Item reset is invoked by cache cleaner for synchronous recovery
from ENOSPC errors. The reset operation removes the cache file and
closes/reopens the downloaders. Although most parts of reset and
other item operations are done with the item mutex held, the mutex
is released during fd.WriteAt and downloaders calls. We used preAccess
and postAccess calls to serialize Reset, ReadAt, and Open, but missed
some other item operations. The patch adds preAccess/postAccess
calls in Sync, Truncate, Close, WriteAt, and rename.
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