[PATCH] sha1_file.c:create_tmpfile(): Fix race when creating loose object dirs

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@herland.net>
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 sha1_file.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)