalAPI) : OnGleanEvents {
  }
 
  override fun cancelUploads() {
- // Cancel any pending workers here so that we don't accidentally upload or
- // collect data after the upload has been disabled.
+ // Cancel any pending metrics ping scheduler tasks
  glean.metricsPingScheduler?.cancel()
  // Cancel any pending workers here so that we don't accidentally upload
  // data after the upload has been disabled.

glean-core/android/src/main/java/mozilla/telemetry/glean/scheduler/MetricsPingScheduler.kt

@@ -103,11 +103,17 @@ internal class MetricsPingScheduler(
  val millisUntilNextDueTime = getMillisecondsUntilDueTime(sendTheNextCalendarDay, now)
  Log.d(LOG_TAG, "Scheduling the 'metrics' ping in ${millisUntilNextDueTime}ms")
 
- // Cancel any existing scheduled work. Does not actually ancel a
- // currently-running task.
+ // Cancel the MPS scheduler, discarding any currently scheduled tasks. This does not
+ // interfere with a currently executing task (if one exists).
  cancel()
 
- timer = Timer("glean.MetricsPingScheduler")
+ // After cancelling the MPS timer, it will no longer be able to schedule new tasks for
+ // execution, so we will need to create a new one in order to schedule more work. This is
+ // done using`isDaemon: true`. We configure the timer to use a daemon-thread because it will
+ // not prevent the application from terminating gracefully, unlike the default user-thread.
+ // See: (https://developer.android.com/reference/java/util/Timer#Timer(boolean))
+ timer = Timer("glean.MetricsPingScheduler", true)
+
  timer?.schedule(MetricsPingTimer(this, reason), millisUntilNextDueTime)
  }
 
@@ -311,8 +317,13 @@ internal class MetricsPingScheduler(
  // Update the collection date: we don't really care if we have data or not, let's
  // always update the sent date.
  updateSentDate(getISOTimeString(now, truncateTo = TimeUnit.DAY))
- // Reschedule the collection.
- schedulePingCollection(now, sendTheNextCalendarDay = true, reason = Pings.metricsReasonCodes.reschedule)
+
+ // Schedule the next metrics ping collection
+ schedulePingCollection(
+ now,
+ sendTheNextCalendarDay = true,
+ reason = Pings.metricsReasonCodes.reschedule,
+ )
  }
 
  /**
@@ -340,6 +351,10 @@ internal class MetricsPingScheduler(
  * Function to cancel any pending metrics ping timers
  */
  fun cancel() {
+ // Terminate the MPS timer object, discarding any currently scheduled tasks. Does not
+ // interfere with a currently executing task (if it exists). Once a timer has been
+ // terminated, its execution thread terminates gracefully, and no more tasks may be
+ // scheduled on it.
  timer?.cancel()
  timer = null
  }