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authorPatrick Steinhardt <ps@pks.im>2025-05-28 14:24:10 +0200
committerJunio C Hamano <gitster@pobox.com>2025-05-28 07:56:29 -0700
commit320572c43d7bc5afbcb8e5faf83b6eccfe6f4e32 (patch)
tree59f08e26a626f6831fe25d4adf8b9b7135be096a /builtin/commit-graph.c
parent8613c2bb6cd16ef530dc5dd74d3b818a1ccbf1c0 (diff)
packfile: explain ordering of how we look up auxiliary pack files
When adding a packfile to an object database we perform four syscalls: - Three calls to access(3p) are done to check for auxiliary data structures. - One call to stat(3p) is done to check for the ".pack" itself. One curious bit is that we perform the access(3p) calls before checking for the packfile itself, but if the packfile doesn't exist we discard all results. The access(3p) calls are thus essentially wasted, so one may be triggered to reorder those calls so that we can short-circuit the other syscalls in case the packfile does not exist. The order in which we look up files is quite important though to help avoid races: - When installing a packfile we move auxiliary data structures into place before we install the ".idx" file. - When deleting a packfile we first delete the ".idx" and ".pack" files before deleting auxiliary data structures. As such, to avoid any races with concurrently created or deleted packs we need to make sure that we _first_ read auxiliary data structures before we read the corresponding ".idx" or ".pack" file. Otherwise it may easily happen that we return a populated but misclassified pack. Add a comment to `add_packed_git()` to make future readers aware of this ordering requirement. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>
Diffstat (limited to 'builtin/commit-graph.c')
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