shallow: verify shallow file after taking lock
Before writing the shallow file, we stat() the existing file to make sure it has not been updated since our operation began. However, we do not do so under a lock, so there is a possible race: 1. Process A takes the lock. 2. Process B calls check_shallow_file_for_update and finds no update. 3. Process A commits the lockfile. 4. Process B takes the lock, then overwrite's process A's changes. We can fix this by doing our check while we hold the lock. Signed-off-by: NJeff King <peff@peff.net> Signed-off-by: NJunio C Hamano <gitster@pobox.com>
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