[PATCH] ref-filter: don't look for objects when outside of a repository

["SZEDER Gábor" <szeder.dev@gmail.com>]

The command 'git ls-remote --sort=authordate <remote>' segfaults when
run outside of a repository, ever since the introduction of its
'--sort' option in 1fb20dfd8e (ls-remote: create '--sort' option,
2018-04-09).

While in general the 'git ls-remote' command can be run outside of a
repository just fine, its '--sort=<key>' option with certain keys does
require access to the referenced objects.  This sorting is implemented
using the generic ref-filter sorting facility, which already handles
missing objects gracefully with the appropriate 'missing object
deadbeef for HEAD' message.  However, being generic means that it
checks replace refs while trying to retrieve an object, and while
doing so it accesses the 'git_replace_ref_base' variable, which has
not been initialized and is still a NULL pointer when outside of a
repository, thus causing the segfault.

Make ref-filter more careful and only attempt to retrieve an object
when we are in a repository.  Also add a test to ensure that 'git
ls-remote --sort' fails gracefully when executed outside of a
repository.

Reported-by: H.Merijn Brand <h.m.brand@xs4all.nl>
Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---

I'm not quite sure that this is the best place to add this check...
but hey, it's a Saturday afternoon after all ;)

 ref-filter.c         | 3 ++-
 t/t5512-ls-remote.sh | 6 ++++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/ref-filter.c b/ref-filter.c
index e1bcb4ca8a..3555bc29e7 100644
--- a/ref-filter.c
+++ b/ref-filter.c
@@ -1473,7 +1473,8 @@ static int get_object(struct ref_array_item *ref, int deref, struct object **obj
 		oi->info.sizep = &oi->size;
 		oi->info.typep = &oi->type;
 	}
-	if (oid_object_info_extended(the_repository, &oi->oid, &oi->info,
+	if (!have_git_dir() ||
+	    oid_object_info_extended(the_repository, &oi->oid, &oi->info,
 				     OBJECT_INFO_LOOKUP_REPLACE))
 		return strbuf_addf_ret(err, -1, _("missing object %s for %s"),
 				       oid_to_hex(&oi->oid), ref->refname);
diff --git a/t/t5512-ls-remote.sh b/t/t5512-ls-remote.sh
index bc5703ff9b..7dd081da01 100755
--- a/t/t5512-ls-remote.sh
+++ b/t/t5512-ls-remote.sh
@@ -302,4 +302,10 @@ test_expect_success 'ls-remote works outside repository' '
 	nongit git ls-remote dst.git
 '
 
+test_expect_success 'ls-remote --sort fails gracefully outside repository' '
+	# Use a sort key that requires access to the referenced objects.
+	nongit test_must_fail git ls-remote --sort=authordate "$TRASH_DIRECTORY" 2>err &&
+	test_i18ngrep "^fatal: missing object" err
+'
+
 test_done
-- 
2.19.0.355.geb876cd9d6