From ca9199300eb0d485f5e03961bbbd9aff3484a1fa Mon Sep 17 00:00:00 2001 From: Jeff King Date: Thu, 20 Jun 2013 10:37:52 +0200 Subject: [PATCH] get_packed_ref_cache: reload packed-refs file when it changes Once we read the packed-refs file into memory, we cache it to save work on future ref lookups. However, our cache may be out of date with respect to what is on disk if another process is simultaneously packing the refs. Normally it is acceptable for us to be a little out of date, since there is no guarantee whether we read the file before or after the simultaneous update. However, there is an important special case: our packed-refs file must be up to date with respect to any loose refs we read. Otherwise, we risk the following race condition: 0. There exists a loose ref refs/heads/master. 1. Process A starts and looks up the ref "master". It first checks $GIT_DIR/master, which does not exist. It then loads (and caches) the packed-refs file to see if "master" exists in it, which it does not. 2. Meanwhile, process B runs "pack-refs --all --prune". It creates a new packed-refs file which contains refs/heads/master, and removes the loose copy at $GIT_DIR/refs/heads/master. 3. Process A continues its lookup, and eventually tries $GIT_DIR/refs/heads/master. It sees that the loose ref is missing, and falls back to the packed-refs file. But it examines its cached version, which does not have refs/heads/master. After trying a few other prefixes, it reports master as a non-existent ref. There are many variants (e.g., step 1 may involve process A looking up another ref entirely, so even a fully qualified refname can fail). One of the most interesting ones is if "refs/heads/master" is already packed. In that case process A will not see it as missing, but rather will report whatever value happened to be in the packed-refs file before process B repacked (which might be an arbitrarily old value). We can fix this by making sure we reload the packed-refs file from disk after looking at any loose refs. That's unacceptably slow, so we can check its stat()-validity as a proxy, and read it only when it appears to have changed. Reading the packed-refs file after performing any loose-ref system calls is sufficient because we know the ordering of the pack-refs process: it always makes sure the newly written packed-refs file is installed into place before pruning any loose refs. As long as those operations by B appear in their executed order to process A, by the time A sees the missing loose ref, the new packed-refs file must be in place. Signed-off-by: Michael Haggerty Signed-off-by: Jeff King Signed-off-by: Junio C Hamano --- refs.c | 21 ++++++++++++++++----- 1 file changed, 16 insertions(+), 5 deletions(-) diff --git a/refs.c b/refs.c index a5a5b5d3bd..60f250769e 100644 --- a/refs.c +++ b/refs.c @@ -825,6 +825,9 @@ struct packed_ref_cache { * when it is unlocked. */ struct lock_file *lock; + + /* The metadata from when this packed-refs cache was read */ + struct stat_validity validity; }; /* @@ -862,6 +865,7 @@ static int release_packed_ref_cache(struct packed_ref_cache *packed_refs) { if (!--packed_refs->referrers) { free_ref_entry(packed_refs->root); + stat_validity_clear(&packed_refs->validity); free(packed_refs); return 1; } else { @@ -1053,19 +1057,26 @@ static void read_packed_refs(FILE *f, struct ref_dir *dir) */ static struct packed_ref_cache *get_packed_ref_cache(struct ref_cache *refs) { + const char *packed_refs_file; + + if (*refs->name) + packed_refs_file = git_path_submodule(refs->name, "packed-refs"); + else + packed_refs_file = git_path("packed-refs"); + + if (refs->packed && + !stat_validity_check(&refs->packed->validity, packed_refs_file)) + clear_packed_ref_cache(refs); + if (!refs->packed) { - const char *packed_refs_file; FILE *f; refs->packed = xcalloc(1, sizeof(*refs->packed)); acquire_packed_ref_cache(refs->packed); refs->packed->root = create_dir_entry(refs, "", 0, 0); - if (*refs->name) - packed_refs_file = git_path_submodule(refs->name, "packed-refs"); - else - packed_refs_file = git_path("packed-refs"); f = fopen(packed_refs_file, "r"); if (f) { + stat_validity_update(&refs->packed->validity, fileno(f)); read_packed_refs(f, get_ref_dir(refs->packed->root)); fclose(f); } -- 2.11.4.GIT