refs: don't dereference on rename

When renaming refs, don't dereference either the origin or the destination
before renaming.

The origin does not need to be dereferenced because it is presently
forbidden to rename symbolic refs.

Not dereferencing the destination fixes a bug where renaming on top of
a broken symref would use the pointed-to ref name for the moved
reflog.

Add a test for the reflog bug.

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

refs/files-backend.c

@@ -2333,7 +2333,8 @@ int rename_ref(const char *oldrefname, const char *newrefname, const char *logms
 	if (log && S_ISLNK(loginfo.st_mode))
 		return error("reflog for %s is a symlink", oldrefname);
 
-	if (!resolve_ref_unsafe(oldrefname, RESOLVE_REF_READING, orig_sha1, &flag))
+	if (!resolve_ref_unsafe(oldrefname, RESOLVE_REF_READING | RESOLVE_REF_NO_RECURSE,
+				orig_sha1, &flag))
 		return error("refname %s not found", oldrefname);
 
 	if (flag & REF_ISSYMREF)
@@ -2351,8 +2352,16 @@ int rename_ref(const char *oldrefname, const char *newrefname, const char *logms
 		goto rollback;
 	}
 
-	if (!read_ref_full(newrefname, RESOLVE_REF_READING, sha1, NULL) &&
-	    delete_ref(newrefname, sha1, REF_NODEREF)) {
+	/*
+	 * Since we are doing a shallow lookup, sha1 is not the
+	 * correct value to pass to delete_ref as old_sha1. But that
+	 * doesn't matter, because an old_sha1 check wouldn't add to
+	 * the safety anyway; we want to delete the reference whatever
+	 * its current value.
+	 */
+	if (!read_ref_full(newrefname, RESOLVE_REF_READING | RESOLVE_REF_NO_RECURSE,
+			   sha1, NULL) &&
+	    delete_ref(newrefname, NULL, REF_NODEREF)) {
 		if (errno==EISDIR) {
 			struct strbuf path = STRBUF_INIT;
 			int result;
@@ -2376,7 +2385,8 @@ int rename_ref(const char *oldrefname, const char *newrefname, const char *logms
 
 	logmoved = log;
 
-	lock = lock_ref_sha1_basic(newrefname, NULL, NULL, NULL, 0, NULL, &err);
+	lock = lock_ref_sha1_basic(newrefname, NULL, NULL, NULL, REF_NODEREF,
+				   NULL, &err);
 	if (!lock) {
 		error("unable to rename '%s' to '%s': %s", oldrefname, newrefname, err.buf);
 		strbuf_release(&err);
@@ -2394,7 +2404,8 @@ int rename_ref(const char *oldrefname, const char *newrefname, const char *logms
 	return 0;
 
  rollback:
-	lock = lock_ref_sha1_basic(oldrefname, NULL, NULL, NULL, 0, NULL, &err);
+	lock = lock_ref_sha1_basic(oldrefname, NULL, NULL, NULL, REF_NODEREF,
+				   NULL, &err);
 	if (!lock) {
 		error("unable to lock %s for rollback: %s", oldrefname, err.buf);
 		strbuf_release(&err);

t/t3200-branch.sh

@@ -79,6 +79,15 @@ test_expect_success 'git branch -m dumps usage' '
 	test_i18ngrep "branch name required" err
 '
 
+test_expect_success 'git branch -m m broken_symref should work' '
+	test_when_finished "git branch -D broken_symref" &&
+	git branch -l m &&
+	git symbolic-ref refs/heads/broken_symref refs/heads/i_am_broken &&
+	git branch -m m broken_symref &&
+	git reflog exists refs/heads/broken_symref &&
+	test_must_fail git reflog exists refs/heads/i_am_broken
+'
+
 test_expect_success 'git branch -m m m/m should work' '
 	git branch -l m &&
 	git branch -m m m/m &&