The downloaders.Close() call acquires the downloaders' mutex before
calling the wait group wait and the main downloaders thread has a
periodical (5 seconds interval) call to kick its waiters and the
waiter dispatch function tries to get the mutex. So a deadlock can
occur if the Close() call starts, gets the mutex, while the main
downloader thread already got the timer's tick and proceeded to
call kickWaiters. The deadlock happens when the Close call gets
the mutex between the timer's kick and the main downloader thread
gets the mutex first. So it's a pretty short period of time and
it probably explains why the problem has not surfaced, maybe
something like tens of nanoseconds out of 5 seconds (~10^^-8).
It took 5 days of continued stressing the Close calls for the
deadlock to appear.
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