There was a race condition such that when the cancel `Intent` is
received, it would queue up a message on the download thread to stop
downloading. This was followed by cancelling the notification on the
thread the `Intent` was received on. As a result of the message taking
some time to get to the other thread, it would likely cause another
progress notification to be shown after we had already removed that
notification - making it show again after being cancelled.
The code in the download thread progress handler, which updates the
notification with progress, is not perfect. It does a check before
deciding to broadcast progress and update the notification. However,
the check happens at the beginning of the group of expressions. As
such, there is likely a small change that `isActive` returns true,
then the notification is cancelled on the other thread, and finally,
the download thread updates the progress again. However this is better
than it was before where the notification didn't go away.
Previously, apks were not being removed from the queue once download
was completed, only when removed. It didn't cause any specific issues
here, but I removed downloads from the queue after completion to prevent
potential problems in the future. Now the queue is a better representation
of what is to be downloaded.
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