ref-filter: fix NULL check for parse object failure

After we run parse_object_buffer() to get an object's contents, we try
to check that the return value wasn't NULL. However, since our "struct
object" is a pointer-to-pointer, and we assign like:

  *obj = parse_object_buffer(...);

it's not correct to check:

  if (!obj)

That will always be true, since our double pointer will continue to
point to the single pointer (which is itself NULL). This is a regression
that was introduced by aa46a0d (ref-filter: use oid_object_info() to
get object, 2018-07-17); since that commit we'll segfault on a parse
failure, as we try to look at the NULL object pointer.

There are many ways a parse could fail, but most of them are hard to set
up in the tests (it's easy to make a bogus object, but update-ref will
refuse to point to it). The test here uses a tag which points to a wrong
object type. A parse of just the broken tag object will succeed, but
seeing both tag objects in the same process will lead to a parse error
(since we'll see the pointed-to object as both types).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: peff

ref-filter.c

@@ -1595,7 +1595,7 @@ static int get_object(struct ref_array_item *ref, int deref, struct object **obj
 
 	if (oi->info.contentp) {
 		*obj = parse_object_buffer(the_repository, &oi->oid, oi->type, oi->size, oi->content, &eaten);
-		if (!obj) {
+		if (!*obj) {
 			if (!eaten)
 				free(oi->content);
 			return strbuf_addf_ret(err, -1, _("parse_object_buffer failed on %s for %s"),

t/t6300-for-each-ref.sh

@@ -1030,4 +1030,14 @@ test_expect_success 'for-each-ref --ignore-case works on multiple sort keys' '
 	test_cmp expect actual
 '
 
+test_expect_success 'for-each-ref reports broken tags' '
+	git tag -m "good tag" broken-tag-good HEAD &&
+	git cat-file tag broken-tag-good >good &&
+	sed s/commit/blob/ <good >bad &&
+	bad=$(git hash-object -w -t tag bad) &&
+	git update-ref refs/tags/broken-tag-bad $bad &&
+	test_must_fail git for-each-ref --format="%(*objectname)" \
+		refs/tags/broken-tag-*
+'
+
 test_done