[PATCH 5/5] rename_ref(): fix a mkdir()/rmdir() race

[Michael Haggerty <mhagger@alum.mit.edu>]

When renaming a reflog file, it was possible that an empty directory
that we just created using safe_create_leading_directories() might get
deleted by another process before we have a chance to move the new
file into it.

So if the rename fails with ENOENT, then retry from the beginning.
Make up to three attempts before giving up.

It could theoretically happen that the ENOENT comes from the
disappearance of TMP_RENAMED_LOG.  In that case three pointless
attempts will be made to move the nonexistent file, but no other harm
should come of it.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
---
 refs.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/refs.c b/refs.c
index 3926136..3ab1491 100644
--- a/refs.c
+++ b/refs.c
@@ -2516,6 +2516,7 @@ int rename_ref(const char *oldrefname, const char *newrefname, const char *logms
 	struct stat loginfo;
 	int log = !lstat(git_path("logs/%s", oldrefname), &loginfo);
 	const char *symref = NULL;
+	int attempts = 3;
 
 	if (log && S_ISLNK(loginfo.st_mode))
 		return error("reflog for %s is a symlink", oldrefname);
@@ -2555,12 +2556,12 @@ int rename_ref(const char *oldrefname, const char *newrefname, const char *logms
 		}
 	}
 
+ retry:
 	if (log && safe_create_leading_directories(git_path("logs/%s", newrefname))) {
 		error("unable to create directory for %s", newrefname);
 		goto rollback;
 	}
 
- retry:
 	if (log && rename(git_path(TMP_RENAMED_LOG), git_path("logs/%s", newrefname))) {
 		if (errno==EISDIR || errno==ENOTDIR) {
 			/*
@@ -2574,6 +2575,13 @@ int rename_ref(const char *oldrefname, const char *newrefname, const char *logms
 			}
 			goto retry;
 		} else {
+			if (errno == ENOENT && --attempts)
+				/*
+				 * Perhaps somebody just pruned the empty
+				 * directory into which we wanted to move the
+				 * file.
+				 */
+				goto retry;
 			error("unable to move logfile "TMP_RENAMED_LOG" to logs/%s: %s",
 				newrefname, strerror(errno));
 			goto rollback;
-- 
1.8.5.1