hash: provide per-algorithm null OIDs

Up until recently, object IDs did not have an algorithm member, only a
hash.  Consequently, it was possible to share one null (all-zeros)
object ID among all hash algorithms.  Now that we're going to be
handling objects from multiple hash algorithms, it's important to make
sure that all object IDs have a correct algorithm field.

Introduce a per-algorithm null OID, and add it to struct hash_algo.
Introduce a wrapper function as well, and use it everywhere we used to
use the null_oid constant.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 5 insertions, 2 deletions

diff --git a/hash.h b/hash.h
index 0e85e448ed..2986f991c6 100644
--- a/hash.h
+++ b/hash.h
@@ -170,6 +170,9 @@ struct git_hash_algo {
 
 	/* The OID of the empty blob. */
 	const struct object_id *empty_blob;
+
+	/* The all-zeros OID. */
+	const struct object_id *null_oid;
 };
 extern const struct git_hash_algo hash_algos[GIT_HASH_NALGOS];
 
@@ -190,7 +193,7 @@ static inline int hash_algo_by_ptr(const struct git_hash_algo *p)
 
 #define the_hash_algo the_repository->hash_algo
 
-extern const struct object_id null_oid;
+const struct object_id *null_oid(void);
 
 static inline int hashcmp_algop(const unsigned char *sha1, const unsigned char *sha2, const struct git_hash_algo *algop)
 {
@@ -246,7 +249,7 @@ static inline int oideq(const struct object_id *oid1, const struct object_id *oi
 
 static inline int is_null_oid(const struct object_id *oid)
 {
-	return oideq(oid, &null_oid);
+	return oideq(oid, null_oid());
 }
 
 static inline void hashcpy(unsigned char *sha_dst, const unsigned char *sha_src)