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 <johan@xxxxxxxxxxx>
---
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
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