The cache file is only ever written to in the `write_file` function,
where it does so by writing to a temp file and performing an atomic
rename of that file. When writing, the target file is never locked.
The cache file is only ever read in the `get` function, and there is no
need for an exclusive lock in that situation.
There is therefore no need to obtain any lock, shared or exclusive, to
read the cache file. Doing so only affects performance of the file sytem
as file locks must be needlessly obtained and written to disk for a read
operation which does not benefit from them.
Andrew Nicols
049288a65d