rename_tmp_log(): handle a possible mkdir/rmdir race

If a directory vanishes while renaming the temporary reflog file,
retry (up to 3 times).  This could happen if another process deletes
the directory created by safe_create_leading_directories() just before
we rename the file into the directory.

As far as I can tell, this race could not occur internal to git.  The
only time that a directory under $GIT_DIR/logs is deleted is if room
has to be made for a log file for a reference with the same name;
for example, in the following sequence:

    git branch foo/bar    # Creates file .git/logs/refs/heads/foo/bar
    git branch -d foo/bar # Deletes file but leaves .git/logs/refs/heads/foo/
    git branch foo        # Deletes .git/logs/refs/heads/foo/

But the only reason the last command deletes the directory is because
it wants to create a file with the same name.  So if another process
(e.g.,

    git branch foo/baz

) wants to create that directory, one of the two is doomed to failure
anyway because of a D/F conflict.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.c

@@ -2530,12 +2530,14 @@ int delete_ref(const char *refname, const unsigned char *sha1, int delopt)
 
 static int rename_tmp_log(const char *newrefname)
 {
+	int attempts_remaining = 3;
+
+ retry:
 	if (safe_create_leading_directories(git_path("logs/%s", newrefname))) {
 		error("unable to create directory for %s", newrefname);
 		return -1;
 	}
 
- retry:
 	if (rename(git_path(TMP_RENAMED_LOG), git_path("logs/%s", newrefname))) {
 		if (errno==EISDIR || errno==ENOTDIR) {
 			/*
@@ -2548,6 +2550,13 @@ static int rename_tmp_log(const char *newrefname)
 				return -1;
 			}
 			goto retry;
+		} else if (errno == ENOENT && --attempts_remaining > 0) {
+			/*
+			 * Maybe another process just deleted one of
+			 * the directories in the path to newrefname.
+			 * Try again from the beginning.
+			 */
+			goto retry;
 		} else {
 			error("unable to move logfile "TMP_RENAMED_LOG" to logs/%s: %s",
 				newrefname, strerror(errno));