From: Johan Herland Date: Fri, 18 Oct 2013 13:17:47 GMT Subject: [PATCH] sha1_file.c:create_tmpfile(): Fix race when creating loose object dirs Message-ID: <1382102267-18151-1-git-send-email-johan@herland.net> There are cases (e.g. when running concurrent fetches in a repo) where multiple Git processes concurrently attempt to create loose objects within the same objects/XX/ dir. The creation of the loose object files is (AFAICS) safe from races, but the creation of the objects/XX/ dir in which the loose objects reside is unsafe, for example: Two concurrent fetches - A and B. As part of its fetch, A needs to store 12aaaaa as a loose object. B, on the other hand, needs to store 12bbbbb as a loose object. The objects/12 directory does not already exist. Concurrently, both A and B determine that they need to create the objects/12 directory (because their first call to git_mkstemp_mode() within create_tmpfile() fails witn ENOENT). One of them - let's say A - executes the following mkdir() call before the other. This first call returns success, and A moves on. When B gets around to calling mkdir(), it fails with EEXIST, because A won the race. The mkdir() error causes B to return -1 from create_tmpfile(), which propagates all the way, resulting in the fetch failing with: error: unable to create temporary file: File exists fatal: failed to write object fatal: unpack-objects failed Although it's hard to add a testcase reproducing this issue, it's easy to reproduce if we insert a sleep after the if (mkdir(buffer, 0777) || adjust_shared_perm(buffer)) return -1; block, and then run two concurrent "git fetch"es against the same repo. The fix is to simply handle mkdir() setting errno = EEXIST as success. Signed-off-by: Johan Herland --- sha1_file.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/sha1_file.c b/sha1_file.c index f80bbe4..00ffffe 100644 --- a/sha1_file.c +++ b/sha1_file.c @@ -2857,7 +2857,9 @@ static int create_tmpfile(char *buffer, size_t bufsiz, const char *filename) /* Make sure the directory exists */ memcpy(buffer, filename, dirlen); buffer[dirlen-1] = 0; - if (mkdir(buffer, 0777) || adjust_shared_perm(buffer)) + if (mkdir(buffer, 0777) && errno != EEXIST) + return -1; + if (adjust_shared_perm(buffer)) return -1; /* Try again */ -- 1.8.4.653.g2df02b3