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authorJeff King <peff@peff.net>2013-06-20 08:37:53 (GMT)
committerJunio C Hamano <gitster@pobox.com>2013-06-20 22:50:17 (GMT)
commit98eeb09e8acb6cbe0b0da3b1772b6676fe6d167f (patch)
tree00e9bbf279111957e80c02d8a537e120df7d33d2 /builtin/ls-files.c
parentca9199300eb0d485f5e03961bbbd9aff3484a1fa (diff)
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for_each_ref: load all loose refs before packed refs
If we are iterating through the refs using for_each_ref (or any of its sister functions), we can get into a race condition with a simultaneous "pack-refs --prune" that looks like this: 0. We have a large number of loose refs, and a few packed refs. refs/heads/z/foo is loose, with no matching entry in the packed-refs file. 1. Process A starts iterating through the refs. It loads the packed-refs file from disk, then starts lazily traversing through the loose ref directories. 2. Process B, running "pack-refs --prune", writes out the new packed-refs file. It then deletes the newly packed refs, including refs/heads/z/foo. 3. Meanwhile, process A has finally gotten to refs/heads/z (it traverses alphabetically). It descends, but finds nothing there. It checks its cached view of the packed-refs file, but it does not mention anything in "refs/heads/z/" at all (it predates the new file written by B in step 2). The traversal completes successfully without mentioning refs/heads/z/foo at all (the name, of course, isn't important; but the more refs you have and the farther down the alphabetical list a ref is, the more likely it is to hit the race). If refs/heads/z/foo did exist in the packed refs file at state 0, we would see an entry for it, but it would show whatever sha1 the ref had the last time it was packed (which could be an arbitrarily long time ago). This can be especially dangerous when process A is "git prune", as it means our set of reachable tips will be incomplete, and we may erroneously prune objects reachable from that tip (the same thing can happen if "repack -ad" is used, as it simply drops unreachable objects that are packed). This patch solves it by loading all of the loose refs for our traversal into our in-memory cache, and then refreshing the packed-refs cache. Because a pack-refs writer will always put the new packed-refs file into place before starting the prune, we know that any loose refs we fail to see will either truly be missing, or will have already been put in the packed-refs file by the time we refresh. Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
Diffstat (limited to 'builtin/ls-files.c')
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