update_ref(): don't read old reference value before delete

If we are deleting the reference, then we don't need to read the
reference's old value. It doesn't provide any race safety, because the
value read just before the delete is no "better" than the value that
would be read under lock during the delete. And even if the reference
previously didn't exist, we can call delete_ref() on it if we don't
provide an old_sha1 value.

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

diff --git a/fast-import.c b/fast-import.c
index 6378726993445694581ac36d6351f817fe488c72..d7ed0653812be7ea047b98f7a2af3a28e0e70207 100644 (file)
--- a/fast-import.c
+++ b/fast-import.c
@@ -1692,13 +1692,13 @@ static int update_branch(struct branch *b)
        unsigned char old_sha1[20];
        struct strbuf err = STRBUF_INIT;
 
-       if (read_ref(b->name, old_sha1))
-               hashclr(old_sha1);
        if (is_null_sha1(b->sha1)) {
                if (b->delete)
-                       delete_ref(b->name, old_sha1, 0);
+                       delete_ref(b->name, NULL, 0);
                return 0;
        }
+       if (read_ref(b->name, old_sha1))
+               hashclr(old_sha1);
        if (!force_update && !is_null_sha1(old_sha1)) {
                struct commit *old_cmit, *new_cmit;