The background chunk cleaner slept for the whole ChunkCleanInterval
(default 1 minute) before checking its stop channel, and only ran
CleanUpCache via the select default branch. This meant a cache that
had been stopped by StopBackgroundRunners could keep running
CleanUpCache for up to an interval afterwards.
The cache backend tests all share a single on-disk chunk store (the
TestInternalCache remote), so a lingering cleaner from a finished test
could call CleanChunksBySize and os.RemoveAll chunks that a later,
unrelated test had just written. The later test would then read a
chunk back and get an unexpected EOF - eg
TestInternalMaxChunkSizeRespected failing intermittently on CI.
Wait on a timer and the stop channel together so a stop is honoured
immediately and the cleaner can never run again once stopped.
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