read_loose_refs(): treat NULL_SHA1 loose references as broken

NULL_SHA1 is used to indicate an "invalid object name" throughout our
code (and the code of other git implementations), so it is vastly more
likely that an on-disk reference was set to this value due to a
software bug than that NULL_SHA1 is the legitimate SHA-1 of an actual
object.  Therefore, if a loose reference has the value NULL_SHA1,
consider it to be broken.

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

refs.c

@@ -1297,6 +1297,16 @@ static void read_loose_refs(const char *dirname, struct ref_dir *dir)
 			if (!read_ok) {
 				hashclr(sha1);
 				flag |= REF_ISBROKEN;
+			} else if (is_null_sha1(sha1)) {
+				/*
+				 * It is so astronomically unlikely
+				 * that NULL_SHA1 is the SHA-1 of an
+				 * actual object that we consider its
+				 * appearance in a loose reference
+				 * file to be repo corruption
+				 * (probably due to a software bug).
+				 */
+				flag |= REF_ISBROKEN;
 			}
 
 			if (check_refname_format(refname.buf,

t/t6301-for-each-ref-errors.sh

@@ -24,7 +24,7 @@ test_expect_success 'Broken refs are reported correctly' '
 	test_cmp broken-err err
 '
 
-test_expect_failure 'NULL_SHA1 refs are reported correctly' '
+test_expect_success 'NULL_SHA1 refs are reported correctly' '
 	r=refs/heads/zeros &&
 	echo $ZEROS >.git/$r &&
 	test_when_finished "rm -f .git/$r" &&