If a queue is stopped and one of its shards happens to hit the
batch_send_deadline at the same time a deadlock can occur where stop
holds the mutex and will not release it until the send is finished, but
the send needs the mutex to retrieve the most recent batch. This is
fixed by using a second mutex just for writing.
In addition, the test I wrote exposed a case where during shutdown a
batch could be sent twice due to concurrent calls to queue.Batch() and
queue.FlushAndShutdown(). Protect these with a mutex as well.
Signed-off-by: Chris Marchbanks <csmarchbanks@gmail.com>
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