The CacheFile.fileLock() method is used to acquire a lock
on a cache file so that the file can't be deleted (or its file
handle closed) during the execution of a read or a write
operation.
Today this lock is obtained by first acquiring the eviction
lock (the write lock of the readwrite lock), then by checking
if the cache file is evicted and the file channel still open,
and finally by obtaining the file lock (the read lock of the
readwrite lock). Acquiring the read lock while the eviction
lock is held ensures that the cache file eviction cannot
start in the meanwhile. But eviction starts (and terminations)
also acquire the eviction lock; and this lock cannot be
obtained while a read lock is held (the write lock of a
readwrite lock is exclusive).
If we were acquiring a read lock and checking the eviction
flag and file channel existence while holding the read lock
we know that no eviction can start or finish until the
read lock is released.
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