Re: Race condition with git-status and git-rebase
- From
Junio C Hamano <gitster@pobox.com>
- Date
- Apr 8, 2014, 22:56 UTC
- Message-ID
- <xmqq1tx72zxo.fsf@gitster.dls.corp.google.com>
- In-Reply-To
- <20140408184421.GD5208@abyss.hitronhub.home>
Yiannis Marangos <yiannis.marangos@gmail.com> writes:
Show 8 quoted lines
> process A calls git-rebase > process A applies the 1st commit > process B calls git-status > process B calls read_cache() > process A applies the 2nd commit > process B holds the index.lock > process B writes back the old index (the one that was read from read_cache()) > process A applies the 3rd commit (now the 3rd commit contains also a revert of the 2nd)
The above sequence shows we are clearly doing it wrong. B's taking of the lock is not protecting anything from anybody.
We try minimizing the lock contention by delaying the taking of the lock by B, but even if we leave it at the current location in the above sequence, at least B must verify that the contents of the index it just took the lock on still matches what it read with its earlier call to read_cache(), which would make it pretend as if it took the lock before it did read_cache(), before writing the refreshed results back.
I think we have a checksum for the entire index file at the end, so one fix might be to teach read_index_from() to read that and store it in a new field in "struct index_state", and then make the "opportunistic update" codepath to verify that the index on the filesystem still has the same checksum.