receive-pack: allow deletion of corrupt refs

Occasionally, in some setups (*cough* forks on repo.or.cz *cough*) some
refs go stale, e.g. when the forkee rebased and lost some objects needed
by the fork.  The quick & dirty way to deal with those refs is to delete
them and push them again.

However, git-push first would first fetch the current commit name for the
ref, would receive a null sha1 since the ref does not point to a valid
object, then tell receive-pack that it should delete the ref with this
commit name.  delete_ref() would be subsequently be called, and check that
resolve_ref() (which does _not_ check for validity of the object) returns
the same commit name.  Which would fail.

The proper fix is to avoid corrupting repositories, but in the meantime
this is a good fix in any case.

Incidentally, some instances of "cd .." in the test cases were fixed, so
that subsequent test cases run in t/trash/ irrespective of the outcome of
the previous test cases.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

receive-pack.c

@@ -200,6 +200,10 @@ static const char *update(struct command *cmd)
 	}
 
 	if (is_null_sha1(new_sha1)) {
+		if (!parse_object(old_sha1)) {
+			warning ("Allowing deletion of corrupt ref.");
+			old_sha1 = NULL;
+		}
 		if (delete_ref(name, old_sha1)) {
 			error("failed to delete %s", name);
 			return "failed to delete";

t/t5516-fetch-push.sh

@@ -247,11 +247,50 @@ test_expect_success 'push with colon-less refspec (4)' '
 test_expect_success 'push with dry-run' '
 
 	mk_test heads/master &&
-	cd testrepo &&
+	(cd testrepo &&
-	old_commit=$(git show-ref -s --verify refs/heads/master) &&
+	 old_commit=$(git show-ref -s --verify refs/heads/master)) &&
-	cd .. &&
 	git push --dry-run testrepo &&
 	check_push_result $old_commit heads/master
 '
 
+test_expect_success 'push updates local refs' '
+
+	rm -rf parent child &&
+	mkdir parent &&
+	(cd parent && git init &&
+		echo one >foo && git add foo && git commit -m one) &&
+	git clone parent child &&
+	(cd child &&
+		echo two >foo && git commit -a -m two &&
+		git push &&
+	test $(git rev-parse master) = $(git rev-parse remotes/origin/master))
+
+'
+
+test_expect_success 'push does not update local refs on failure' '
+
+	rm -rf parent child &&
+	mkdir parent &&
+	(cd parent && git init &&
+		echo one >foo && git add foo && git commit -m one &&
+		echo exit 1 >.git/hooks/pre-receive &&
+		chmod +x .git/hooks/pre-receive) &&
+	git clone parent child &&
+	(cd child &&
+		echo two >foo && git commit -a -m two &&
+		! git push &&
+		test $(git rev-parse master) != \
+			$(git rev-parse remotes/origin/master))
+
+'
+
+test_expect_success 'allow deleting an invalid remote ref' '
+
+	pwd &&
+	rm -f testrepo/.git/objects/??/* &&
+	git push testrepo :refs/heads/master &&
+	(cd testrepo && ! git rev-parse --verify refs/heads/master)
+
+'
+
 test_done