[PATCH v2 3/3] read_loose_refs(): treat NULL_SHA1 loose references as broken

[Michael Haggerty <mhagger@alum.mit.edu>]

NULL_SHA1 is never a valid value for a reference. If a loose reference
has that value, mark it as broken.

Why check NULL_SHA1 and not the nearly 2^160 other SHA-1s that are
also invalid in a given repository? Because (a) it is cheap to test
for NULL_SHA1, and (b) NULL_SHA1 is often used as a "SHA-1 is invalid"
value inside of Git client source code (not only ours!), and
accidentally writing it to a loose reference file would be an easy
mistake to make.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
---
 refs.c                         | 7 +++++++
 t/t6301-for-each-ref-errors.sh | 2 +-
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/refs.c b/refs.c
index 47e4e53..c28fde1 100644
--- a/refs.c
+++ b/refs.c
@@ -1321,6 +1321,13 @@ static void read_loose_refs(const char *dirname, struct ref_dir *dir)
 				hashclr(sha1);
 				flag |= REF_ISBROKEN;
 			}
+
+			if (!(flag & REF_ISBROKEN) && is_null_sha1(sha1)) {
+				/* NULL_SHA1 is never a valid reference value. */
+				hashclr(sha1);
+				flag |= REF_ISBROKEN;
+			}
+
 			if (check_refname_format(refname.buf,
 						 REFNAME_ALLOW_ONELEVEL)) {
 				hashclr(sha1);
diff --git a/t/t6301-for-each-ref-errors.sh b/t/t6301-for-each-ref-errors.sh
index 72d2397..cdb67a0 100755
--- a/t/t6301-for-each-ref-errors.sh
+++ b/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" &&
-- 
2.1.4