From c715f783696346ff325dbdb88a5db69825f0d672 Mon Sep 17 00:00:00 2001 From: "Shawn O. Pearce" Date: Thu, 1 Feb 2007 15:52:38 -0500 Subject: [PATCH] Don't find objects in packs which aren't available anymore. Matthias Lederhofer identified a race condition where a Git reader process was able to locate an object in a packed_git index, but was then preempted while a `git repack -a -d` ran and completed. By the time the reader was able to seek in the packfile to get the object data, the packfile no longer existed on disk. In this particular case the reader process did not attempt to open the packfile before it was deleted, so it did not already have the pack_fd field popuplated. With the packfile itself gone, there was no way for the reader to open it and fetch the data. I'm fixing the race condition by teaching find_pack_entry to ignore a packed_git whose packfile is not currently open and which cannot be opened. If none of the currently known packs can supply the object, we will return 0 and the caller will decide the object is not available. If this is the first attempt at finding an object, the caller will reprepare_packed_git and try again. If it was the second attempt, the caller will typically return NULL back, and an error message about a missing object will be reported. This patch does not address the situation of a reader which is being starved out by a tight sequence of `git repack -a -d` runs. In this particular case the reader will try twice, probably fail both times, and declare the object in question cannot be found. As it is highly unlikely that a real world `git repack -a -d` can complete faster than a reader can open a packfile, so I don't think this is a huge concern. Signed-off-by: Shawn O. Pearce Signed-off-by: Junio C Hamano --- sha1_file.c | 12 ++++++++++++ 1 file changed, 12 insertions(+) diff --git a/sha1_file.c b/sha1_file.c index eec4f418b..2eff14ac8 100644 --- a/sha1_file.c +++ b/sha1_file.c @@ -1408,6 +1408,18 @@ static int find_pack_entry(const unsigned char *sha1, struct pack_entry *e, cons } offset = find_pack_entry_one(sha1, p); if (offset) { + /* + * We are about to tell the caller where they can + * locate the requested object. We better make + * sure the packfile is still here and can be + * accessed before supplying that answer, as + * it may have been deleted since the index + * was loaded! + */ + if (p->pack_fd == -1 && open_packed_git(p)) { + error("packfile %s cannot be accessed", p->pack_name); + continue; + } e->offset = offset; e->p = p; hashcpy(e->sha1, sha1); -- 2.30.2