Merge branch 'bc/hash-transition-interop-part-1'

SHA-256 transition.

* bc/hash-transition-interop-part-1:
  hex: print objects using the hash algorithm member
  hex: default to the_hash_algo on zero algorithm value
  builtin/pack-objects: avoid using struct object_id for pack hash
  commit-graph: don't store file hashes as struct object_id
  builtin/show-index: set the algorithm for object IDs
  hash: provide per-algorithm null OIDs
  hash: set, copy, and use algo field in struct object_id
  builtin/pack-redundant: avoid casting buffers to struct object_id
  Use the final_oid_fn to finalize hashing of object IDs
  hash: add a function to finalize object IDs
  http-push: set algorithm when reading object ID
  Always use oidread to read into struct object_id
  hash: add an algo member to struct object_id

archive.c

@@ -203,7 +203,7 @@ static void queue_directory(const unsigned char *sha1,
 	d->mode	   = mode;
 	c->bottom  = d;
 	d->len = xsnprintf(d->path, len, "%.*s%s/", (int)base->len, base->buf, filename);
-	hashcpy(d->oid.hash, sha1);
+	oidread(&d->oid, sha1);
 }
 
 static int write_directory(struct archiver_context *c)
@@ -275,9 +275,11 @@ int write_archive_entries(struct archiver_args *args,
 	int err;
 	struct strbuf path_in_archive = STRBUF_INIT;
 	struct strbuf content = STRBUF_INIT;
-	struct object_id fake_oid = null_oid;
+	struct object_id fake_oid;
 	int i;
 
+	oidcpy(&fake_oid, null_oid());
+
 	if (args->baselen > 0 && args->base[args->baselen - 1] == '/') {
 		size_t len = args->baselen;
 

blame.c

@@ -242,7 +242,7 @@ static struct commit *fake_working_tree_commit(struct repository *r,
 		switch (st.st_mode & S_IFMT) {
 		case S_IFREG:
 			if (opt->flags.allow_textconv &&
-			    textconv_object(r, read_from, mode, &null_oid, 0, &buf_ptr, &buf_len))
+			    textconv_object(r, read_from, mode, null_oid(), 0, &buf_ptr, &buf_len))
 				strbuf_attach(&buf, buf_ptr, buf_len, buf_len + 1);
 			else if (strbuf_read_file(&buf, read_from, st.st_size) != st.st_size)
 				die_errno("cannot open or read '%s'", read_from);

branch.c

@@ -322,7 +322,7 @@ void create_branch(struct repository *r,
 		transaction = ref_transaction_begin(&err);
 		if (!transaction ||
 		    ref_transaction_update(transaction, ref.buf,
-					   &oid, forcing ? NULL : &null_oid,
+					   &oid, forcing ? NULL : null_oid(),
 					   0, msg, &err) ||
 		    ref_transaction_commit(transaction, &err))
 			die("%s", err.buf);

builtin/checkout.c

@@ -106,8 +106,8 @@ static int post_checkout_hook(struct commit *old_commit, struct commit *new_comm
 			      int changed)
 {
 	return run_hook_le(NULL, "post-checkout",
-			   oid_to_hex(old_commit ? &old_commit->object.oid : &null_oid),
+			   oid_to_hex(old_commit ? &old_commit->object.oid : null_oid()),
-			   oid_to_hex(new_commit ? &new_commit->object.oid : &null_oid),
+			   oid_to_hex(new_commit ? &new_commit->object.oid : null_oid()),
 			   changed ? "1" : "0", NULL);
 	/* "new_commit" can be NULL when checking out from the index before
 	   a commit exists. */
@@ -644,7 +644,7 @@ static int reset_tree(struct tree *tree, const struct checkout_opts *o,
 	opts.src_index = &the_index;
 	opts.dst_index = &the_index;
 	init_checkout_metadata(&opts.meta, info->refname,
-			       info->commit ? &info->commit->object.oid : &null_oid,
+			       info->commit ? &info->commit->object.oid : null_oid(),
 			       NULL);
 	parse_tree(tree);
 	init_tree_desc(&tree_desc, tree->buffer, tree->size);

builtin/clone.c

@@ -820,7 +820,7 @@ static int checkout(int submodule_progress)
 	if (write_locked_index(&the_index, &lock_file, COMMIT_LOCK))
 		die(_("unable to write new index file"));
 
-	err |= run_hook_le(NULL, "post-checkout", oid_to_hex(&null_oid),
+	err |= run_hook_le(NULL, "post-checkout", oid_to_hex(null_oid()),
 			   oid_to_hex(&oid), "1", NULL);
 
 	if (!err && (option_recurse_submodules.nr > 0)) {

builtin/describe.c

@@ -502,7 +502,7 @@ static void describe_blob(struct object_id oid, struct strbuf *dst)
 {
 	struct rev_info revs;
 	struct strvec args = STRVEC_INIT;
-	struct process_commit_data pcd = { null_oid, oid, dst, &revs};
+	struct process_commit_data pcd = { *null_oid(), oid, dst, &revs};
 
 	strvec_pushl(&args, "internal: The first arg is not parsed",
 		     "--objects", "--in-commit-order", "--reverse", "HEAD",

builtin/diff.c

@@ -98,7 +98,7 @@ static int builtin_diff_b_f(struct rev_info *revs,
 
 	stuff_change(&revs->diffopt,
 		     blob[0]->mode, canon_mode(st.st_mode),
-		     &blob[0]->item->oid, &null_oid,
+		     &blob[0]->item->oid, null_oid(),
 		     1, 0,
 		     blob[0]->path ? blob[0]->path : path,
 		     path);

builtin/fast-export.c

@@ -870,7 +870,7 @@ static void handle_tag(const char *name, struct tag *tag)
 				p = rewrite_commit((struct commit *)tagged);
 				if (!p) {
 					printf("reset %s\nfrom %s\n\n",
-					       name, oid_to_hex(&null_oid));
+					       name, oid_to_hex(null_oid()));
 					free(buf);
 					return;
 				}
@@ -884,7 +884,7 @@ static void handle_tag(const char *name, struct tag *tag)
 
 	if (tagged->type == OBJ_TAG) {
 		printf("reset %s\nfrom %s\n\n",
-		       name, oid_to_hex(&null_oid));
+		       name, oid_to_hex(null_oid()));
 	}
 	skip_prefix(name, "refs/tags/", &name);
 	printf("tag %s\n", name);
@@ -1016,7 +1016,7 @@ static void handle_tags_and_duplicates(struct string_list *extras)
 				 * it.
 				 */
 				printf("reset %s\nfrom %s\n\n",
-				       name, oid_to_hex(&null_oid));
+				       name, oid_to_hex(null_oid()));
 				continue;
 			}
 
@@ -1035,7 +1035,7 @@ static void handle_tags_and_duplicates(struct string_list *extras)
 				if (!reference_excluded_commits) {
 					/* delete the ref */
 					printf("reset %s\nfrom %s\n\n",
-					       name, oid_to_hex(&null_oid));
+					       name, oid_to_hex(null_oid()));
 					continue;
 				}
 				/* set ref to commit using oid, not mark */
@@ -1146,7 +1146,7 @@ static void handle_deletes(void)
 			continue;
 
 		printf("reset %s\nfrom %s\n\n",
-				refspec->dst, oid_to_hex(&null_oid));
+				refspec->dst, oid_to_hex(null_oid()));
 	}
 }
 

builtin/fast-import.c

@@ -940,7 +940,7 @@ static int store_object(
 	the_hash_algo->init_fn(&c);
 	the_hash_algo->update_fn(&c, hdr, hdrlen);
 	the_hash_algo->update_fn(&c, dat->buf, dat->len);
-	the_hash_algo->final_fn(oid.hash, &c);
+	the_hash_algo->final_oid_fn(&oid, &c);
 	if (oidout)
 		oidcpy(oidout, &oid);
 
@@ -1136,7 +1136,7 @@ static void stream_blob(uintmax_t len, struct object_id *oidout, uintmax_t mark)
 		}
 	}
 	git_deflate_end(&s);
-	the_hash_algo->final_fn(oid.hash, &c);
+	the_hash_algo->final_oid_fn(&oid, &c);
 
 	if (oidout)
 		oidcpy(oidout, &oid);
@@ -1276,8 +1276,8 @@ static void load_tree(struct tree_entry *root)
 		e->versions[0].mode = e->versions[1].mode;
 		e->name = to_atom(c, strlen(c));
 		c += e->name->str_len + 1;
-		hashcpy(e->versions[0].oid.hash, (unsigned char *)c);
+		oidread(&e->versions[0].oid, (unsigned char *)c);
-		hashcpy(e->versions[1].oid.hash, (unsigned char *)c);
+		oidread(&e->versions[1].oid, (unsigned char *)c);
 		c += the_hash_algo->rawsz;
 	}
 	free(buf);

builtin/grep.c

@@ -421,7 +421,7 @@ static int grep_submodule(struct grep_opt *opt,
 	struct grep_opt subopt;
 	int hit;
 
-	sub = submodule_from_path(superproject, &null_oid, path);
+	sub = submodule_from_path(superproject, null_oid(), path);
 
 	if (!is_submodule_active(superproject, path))
 		return 0;

builtin/index-pack.c

@@ -489,7 +489,7 @@ static void *unpack_entry_data(off_t offset, unsigned long size,
 		bad_object(offset, _("inflate returned %d"), status);
 	git_inflate_end(&stream);
 	if (oid)
-		the_hash_algo->final_fn(oid->hash, &c);
+		the_hash_algo->final_oid_fn(oid, &c);
 	return buf == fixed_buf ? NULL : buf;
 }
 
@@ -524,7 +524,7 @@ static void *unpack_raw_entry(struct object_entry *obj,
 
 	switch (obj->type) {
 	case OBJ_REF_DELTA:
-		hashcpy(ref_oid->hash, fill(the_hash_algo->rawsz));
+		oidread(ref_oid, fill(the_hash_algo->rawsz));
 		use(the_hash_algo->rawsz);
 		break;
 	case OBJ_OFS_DELTA:
@@ -1358,7 +1358,7 @@ static struct object_entry *append_obj_to_pack(struct hashfile *f,
 	obj[1].idx.offset += write_compressed(f, buf, size);
 	obj[0].idx.crc32 = crc32_end(f);
 	hashflush(f);
-	hashcpy(obj->idx.oid.hash, sha1);
+	oidread(&obj->idx.oid, sha1);
 	return obj;
 }
 

builtin/ls-files.c

@@ -210,7 +210,7 @@ static void show_submodule(struct repository *superproject,
 {
 	struct repository subrepo;
 	const struct submodule *sub = submodule_from_path(superproject,
-							  &null_oid, path);
+							  null_oid(), path);
 
 	if (repo_submodule_init(&subrepo, superproject, sub))
 		return;

builtin/pack-objects.c

@@ -1030,7 +1030,7 @@ static void write_pack_file(void)
 	write_order = compute_write_order();
 
 	do {
-		struct object_id oid;
+		unsigned char hash[GIT_MAX_RAWSZ];
 		char *pack_tmp_name = NULL;
 
 		if (pack_to_stdout)
@@ -1059,13 +1059,13 @@ static void write_pack_file(void)
 		 * If so, rewrite it like in fast-import
 		 */
 		if (pack_to_stdout) {
-			finalize_hashfile(f, oid.hash, CSUM_HASH_IN_STREAM | CSUM_CLOSE);
+			finalize_hashfile(f, hash, CSUM_HASH_IN_STREAM | CSUM_CLOSE);
 		} else if (nr_written == nr_remaining) {
-			finalize_hashfile(f, oid.hash, CSUM_HASH_IN_STREAM | CSUM_FSYNC | CSUM_CLOSE);
+			finalize_hashfile(f, hash, CSUM_HASH_IN_STREAM | CSUM_FSYNC | CSUM_CLOSE);
 		} else {
-			int fd = finalize_hashfile(f, oid.hash, 0);
+			int fd = finalize_hashfile(f, hash, 0);
-			fixup_pack_header_footer(fd, oid.hash, pack_tmp_name,
+			fixup_pack_header_footer(fd, hash, pack_tmp_name,
-						 nr_written, oid.hash, offset);
+						 nr_written, hash, offset);
 			close(fd);
 			if (write_bitmap_index) {
 				if (write_bitmap_index != WRITE_BITMAP_QUIET)
@@ -1100,17 +1100,17 @@ static void write_pack_file(void)
 			strbuf_addf(&tmpname, "%s-", base_name);
 
 			if (write_bitmap_index) {
-				bitmap_writer_set_checksum(oid.hash);
+				bitmap_writer_set_checksum(hash);
 				bitmap_writer_build_type_index(
 					&to_pack, written_list, nr_written);
 			}
 
 			finish_tmp_packfile(&tmpname, pack_tmp_name,
 					    written_list, nr_written,
-					    &pack_idx_opts, oid.hash);
+					    &pack_idx_opts, hash);
 
 			if (write_bitmap_index) {
-				strbuf_addf(&tmpname, "%s.bitmap", oid_to_hex(&oid));
+				strbuf_addf(&tmpname, "%s.bitmap", hash_to_hex(hash));
 
 				stop_progress(&progress_state);
 
@@ -1124,7 +1124,7 @@ static void write_pack_file(void)
 
 			strbuf_release(&tmpname);
 			free(pack_tmp_name);
-			puts(oid_to_hex(&oid));
+			puts(hash_to_hex(hash));
 		}
 
 		/* mark written objects as written to previous pack */

builtin/pack-redundant.c

@@ -20,7 +20,7 @@ static int load_all_packs, verbose, alt_odb;
 
 struct llist_item {
 	struct llist_item *next;
-	const struct object_id *oid;
+	struct object_id oid;
 };
 static struct llist {
 	struct llist_item *front;
@@ -95,10 +95,10 @@ static struct llist * llist_copy(struct llist *list)
 
 static inline struct llist_item *llist_insert(struct llist *list,
 					      struct llist_item *after,
-					      const struct object_id *oid)
+					      const unsigned char *oid)
 {
 	struct llist_item *new_item = llist_item_get();
-	new_item->oid = oid;
+	oidread(&new_item->oid, oid);
 	new_item->next = NULL;
 
 	if (after != NULL) {
@@ -118,7 +118,7 @@ static inline struct llist_item *llist_insert(struct llist *list,
 }
 
 static inline struct llist_item *llist_insert_back(struct llist *list,
-						   const struct object_id *oid)
+						   const unsigned char *oid)
 {
 	return llist_insert(list, list->back, oid);
 }
@@ -130,9 +130,9 @@ static inline struct llist_item *llist_insert_sorted_unique(struct llist *list,
 
 	l = (hint == NULL) ? list->front : hint;
 	while (l) {
-		int cmp = oidcmp(l->oid, oid);
+		int cmp = oidcmp(&l->oid, oid);
 		if (cmp > 0) { /* we insert before this entry */
-			return llist_insert(list, prev, oid);
+			return llist_insert(list, prev, oid->hash);
 		}
 		if (!cmp) { /* already exists */
 			return l;
@@ -141,11 +141,11 @@ static inline struct llist_item *llist_insert_sorted_unique(struct llist *list,
 		l = l->next;
 	}
 	/* insert at the end */
-	return llist_insert_back(list, oid);
+	return llist_insert_back(list, oid->hash);
 }
 
 /* returns a pointer to an item in front of sha1 */
-static inline struct llist_item * llist_sorted_remove(struct llist *list, const struct object_id *oid, struct llist_item *hint)
+static inline struct llist_item * llist_sorted_remove(struct llist *list, const unsigned char *oid, struct llist_item *hint)
 {
 	struct llist_item *prev, *l;
 
@@ -153,7 +153,7 @@ redo_from_start:
 	l = (hint == NULL) ? list->front : hint;
 	prev = NULL;
 	while (l) {
-		const int cmp = oidcmp(l->oid, oid);
+		const int cmp = hashcmp(l->oid.hash, oid);
 		if (cmp > 0) /* not in list, since sorted */
 			return prev;
 		if (!cmp) { /* found */
@@ -188,7 +188,7 @@ static void llist_sorted_difference_inplace(struct llist *A,
 	b = B->front;
 
 	while (b) {
-		hint = llist_sorted_remove(A, b->oid, hint);
+		hint = llist_sorted_remove(A, b->oid.hash, hint);
 		b = b->next;
 	}
 }
@@ -260,10 +260,10 @@ static void cmp_two_packs(struct pack_list *p1, struct pack_list *p2)
 		/* cmp ~ p1 - p2 */
 		if (cmp == 0) {
 			p1_hint = llist_sorted_remove(p1->unique_objects,
-					(const struct object_id *)(p1_base + p1_off),
+					p1_base + p1_off,
 					p1_hint);
 			p2_hint = llist_sorted_remove(p2->unique_objects,
-					(const struct object_id *)(p1_base + p1_off),
+					p1_base + p1_off,
 					p2_hint);
 			p1_off += p1_step;
 			p2_off += p2_step;
@@ -455,7 +455,7 @@ static void load_all_objects(void)
 		l = pl->remaining_objects->front;
 		while (l) {
 			hint = llist_insert_sorted_unique(all_objects,
-							  l->oid, hint);
+							  &l->oid, hint);
 			l = l->next;
 		}
 		pl = pl->next;
@@ -521,7 +521,7 @@ static struct pack_list * add_pack(struct packed_git *p)
 	base += 256 * 4 + ((p->index_version < 2) ? 4 : 8);
 	step = the_hash_algo->rawsz + ((p->index_version < 2) ? 4 : 0);
 	while (off < p->num_objects * step) {
-		llist_insert_back(l.remaining_objects, (const struct object_id *)(base + off));
+		llist_insert_back(l.remaining_objects, base + off);
 		off += step;
 	}
 	l.all_objects_size = l.remaining_objects->size;

builtin/rebase.c

@@ -485,7 +485,7 @@ static const char * const builtin_rebase_interactive_usage[] = {
 int cmd_rebase__interactive(int argc, const char **argv, const char *prefix)
 {
 	struct rebase_options opts = REBASE_OPTIONS_INIT;
-	struct object_id squash_onto = null_oid;
+	struct object_id squash_onto = *null_oid();
 	enum action command = ACTION_NONE;
 	struct option options[] = {
 		OPT_NEGBIT(0, "ff", &opts.flags, N_("allow fast-forward"),
@@ -1140,7 +1140,7 @@ static int can_fast_forward(struct commit *onto, struct commit *upstream,
 
 	merge_bases = get_merge_bases(onto, head);
 	if (!merge_bases || merge_bases->next) {
-		oidcpy(merge_base, &null_oid);
+		oidcpy(merge_base, null_oid());
 		goto done;
 	}
 

builtin/receive-pack.c

@@ -329,7 +329,7 @@ static void write_head_info(void)
 	for_each_alternate_ref(show_one_alternate_ref, &seen);
 	oidset_clear(&seen);
 	if (!sent_capabilities)
-		show_ref("capabilities^{}", &null_oid);
+		show_ref("capabilities^{}", null_oid());
 
 	advertise_shallow_grafts(1);
 

builtin/show-index.c

@@ -71,9 +71,11 @@ int cmd_show_index(int argc, const char **argv, const char *prefix)
 			uint32_t off;
 		} *entries;
 		ALLOC_ARRAY(entries, nr);
-		for (i = 0; i < nr; i++)
+		for (i = 0; i < nr; i++) {
 			if (fread(entries[i].oid.hash, hashsz, 1, stdin) != 1)
 				die("unable to read sha1 %u/%u", i, nr);
+			entries[i].oid.algo = hash_algo_by_ptr(the_hash_algo);
+		}
 		for (i = 0; i < nr; i++)
 			if (fread(&entries[i].crc, 4, 1, stdin) != 1)
 				die("unable to read crc %u/%u", i, nr);

builtin/submodule--helper.c

@@ -426,7 +426,8 @@ static int module_list(int argc, const char **argv, const char *prefix)
 		const struct cache_entry *ce = list.entries[i];
 
 		if (ce_stage(ce))
-			printf("%06o %s U\t", ce->ce_mode, oid_to_hex(&null_oid));
+			printf("%06o %s U\t", ce->ce_mode,
+			       oid_to_hex(null_oid()));
 		else
 			printf("%06o %s %d\t", ce->ce_mode,
 			       oid_to_hex(&ce->oid), ce_stage(ce));
@@ -466,7 +467,7 @@ static void runcommand_in_submodule_cb(const struct cache_entry *list_item,
 
 	displaypath = get_submodule_displaypath(path, info->prefix);
 
-	sub = submodule_from_path(the_repository, &null_oid, path);
+	sub = submodule_from_path(the_repository, null_oid(), path);
 
 	if (!sub)
 		die(_("No url found for submodule path '%s' in .gitmodules"),
@@ -623,7 +624,7 @@ static void init_submodule(const char *path, const char *prefix,
 
 	displaypath = get_submodule_displaypath(path, prefix);
 
-	sub = submodule_from_path(the_repository, &null_oid, path);
+	sub = submodule_from_path(the_repository, null_oid(), path);
 
 	if (!sub)
 		die(_("No url found for submodule path '%s' in .gitmodules"),
@@ -783,14 +784,14 @@ static void status_submodule(const char *path, const struct object_id *ce_oid,
 	struct strbuf buf = STRBUF_INIT;
 	const char *git_dir;
 
-	if (!submodule_from_path(the_repository, &null_oid, path))
+	if (!submodule_from_path(the_repository, null_oid(), path))
 		die(_("no submodule mapping found in .gitmodules for path '%s'"),
 		      path);
 
 	displaypath = get_submodule_displaypath(path, prefix);
 
 	if ((CE_STAGEMASK & ce_flags) >> CE_STAGESHIFT) {
-		print_status(flags, 'U', path, &null_oid, displaypath);
+		print_status(flags, 'U', path, null_oid(), displaypath);
 		goto cleanup;
 	}
 
@@ -916,7 +917,7 @@ static int module_name(int argc, const char **argv, const char *prefix)
 	if (argc != 2)
 		usage(_("git submodule--helper name <path>"));
 
-	sub = submodule_from_path(the_repository, &null_oid, argv[1]);
+	sub = submodule_from_path(the_repository, null_oid(), argv[1]);
 
 	if (!sub)
 		die(_("no submodule mapping found in .gitmodules for path '%s'"),
@@ -1040,7 +1041,7 @@ static void generate_submodule_summary(struct summary_cb *info,
 	char *errmsg = NULL;
 	int total_commits = -1;
 
-	if (!info->cached && oideq(&p->oid_dst, &null_oid)) {
+	if (!info->cached && oideq(&p->oid_dst, null_oid())) {
 		if (S_ISGITLINK(p->mod_dst)) {
 			struct ref_store *refs = get_submodule_ref_store(p->sm_path);
 			if (refs)
@@ -1177,7 +1178,7 @@ static void prepare_submodule_summary(struct summary_cb *info,
 
 		if (info->for_status && p->status != 'A' &&
 		    (sub = submodule_from_path(the_repository,
-					       &null_oid, p->sm_path))) {
+					       null_oid(), p->sm_path))) {
 			char *config_key = NULL;
 			const char *value;
 			int ignore_all = 0;
@@ -1373,7 +1374,7 @@ static void sync_submodule(const char *path, const char *prefix,
 	if (!is_submodule_active(the_repository, path))
 		return;
 
-	sub = submodule_from_path(the_repository, &null_oid, path);
+	sub = submodule_from_path(the_repository, null_oid(), path);
 
 	if (sub && sub->url) {
 		if (starts_with_dot_dot_slash(sub->url) ||
@@ -1525,7 +1526,7 @@ static void deinit_submodule(const char *path, const char *prefix,
 	struct strbuf sb_config = STRBUF_INIT;
 	char *sub_git_dir = xstrfmt("%s/.git", path);
 
-	sub = submodule_from_path(the_repository, &null_oid, path);
+	sub = submodule_from_path(the_repository, null_oid(), path);
 
 	if (!sub || !sub->name)
 		goto cleanup;
@@ -1925,7 +1926,7 @@ static void determine_submodule_update_strategy(struct repository *r,
 						const char *update,
 						struct submodule_update_strategy *out)
 {
-	const struct submodule *sub = submodule_from_path(r, &null_oid, path);
+	const struct submodule *sub = submodule_from_path(r, null_oid(), path);
 	char *key;
 	const char *val;
 
@@ -2077,7 +2078,7 @@ static int prepare_to_clone_next_submodule(const struct cache_entry *ce,
 		goto cleanup;
 	}
 
-	sub = submodule_from_path(the_repository, &null_oid, ce->name);
+	sub = submodule_from_path(the_repository, null_oid(), ce->name);
 
 	if (suc->recursive_prefix)
 		displaypath = relative_path(suc->recursive_prefix,
@@ -2395,7 +2396,7 @@ static const char *remote_submodule_branch(const char *path)
 	const char *branch = NULL;
 	char *key;
 
-	sub = submodule_from_path(the_repository, &null_oid, path);
+	sub = submodule_from_path(the_repository, null_oid(), path);
 	if (!sub)
 		return NULL;
 
@@ -2533,7 +2534,7 @@ static int ensure_core_worktree(int argc, const char **argv, const char *prefix)
 
 	path = argv[1];
 
-	sub = submodule_from_path(the_repository, &null_oid, path);
+	sub = submodule_from_path(the_repository, null_oid(), path);
 	if (!sub)
 		BUG("We could get the submodule handle before?");
 

builtin/unpack-objects.c

@@ -355,7 +355,7 @@ static void unpack_delta_entry(enum object_type type, unsigned long delta_size,
 	struct object_id base_oid;
 
 	if (type == OBJ_REF_DELTA) {
-		hashcpy(base_oid.hash, fill(the_hash_algo->rawsz));
+		oidread(&base_oid, fill(the_hash_algo->rawsz));
 		use(the_hash_algo->rawsz);
 		delta_data = get_data(delta_size);
 		if (dry_run || !delta_data) {
@@ -421,7 +421,8 @@ static void unpack_delta_entry(enum object_type type, unsigned long delta_size,
 			 * has not been resolved yet.
 			 */
 			oidclr(&obj_list[nr].oid);
-			add_delta_to_list(nr, &null_oid, base_offset, delta_data, delta_size);
+			add_delta_to_list(nr, null_oid(), base_offset,
+					  delta_data, delta_size);
 			return;
 		}
 	}
@@ -576,7 +577,7 @@ int cmd_unpack_objects(int argc, const char **argv, const char *prefix)
 	the_hash_algo->init_fn(&ctx);
 	unpack_all();
 	the_hash_algo->update_fn(&ctx, buffer, offset);
-	the_hash_algo->final_fn(oid.hash, &ctx);
+	the_hash_algo->final_oid_fn(&oid, &ctx);
 	if (strict) {
 		write_rest();
 		if (fsck_finish(&fsck_options))

builtin/worktree.c

@@ -331,7 +331,7 @@ static int add_worktree(const char *path, const char *refname,
 	 */
 	strbuf_reset(&sb);
 	strbuf_addf(&sb, "%s/HEAD", sb_repo.buf);
-	write_file(sb.buf, "%s", oid_to_hex(&null_oid));
+	write_file(sb.buf, "%s", oid_to_hex(null_oid()));
 	strbuf_reset(&sb);
 	strbuf_addf(&sb, "%s/commondir", sb_repo.buf);
 	write_file(sb.buf, "../..");
@@ -394,7 +394,7 @@ done:
 			cp.argv = NULL;
 			cp.trace2_hook_name = "post-checkout";
 			strvec_pushl(&cp.args, absolute_path(hook),
-				     oid_to_hex(&null_oid),
+				     oid_to_hex(null_oid()),
 				     oid_to_hex(&commit->object.oid),
 				     "1", NULL);
 			ret = run_command(&cp);

bulk-checkin.c

@@ -238,7 +238,7 @@ static int deflate_to_pack(struct bulk_checkin_state *state,
 		if (lseek(fd, seekback, SEEK_SET) == (off_t) -1)
 			return error("cannot seek back");
 	}
-	the_hash_algo->final_fn(result_oid->hash, &ctx);
+	the_hash_algo->final_oid_fn(result_oid, &ctx);
 	if (!idx)
 		return 0;
 

combine-diff.c

@@ -1068,7 +1068,7 @@ static void show_patch_diff(struct combine_diff_path *elem, int num_parent,
 						   &result_size, NULL, NULL);
 		} else if (textconv) {
 			struct diff_filespec *df = alloc_filespec(elem->path);
-			fill_filespec(df, &null_oid, 0, st.st_mode);
+			fill_filespec(df, null_oid(), 0, st.st_mode);
 			result_size = fill_textconv(opt->repo, textconv, df, &result);
 			free_filespec(df);
 		} else if (0 <= (fd = open(elem->path, O_RDONLY))) {

commit-graph.c

@@ -425,7 +425,7 @@ struct commit_graph *parse_commit_graph(struct repository *r,
 		FREE_AND_NULL(graph->bloom_filter_settings);
 	}
 
-	hashcpy(graph->oid.hash, graph->data + graph->data_len - graph->hash_len);
+	oidread(&graph->oid, graph->data + graph->data_len - graph->hash_len);
 
 	if (verify_commit_graph_lite(graph))
 		goto free_and_return;
@@ -746,7 +746,7 @@ static void load_oid_from_graph(struct commit_graph *g,
 
 	lex_index = pos - g->num_commits_in_base;
 
-	hashcpy(oid->hash, g->chunk_oid_lookup + g->hash_len * lex_index);
+	oidread(oid, g->chunk_oid_lookup + g->hash_len * lex_index);
 }
 
 static struct commit_list **insert_parent_or_die(struct repository *r,
@@ -939,7 +939,7 @@ static struct tree *load_tree_for_commit(struct repository *r,
 	commit_data = g->chunk_commit_data +
 			GRAPH_DATA_WIDTH * (graph_pos - g->num_commits_in_base);
 
-	hashcpy(oid.hash, commit_data);
+	oidread(&oid, commit_data);
 	set_commit_tree(c, lookup_tree(r, &oid));
 
 	return c->maybe_tree;
@@ -1793,8 +1793,8 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx)
 	struct lock_file lk = LOCK_INIT;
 	const unsigned hashsz = the_hash_algo->rawsz;
 	struct strbuf progress_title = STRBUF_INIT;
-	struct object_id file_hash;
 	struct chunkfile *cf;
+	unsigned char file_hash[GIT_MAX_RAWSZ];
 
 	if (ctx->split) {
 		struct strbuf tmp_file = STRBUF_INIT;
@@ -1909,7 +1909,7 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx)
 	}
 
 	close_commit_graph(ctx->r->objects);
-	finalize_hashfile(f, file_hash.hash, CSUM_HASH_IN_STREAM | CSUM_FSYNC);
+	finalize_hashfile(f, file_hash, CSUM_HASH_IN_STREAM | CSUM_FSYNC);
 	free_chunkfile(cf);
 
 	if (ctx->split) {
@@ -1945,7 +1945,7 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx)
 			unlink(graph_name);
 		}
 
-		ctx->commit_graph_hash_after[ctx->num_commit_graphs_after - 1] = xstrdup(oid_to_hex(&file_hash));
+		ctx->commit_graph_hash_after[ctx->num_commit_graphs_after - 1] = xstrdup(hash_to_hex(file_hash));
 		final_graph_name = get_split_graph_filename(ctx->odb,
 					ctx->commit_graph_hash_after[ctx->num_commit_graphs_after - 1]);
 		ctx->commit_graph_filenames_after[ctx->num_commit_graphs_after - 1] = final_graph_name;
@@ -2322,7 +2322,7 @@ int write_commit_graph(struct object_directory *odb,
 		struct commit_graph *g = ctx->r->objects->commit_graph;
 		for (i = 0; i < g->num_commits; i++) {
 			struct object_id oid;
-			hashcpy(oid.hash, g->chunk_oid_lookup + g->hash_len * i);
+			oidread(&oid, g->chunk_oid_lookup + g->hash_len * i);
 			oid_array_append(&ctx->oids, &oid);
 		}
 	}
@@ -2425,7 +2425,8 @@ static void graph_report(const char *fmt, ...)
 int verify_commit_graph(struct repository *r, struct commit_graph *g, int flags)
 {
 	uint32_t i, cur_fanout_pos = 0;
-	struct object_id prev_oid, cur_oid, checksum;
+	struct object_id prev_oid, cur_oid;
+	unsigned char checksum[GIT_MAX_HEXSZ];
 	int generation_zero = 0;
 	struct hashfile *f;
 	int devnull;
@@ -2444,8 +2445,8 @@ int verify_commit_graph(struct repository *r, struct commit_graph *g, int flags)
 	devnull = open("/dev/null", O_WRONLY);
 	f = hashfd(devnull, NULL);
 	hashwrite(f, g->data, g->data_len - g->hash_len);
-	finalize_hashfile(f, checksum.hash, CSUM_CLOSE);
+	finalize_hashfile(f, checksum, CSUM_CLOSE);
-	if (!hasheq(checksum.hash, g->data + g->data_len - g->hash_len)) {
+	if (!hasheq(checksum, g->data + g->data_len - g->hash_len)) {
 		graph_report(_("the commit-graph file has incorrect checksum and is likely corrupt"));
 		verify_commit_graph_error = VERIFY_COMMIT_GRAPH_ERROR_HASH;
 	}
@@ -2453,7 +2454,7 @@ int verify_commit_graph(struct repository *r, struct commit_graph *g, int flags)
 	for (i = 0; i < g->num_commits; i++) {
 		struct commit *graph_commit;
 
-		hashcpy(cur_oid.hash, g->chunk_oid_lookup + g->hash_len * i);
+		oidread(&cur_oid, g->chunk_oid_lookup + g->hash_len * i);
 
 		if (i && oidcmp(&prev_oid, &cur_oid) >= 0)
 			graph_report(_("commit-graph has incorrect OID order: %s then %s"),
@@ -2501,7 +2502,7 @@ int verify_commit_graph(struct repository *r, struct commit_graph *g, int flags)
 		timestamp_t generation;
 
 		display_progress(progress, i + 1);
-		hashcpy(cur_oid.hash, g->chunk_oid_lookup + g->hash_len * i);
+		oidread(&cur_oid, g->chunk_oid_lookup + g->hash_len * i);
 
 		graph_commit = lookup_commit(r, &cur_oid);
 		odb_commit = (struct commit *)create_object(r, &cur_oid, alloc_commit_node(r));

connect.c

@@ -254,7 +254,7 @@ static int process_dummy_ref(const struct packet_reader *reader)
 		return 0;
 	name++;
 
-	return oideq(&null_oid, &oid) && !strcmp(name, "capabilities^{}");
+	return oideq(null_oid(), &oid) && !strcmp(name, "capabilities^{}");
 }
 
 static void check_no_capabilities(const char *line, int len)

diff-lib.c

@@ -232,7 +232,7 @@ int run_diff_files(struct rev_info *revs, unsigned int option)
 				   ce_intent_to_add(ce)) {
 				newmode = ce_mode_from_stat(ce, st.st_mode);
 				diff_addremove(&revs->diffopt, '+', newmode,
-					       &null_oid, 0, ce->name, 0);
+					       null_oid(), 0, ce->name, 0);
 				continue;
 			}
 
@@ -249,7 +249,7 @@ int run_diff_files(struct rev_info *revs, unsigned int option)
 		}
 		oldmode = ce->ce_mode;
 		old_oid = &ce->oid;
-		new_oid = changed ? &null_oid : &ce->oid;
+		new_oid = changed ? null_oid() : &ce->oid;
 		diff_change(&revs->diffopt, oldmode, newmode,
 			    old_oid, new_oid,
 			    !is_null_oid(old_oid),
@@ -307,7 +307,7 @@ static int get_stat_data(const struct index_state *istate,
 						    0, dirty_submodule);
 		if (changed) {
 			mode = ce_mode_from_stat(ce, st.st_mode);
-			oid = &null_oid;
+			oid = null_oid();
 		}
 	}
 

diff-no-index.c

@@ -83,7 +83,7 @@ static struct diff_filespec *noindex_filespec(const char *name, int mode)
 	if (!name)
 		name = "/dev/null";
 	s = alloc_filespec(name);
-	fill_filespec(s, &null_oid, 0, mode);
+	fill_filespec(s, null_oid(), 0, mode);
 	if (name == file_from_standard_input)
 		populate_from_stdin(s);
 	return s;

diff.c

@@ -4190,7 +4190,7 @@ static struct diff_tempfile *prepare_temp_file(struct repository *r,
 				die_errno("readlink(%s)", name);
 			prep_temp_blob(r->index, name, temp, sb.buf, sb.len,
 				       (one->oid_valid ?
-					&one->oid : &null_oid),
+					&one->oid : null_oid()),
 				       (one->oid_valid ?
 					one->mode : S_IFLNK));
 			strbuf_release(&sb);
@@ -4199,7 +4199,7 @@ static struct diff_tempfile *prepare_temp_file(struct repository *r,
 			/* we can borrow from the file in the work tree */
 			temp->name = name;
 			if (!one->oid_valid)
-				oid_to_hex_r(temp->hex, &null_oid);
+				oid_to_hex_r(temp->hex, null_oid());
 			else
 				oid_to_hex_r(temp->hex, &one->oid);
 			/* Even though we may sometimes borrow the
@@ -6234,7 +6234,7 @@ static int diff_get_patch_id(struct diff_options *options, struct object_id *oid
 	}
 
 	if (!stable)
-		the_hash_algo->final_fn(oid->hash, &ctx);
+		the_hash_algo->final_oid_fn(oid, &ctx);
 
 	return 0;
 }

dir.c

@@ -3344,7 +3344,7 @@ static void read_oid(size_t pos, void *cb)
 		rd->data = rd->end + 1;
 		return;
 	}
-	hashcpy(ud->exclude_oid.hash, rd->data);
+	oidread(&ud->exclude_oid, rd->data);
 	rd->data += the_hash_algo->rawsz;
 }
 
@@ -3352,7 +3352,7 @@ static void load_oid_stat(struct oid_stat *oid_stat, const unsigned char *data,
 			  const unsigned char *sha1)
 {
 	stat_data_from_disk(&oid_stat->stat, data);
-	hashcpy(oid_stat->oid.hash, sha1);
+	oidread(&oid_stat->oid, sha1);
 	oid_stat->valid = 1;
 }
 
@@ -3558,7 +3558,7 @@ static void connect_wt_gitdir_in_nested(const char *sub_worktree,
 			 */
 			i++;
 
-		sub = submodule_from_path(&subrepo, &null_oid, ce->name);
+		sub = submodule_from_path(&subrepo, null_oid(), ce->name);
 		if (!sub || !is_submodule_active(&subrepo, ce->name))
 			/* .gitmodules broken or inactive sub */
 			continue;

grep.c

@@ -1494,7 +1494,7 @@ static int fill_textconv_grep(struct repository *r,
 		fill_filespec(df, gs->identifier, 1, 0100644);
 		break;
 	case GREP_SOURCE_FILE:
-		fill_filespec(df, &null_oid, 0, 0100644);
+		fill_filespec(df, null_oid(), 0, 0100644);
 		break;
 	default:
 		BUG("attempt to textconv something without a path?");

hash.h

@@ -95,6 +95,29 @@ static inline void git_SHA256_Clone(git_SHA256_CTX *dst, const git_SHA256_CTX *s
 /* Number of algorithms supported (including unknown). */
 #define GIT_HASH_NALGOS (GIT_HASH_SHA256 + 1)
 
+/* The length in bytes and in hex digits of an object name (SHA-1 value). */
+#define GIT_SHA1_RAWSZ 20
+#define GIT_SHA1_HEXSZ (2 * GIT_SHA1_RAWSZ)
+/* The block size of SHA-1. */
+#define GIT_SHA1_BLKSZ 64
+
+/* The length in bytes and in hex digits of an object name (SHA-256 value). */
+#define GIT_SHA256_RAWSZ 32
+#define GIT_SHA256_HEXSZ (2 * GIT_SHA256_RAWSZ)
+/* The block size of SHA-256. */
+#define GIT_SHA256_BLKSZ 64
+
+/* The length in byte and in hex digits of the largest possible hash value. */
+#define GIT_MAX_RAWSZ GIT_SHA256_RAWSZ
+#define GIT_MAX_HEXSZ GIT_SHA256_HEXSZ
+/* The largest possible block size for any supported hash. */
+#define GIT_MAX_BLKSZ GIT_SHA256_BLKSZ
+
+struct object_id {
+	unsigned char hash[GIT_MAX_RAWSZ];
+	int algo;
+};
+
 /* A suitably aligned type for stack allocations of hash contexts. */
 union git_hash_ctx {
 	git_SHA_CTX sha1;
@@ -106,6 +129,7 @@ typedef void (*git_hash_init_fn)(git_hash_ctx *ctx);
 typedef void (*git_hash_clone_fn)(git_hash_ctx *dst, const git_hash_ctx *src);
 typedef void (*git_hash_update_fn)(git_hash_ctx *ctx, const void *in, size_t len);
 typedef void (*git_hash_final_fn)(unsigned char *hash, git_hash_ctx *ctx);
+typedef void (*git_hash_final_oid_fn)(struct object_id *oid, git_hash_ctx *ctx);
 
 struct git_hash_algo {
 	/*
@@ -138,11 +162,17 @@ struct git_hash_algo {
 	/* The hash finalization function. */
 	git_hash_final_fn final_fn;
 
+	/* The hash finalization function for object IDs. */
+	git_hash_final_oid_fn final_oid_fn;
+
 	/* The OID of the empty tree. */
 	const struct object_id *empty_tree;
 
 	/* 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];
 
@@ -161,67 +191,65 @@ static inline int hash_algo_by_ptr(const struct git_hash_algo *p)
 	return p - hash_algos;
 }
 
-/* The length in bytes and in hex digits of an object name (SHA-1 value). */
-#define GIT_SHA1_RAWSZ 20
-#define GIT_SHA1_HEXSZ (2 * GIT_SHA1_RAWSZ)
-/* The block size of SHA-1. */
-#define GIT_SHA1_BLKSZ 64
-
-/* The length in bytes and in hex digits of an object name (SHA-256 value). */
-#define GIT_SHA256_RAWSZ 32
-#define GIT_SHA256_HEXSZ (2 * GIT_SHA256_RAWSZ)
-/* The block size of SHA-256. */
-#define GIT_SHA256_BLKSZ 64
-
-/* The length in byte and in hex digits of the largest possible hash value. */
-#define GIT_MAX_RAWSZ GIT_SHA256_RAWSZ
-#define GIT_MAX_HEXSZ GIT_SHA256_HEXSZ
-/* The largest possible block size for any supported hash. */
-#define GIT_MAX_BLKSZ GIT_SHA256_BLKSZ
-
-struct object_id {
-	unsigned char hash[GIT_MAX_RAWSZ];
-};
-
 #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(const unsigned char *sha1, const unsigned char *sha2)
+static inline int hashcmp_algop(const unsigned char *sha1, const unsigned char *sha2, const struct git_hash_algo *algop)
 {
 	/*
 	 * Teach the compiler that there are only two possibilities of hash size
 	 * here, so that it can optimize for this case as much as possible.
 	 */
-	if (the_hash_algo->rawsz == GIT_MAX_RAWSZ)
+	if (algop->rawsz == GIT_MAX_RAWSZ)
 		return memcmp(sha1, sha2, GIT_MAX_RAWSZ);
 	return memcmp(sha1, sha2, GIT_SHA1_RAWSZ);
 }
 
-static inline int oidcmp(const struct object_id *oid1, const struct object_id *oid2)
+static inline int hashcmp(const unsigned char *sha1, const unsigned char *sha2)
 {
-	return hashcmp(oid1->hash, oid2->hash);
+	return hashcmp_algop(sha1, sha2, the_hash_algo);
 }
 
-static inline int hasheq(const unsigned char *sha1, const unsigned char *sha2)
+static inline int oidcmp(const struct object_id *oid1, const struct object_id *oid2)
+{
+	const struct git_hash_algo *algop;
+	if (!oid1->algo)
+		algop = the_hash_algo;
+	else
+		algop = &hash_algos[oid1->algo];
+	return hashcmp_algop(oid1->hash, oid2->hash, algop);
+}
+
+static inline int hasheq_algop(const unsigned char *sha1, const unsigned char *sha2, const struct git_hash_algo *algop)
 {
 	/*
 	 * We write this here instead of deferring to hashcmp so that the
 	 * compiler can properly inline it and avoid calling memcmp.
 	 */
-	if (the_hash_algo->rawsz == GIT_MAX_RAWSZ)
+	if (algop->rawsz == GIT_MAX_RAWSZ)
 		return !memcmp(sha1, sha2, GIT_MAX_RAWSZ);
 	return !memcmp(sha1, sha2, GIT_SHA1_RAWSZ);
 }
 
+static inline int hasheq(const unsigned char *sha1, const unsigned char *sha2)
+{
+	return hasheq_algop(sha1, sha2, the_hash_algo);
+}
+
 static inline int oideq(const struct object_id *oid1, const struct object_id *oid2)
 {
-	return hasheq(oid1->hash, oid2->hash);
+	const struct git_hash_algo *algop;
+	if (!oid1->algo)
+		algop = the_hash_algo;
+	else
+		algop = &hash_algos[oid1->algo];
+	return hasheq_algop(oid1->hash, oid2->hash, algop);
 }
 
 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)
@@ -232,6 +260,7 @@ static inline void hashcpy(unsigned char *sha_dst, const unsigned char *sha_src)
 static inline void oidcpy(struct object_id *dst, const struct object_id *src)
 {
 	memcpy(dst->hash, src->hash, GIT_MAX_RAWSZ);
+	dst->algo = src->algo;
 }
 
 static inline struct object_id *oiddup(const struct object_id *src)
@@ -249,11 +278,13 @@ static inline void hashclr(unsigned char *hash)
 static inline void oidclr(struct object_id *oid)
 {
 	memset(oid->hash, 0, GIT_MAX_RAWSZ);
+	oid->algo = hash_algo_by_ptr(the_hash_algo);
 }
 
 static inline void oidread(struct object_id *oid, const unsigned char *hash)
 {
 	memcpy(oid->hash, hash, the_hash_algo->rawsz);
+	oid->algo = hash_algo_by_ptr(the_hash_algo);
 }
 
 static inline int is_empty_blob_sha1(const unsigned char *sha1)
@@ -276,6 +307,11 @@ static inline int is_empty_tree_oid(const struct object_id *oid)
 	return oideq(oid, the_hash_algo->empty_tree);
 }
 
+static inline void oid_set_algo(struct object_id *oid, const struct git_hash_algo *algop)
+{
+	oid->algo = hash_algo_by_ptr(algop);
+}
+
 const char *empty_tree_oid_hex(void);
 const char *empty_blob_oid_hex(void);
 

hex.c

@@ -69,7 +69,10 @@ int get_sha1_hex(const char *hex, unsigned char *sha1)
 int get_oid_hex_algop(const char *hex, struct object_id *oid,
 		      const struct git_hash_algo *algop)
 {
-	return get_hash_hex_algop(hex, oid->hash, algop);
+	int ret = get_hash_hex_algop(hex, oid->hash, algop);
+	if (!ret)
+		oid_set_algo(oid, algop);
+	return ret;
 }
 
 /*
@@ -80,7 +83,7 @@ int get_oid_hex_any(const char *hex, struct object_id *oid)
 {
 	int i;
 	for (i = GIT_HASH_NALGOS - 1; i > 0; i--) {
-		if (!get_hash_hex_algop(hex, oid->hash, &hash_algos[i]))
+		if (!get_oid_hex_algop(hex, oid, &hash_algos[i]))
 			return i;
 	}
 	return GIT_HASH_UNKNOWN;
@@ -95,7 +98,7 @@ int parse_oid_hex_algop(const char *hex, struct object_id *oid,
 			const char **end,
 			const struct git_hash_algo *algop)
 {
-	int ret = get_hash_hex_algop(hex, oid->hash, algop);
+	int ret = get_oid_hex_algop(hex, oid, algop);
 	if (!ret)
 		*end = hex + algop->hexsz;
 	return ret;
@@ -121,6 +124,13 @@ char *hash_to_hex_algop_r(char *buffer, const unsigned char *hash,
 	char *buf = buffer;
 	int i;
 
+	/*
+	 * Our struct object_id has been memset to 0, so default to printing
+	 * using the default hash.
+	 */
+	if (algop == &hash_algos[0])
+		algop = the_hash_algo;
+
 	for (i = 0; i < algop->rawsz; i++) {
 		unsigned int val = *hash++;
 		*buf++ = hex[val >> 4];
@@ -133,7 +143,7 @@ char *hash_to_hex_algop_r(char *buffer, const unsigned char *hash,
 
 char *oid_to_hex_r(char *buffer, const struct object_id *oid)
 {
-	return hash_to_hex_algop_r(buffer, oid->hash, the_hash_algo);
+	return hash_to_hex_algop_r(buffer, oid->hash, &hash_algos[oid->algo]);
 }
 
 char *hash_to_hex_algop(const unsigned char *hash, const struct git_hash_algo *algop)
@@ -151,5 +161,5 @@ char *hash_to_hex(const unsigned char *hash)
 
 char *oid_to_hex(const struct object_id *oid)
 {
-	return hash_to_hex_algop(oid->hash, the_hash_algo);
+	return hash_to_hex_algop(oid->hash, &hash_algos[oid->algo]);
 }

http-push.c

@@ -1022,6 +1022,8 @@ static void remote_ls(const char *path, int flags,
 /* extract hex from sharded "xx/x{38}" filename */
 static int get_oid_hex_from_objpath(const char *path, struct object_id *oid)
 {
+	oid->algo = hash_algo_by_ptr(the_hash_algo);
+
 	if (strlen(path) != the_hash_algo->hexsz + 1)
 		return -1;
 

http-walker.c

@@ -155,7 +155,7 @@ static void prefetch(struct walker *walker, unsigned char *sha1)
 
 	newreq = xmalloc(sizeof(*newreq));
 	newreq->walker = walker;
-	hashcpy(newreq->oid.hash, sha1);
+	oidread(&newreq->oid, sha1);
 	newreq->repo = data->alt;
 	newreq->state = WAITING;
 	newreq->req = NULL;

http.c

@@ -2576,7 +2576,7 @@ int finish_http_object_request(struct http_object_request *freq)
 	}
 
 	git_inflate_end(&freq->stream);
-	the_hash_algo->final_fn(freq->real_oid.hash, &freq->c);
+	the_hash_algo->final_oid_fn(&freq->real_oid, &freq->c);
 	if (freq->zret != Z_STREAM_END) {
 		unlink_or_warn(freq->tmpfile.buf);
 		return -1;

log-tree.c

@@ -419,7 +419,7 @@ void log_write_email_headers(struct rev_info *opt, struct commit *commit,
 {
 	const char *extra_headers = opt->extra_headers;
 	const char *name = oid_to_hex(opt->zero_commit ?
-				      &null_oid : &commit->object.oid);
+				      null_oid() : &commit->object.oid);
 
 	*need_8bit_cte_p = 0; /* unknown */
 

match-trees.c

@@ -226,7 +226,7 @@ static int splice_tree(const struct object_id *oid1, const char *prefix,
 		    oid_to_hex(oid1));
 	if (*subpath) {
 		struct object_id tree_oid;
-		hashcpy(tree_oid.hash, rewrite_here);
+		oidread(&tree_oid, rewrite_here);
 		status = splice_tree(&tree_oid, subpath, oid2, &subtree);
 		if (status)
 			return status;

merge-ort.c

@@ -1406,7 +1406,7 @@ static int handle_content_merge(struct merge_options *opt,
 		two_way = ((S_IFMT & o->mode) != (S_IFMT & a->mode));
 
 		merge_status = merge_3way(opt, path,
-					  two_way ? &null_oid : &o->oid,
+					  two_way ? null_oid() : &o->oid,
 					  &a->oid, &b->oid,
 					  pathnames, extra_marker_size,
 					  &result_buf);
@@ -1428,7 +1428,7 @@ static int handle_content_merge(struct merge_options *opt,
 	} else if (S_ISGITLINK(a->mode)) {
 		int two_way = ((S_IFMT & o->mode) != (S_IFMT & a->mode));
 		clean = merge_submodule(opt, pathnames[0],
-					two_way ? &null_oid : &o->oid,
+					two_way ? null_oid() : &o->oid,
 					&a->oid, &b->oid, &result->oid);
 		if (opt->priv->call_depth && two_way && !clean) {
 			result->mode = o->mode;
@@ -2230,7 +2230,7 @@ static int process_renames(struct merge_options *opt,
 			if (type_changed) {
 				/* rename vs. typechange */
 				/* Mark the original as resolved by removal */
-				memcpy(&oldinfo->stages[0].oid, &null_oid,
+				memcpy(&oldinfo->stages[0].oid, null_oid(),
 				       sizeof(oldinfo->stages[0].oid));
 				oldinfo->stages[0].mode = 0;
 				oldinfo->filemask &= 0x06;
@@ -2917,7 +2917,7 @@ static void process_entry(struct merge_options *opt,
 			if (ci->filemask & (1 << i))
 				continue;
 			ci->stages[i].mode = 0;
-			oidcpy(&ci->stages[i].oid, &null_oid);
+			oidcpy(&ci->stages[i].oid, null_oid());
 		}
 	} else if (ci->df_conflict && ci->merged.result.mode != 0) {
 		/*
@@ -2963,7 +2963,7 @@ static void process_entry(struct merge_options *opt,
 				continue;
 			/* zero out any entries related to directories */
 			new_ci->stages[i].mode = 0;
-			oidcpy(&new_ci->stages[i].oid, &null_oid);
+			oidcpy(&new_ci->stages[i].oid, null_oid());
 		}
 
 		/*
@@ -3064,11 +3064,11 @@ static void process_entry(struct merge_options *opt,
 			new_ci->merged.result.mode = ci->stages[2].mode;
 			oidcpy(&new_ci->merged.result.oid, &ci->stages[2].oid);
 			new_ci->stages[1].mode = 0;
-			oidcpy(&new_ci->stages[1].oid, &null_oid);
+			oidcpy(&new_ci->stages[1].oid, null_oid());
 			new_ci->filemask = 5;
 			if ((S_IFMT & b_mode) != (S_IFMT & o_mode)) {
 				new_ci->stages[0].mode = 0;
-				oidcpy(&new_ci->stages[0].oid, &null_oid);
+				oidcpy(&new_ci->stages[0].oid, null_oid());
 				new_ci->filemask = 4;
 			}
 
@@ -3076,11 +3076,11 @@ static void process_entry(struct merge_options *opt,
 			ci->merged.result.mode = ci->stages[1].mode;
 			oidcpy(&ci->merged.result.oid, &ci->stages[1].oid);
 			ci->stages[2].mode = 0;
-			oidcpy(&ci->stages[2].oid, &null_oid);
+			oidcpy(&ci->stages[2].oid, null_oid());
 			ci->filemask = 3;
 			if ((S_IFMT & a_mode) != (S_IFMT & o_mode)) {
 				ci->stages[0].mode = 0;
-				oidcpy(&ci->stages[0].oid, &null_oid);
+				oidcpy(&ci->stages[0].oid, null_oid());
 				ci->filemask = 2;
 			}
 
@@ -3202,7 +3202,7 @@ static void process_entry(struct merge_options *opt,
 		/* Deleted on both sides */
 		ci->merged.is_null = 1;
 		ci->merged.result.mode = 0;
-		oidcpy(&ci->merged.result.oid, &null_oid);
+		oidcpy(&ci->merged.result.oid, null_oid());
 		ci->merged.clean = !ci->path_conflict;
 	}
 

merge-recursive.c

@@ -486,7 +486,7 @@ static int get_tree_entry_if_blob(struct repository *r,
 
 	ret = get_tree_entry(r, tree, path, &dfs->oid, &dfs->mode);
 	if (S_ISDIR(dfs->mode)) {
-		oidcpy(&dfs->oid, &null_oid);
+		oidcpy(&dfs->oid, null_oid());
 		dfs->mode = 0;
 	}
 	return ret;
@@ -1626,7 +1626,7 @@ static int handle_file_collision(struct merge_options *opt,
 
 	/* Store things in diff_filespecs for functions that need it */
 	null.path = (char *)collide_path;
-	oidcpy(&null.oid, &null_oid);
+	oidcpy(&null.oid, null_oid());
 	null.mode = 0;
 
 	if (merge_mode_and_contents(opt, &null, a, b, collide_path,
@@ -2819,11 +2819,11 @@ static int process_renames(struct merge_options *opt,
 			dst_other.mode = ren1->dst_entry->stages[other_stage].mode;
 			try_merge = 0;
 
-			if (oideq(&src_other.oid, &null_oid) &&
+			if (oideq(&src_other.oid, null_oid()) &&
 			    ren1->dir_rename_original_type == 'A') {
 				setup_rename_conflict_info(RENAME_VIA_DIR,
 							   opt, ren1, NULL);
-			} else if (oideq(&src_other.oid, &null_oid)) {
+			} else if (oideq(&src_other.oid, null_oid())) {
 				setup_rename_conflict_info(RENAME_DELETE,
 							   opt, ren1, NULL);
 			} else if ((dst_other.mode == ren1->pair->two->mode) &&
@@ -2842,7 +2842,7 @@ static int process_renames(struct merge_options *opt,
 						      1, /* update_cache */
 						      0  /* update_wd    */))
 					clean_merge = -1;
-			} else if (!oideq(&dst_other.oid, &null_oid)) {
+			} else if (!oideq(&dst_other.oid, null_oid())) {
 				/*
 				 * Probably not a clean merge, but it's
 				 * premature to set clean_merge to 0 here,

midx.c

@@ -247,7 +247,7 @@ struct object_id *nth_midxed_object_oid(struct object_id *oid,
 	if (n >= m->num_objects)
 		return NULL;
 
-	hashcpy(oid->hash, m->chunk_oid_lookup + m->hash_len * n);
+	oidread(oid, m->chunk_oid_lookup + m->hash_len * n);
 	return oid;
 }
 

notes-merge.c

@@ -600,7 +600,7 @@ int notes_merge(struct notes_merge_options *o,
 	/* Find merge bases */
 	bases = get_merge_bases(local, remote);
 	if (!bases) {
-		base_oid = &null_oid;
+		base_oid = null_oid();
 		base_tree_oid = the_hash_algo->empty_tree;
 		if (o->verbosity >= 4)
 			printf("No merge base found; doing history-less merge\n");

notes.c

@@ -352,7 +352,7 @@ static void add_non_note(struct notes_tree *t, char *path,
 	n->next = NULL;
 	n->path = path;
 	n->mode = mode;
-	hashcpy(n->oid.hash, sha1);
+	oidread(&n->oid, sha1);
 	t->prev_non_note = n;
 
 	if (!t->first_non_note) {
@@ -455,6 +455,8 @@ static void load_subtree(struct notes_tree *t, struct leaf_node *subtree,
 		CALLOC_ARRAY(l, 1);
 		oidcpy(&l->key_oid, &object_oid);
 		oidcpy(&l->val_oid, &entry.oid);
+		oid_set_algo(&l->key_oid, the_hash_algo);
+		oid_set_algo(&l->val_oid, the_hash_algo);
 		if (note_tree_insert(t, node, n, l, type,
 				     combine_notes_concatenate))
 			die("Failed to load %s %s into notes tree "
@@ -484,6 +486,7 @@ handle_non_note:
 				strbuf_addch(&non_note_path, '/');
 			}
 			strbuf_addstr(&non_note_path, entry.path);
+			oid_set_algo(&entry.oid, the_hash_algo);
 			add_non_note(t, strbuf_detach(&non_note_path, NULL),
 				     entry.mode, entry.oid.hash);
 		}
@@ -1134,7 +1137,7 @@ int remove_note(struct notes_tree *t, const unsigned char *object_sha1)
 	if (!t)
 		t = &default_notes_tree;
 	assert(t->initialized);
-	hashcpy(l.key_oid.hash, object_sha1);
+	oidread(&l.key_oid, object_sha1);
 	oidclr(&l.val_oid);
 	note_tree_remove(t, t->root, 0, &l);
 	if (is_null_oid(&l.val_oid)) /* no note was removed */
@@ -1324,7 +1327,7 @@ int copy_note(struct notes_tree *t,
 	if (note)
 		return add_note(t, to_obj, note, combine_notes);
 	else if (existing_note)
-		return add_note(t, to_obj, &null_oid, combine_notes);
+		return add_note(t, to_obj, null_oid(), combine_notes);
 
 	return 0;
 }

object-file.c

@@ -55,18 +55,29 @@
 	"\x6f\xe1\x41\xf7\x74\x91\x20\xa3\x03\x72" \
 	"\x18\x13"
 
-const struct object_id null_oid;
 static const struct object_id empty_tree_oid = {
-	EMPTY_TREE_SHA1_BIN_LITERAL
+	.hash = EMPTY_TREE_SHA1_BIN_LITERAL,
+	.algo = GIT_HASH_SHA1,
 };
 static const struct object_id empty_blob_oid = {
-	EMPTY_BLOB_SHA1_BIN_LITERAL
+	.hash = EMPTY_BLOB_SHA1_BIN_LITERAL,
+	.algo = GIT_HASH_SHA1,
+};
+static const struct object_id null_oid_sha1 = {
+	.hash = {0},
+	.algo = GIT_HASH_SHA1,
 };
 static const struct object_id empty_tree_oid_sha256 = {
-	EMPTY_TREE_SHA256_BIN_LITERAL
+	.hash = EMPTY_TREE_SHA256_BIN_LITERAL,
+	.algo = GIT_HASH_SHA256,
 };
 static const struct object_id empty_blob_oid_sha256 = {
-	EMPTY_BLOB_SHA256_BIN_LITERAL
+	.hash = EMPTY_BLOB_SHA256_BIN_LITERAL,
+	.algo = GIT_HASH_SHA256,
+};
+static const struct object_id null_oid_sha256 = {
+	.hash = {0},
+	.algo = GIT_HASH_SHA256,
 };
 
 static void git_hash_sha1_init(git_hash_ctx *ctx)
@@ -89,6 +100,13 @@ static void git_hash_sha1_final(unsigned char *hash, git_hash_ctx *ctx)
 	git_SHA1_Final(hash, &ctx->sha1);
 }
 
+static void git_hash_sha1_final_oid(struct object_id *oid, git_hash_ctx *ctx)
+{
+	git_SHA1_Final(oid->hash, &ctx->sha1);
+	memset(oid->hash + GIT_SHA1_RAWSZ, 0, GIT_MAX_RAWSZ - GIT_SHA1_RAWSZ);
+	oid->algo = GIT_HASH_SHA1;
+}
+
 
 static void git_hash_sha256_init(git_hash_ctx *ctx)
 {
@@ -110,6 +128,17 @@ static void git_hash_sha256_final(unsigned char *hash, git_hash_ctx *ctx)
 	git_SHA256_Final(hash, &ctx->sha256);
 }
 
+static void git_hash_sha256_final_oid(struct object_id *oid, git_hash_ctx *ctx)
+{
+	git_SHA256_Final(oid->hash, &ctx->sha256);
+	/*
+	 * This currently does nothing, so the compiler should optimize it out,
+	 * but keep it in case we extend the hash size again.
+	 */
+	memset(oid->hash + GIT_SHA256_RAWSZ, 0, GIT_MAX_RAWSZ - GIT_SHA256_RAWSZ);
+	oid->algo = GIT_HASH_SHA256;
+}
+
 static void git_hash_unknown_init(git_hash_ctx *ctx)
 {
 	BUG("trying to init unknown hash");
@@ -130,6 +159,12 @@ static void git_hash_unknown_final(unsigned char *hash, git_hash_ctx *ctx)
 	BUG("trying to finalize unknown hash");
 }
 
+static void git_hash_unknown_final_oid(struct object_id *oid, git_hash_ctx *ctx)
+{
+	BUG("trying to finalize unknown hash");
+}
+
+
 const struct git_hash_algo hash_algos[GIT_HASH_NALGOS] = {
 	{
 		NULL,
@@ -141,6 +176,8 @@ const struct git_hash_algo hash_algos[GIT_HASH_NALGOS] = {
 		git_hash_unknown_clone,
 		git_hash_unknown_update,
 		git_hash_unknown_final,
+		git_hash_unknown_final_oid,
+		NULL,
 		NULL,
 		NULL,
 	},
@@ -155,8 +192,10 @@ const struct git_hash_algo hash_algos[GIT_HASH_NALGOS] = {
 		git_hash_sha1_clone,
 		git_hash_sha1_update,
 		git_hash_sha1_final,
+		git_hash_sha1_final_oid,
 		&empty_tree_oid,
 		&empty_blob_oid,
+		&null_oid_sha1,
 	},
 	{
 		"sha256",
@@ -169,11 +208,18 @@ const struct git_hash_algo hash_algos[GIT_HASH_NALGOS] = {
 		git_hash_sha256_clone,
 		git_hash_sha256_update,
 		git_hash_sha256_final,
+		git_hash_sha256_final_oid,
 		&empty_tree_oid_sha256,
 		&empty_blob_oid_sha256,
+		&null_oid_sha256,
 	}
 };
 
+const struct object_id *null_oid(void)
+{
+	return the_hash_algo->null_oid;
+}
+
 const char *empty_tree_oid_hex(void)
 {
 	static char buf[GIT_MAX_HEXSZ + 1];
@@ -1029,7 +1075,7 @@ int check_object_signature(struct repository *r, const struct object_id *oid,
 			break;
 		r->hash_algo->update_fn(&c, buf, readlen);
 	}
-	r->hash_algo->final_fn(real_oid.hash, &c);
+	r->hash_algo->final_oid_fn(&real_oid, &c);
 	close_istream(st);
 	return !oideq(oid, &real_oid) ? -1 : 0;
 }
@@ -1730,7 +1776,7 @@ static void write_object_file_prepare(const struct git_hash_algo *algo,
 	algo->init_fn(&c);
 	algo->update_fn(&c, hdr, *hdrlen);
 	algo->update_fn(&c, buf, len);
-	algo->final_fn(oid->hash, &c);
+	algo->final_oid_fn(oid, &c);
 }
 
 /*
@@ -1902,7 +1948,7 @@ static int write_loose_object(const struct object_id *oid, char *hdr,
 	if (ret != Z_OK)
 		die(_("deflateEnd on object %s failed (%d)"), oid_to_hex(oid),
 		    ret);
-	the_hash_algo->final_fn(parano_oid.hash, &c);
+	the_hash_algo->final_oid_fn(&parano_oid, &c);
 	if (!oideq(oid, &parano_oid))
 		die(_("confused by unstable object source data for %s"),
 		    oid_to_hex(oid));
@@ -2315,6 +2361,7 @@ int for_each_file_in_obj_subdir(unsigned int subdir_nr,
 		if (namelen == the_hash_algo->hexsz - 2 &&
 		    !hex_to_bytes(oid.hash + 1, de->d_name,
 				  the_hash_algo->rawsz - 1)) {
+			oid_set_algo(&oid, the_hash_algo);
 			if (obj_cb) {
 				r = obj_cb(&oid, path->buf, data);
 				if (r)
@@ -2483,7 +2530,7 @@ static int check_stream_oid(git_zstream *stream,
 		return -1;
 	}
 
-	the_hash_algo->final_fn(real_oid.hash, &c);
+	the_hash_algo->final_oid_fn(&real_oid, &c);
 	if (!oideq(expected_oid, &real_oid)) {
 		error(_("hash mismatch for %s (expected %s)"), path,
 		      oid_to_hex(expected_oid));

parse-options-cb.c

@@ -140,7 +140,7 @@ int parse_opt_object_id(const struct option *opt, const char *arg, int unset)
 	struct object_id *target = opt->value;
 
 	if (unset) {
-		*target = null_oid;
+		oidcpy(target, null_oid());
 		return 0;
 	}
 	if (!arg)

range-diff.c

@@ -445,7 +445,7 @@ static struct diff_filespec *get_filespec(const char *name, const char *p)
 {
 	struct diff_filespec *spec = alloc_filespec(name);
 
-	fill_filespec(spec, &null_oid, 0, 0100644);
+	fill_filespec(spec, null_oid(), 0, 0100644);
 	spec->data = (char *)p;
 	spec->size = strlen(p);
 	spec->should_munmap = 0;

read-cache.c

@@ -1886,7 +1886,7 @@ static struct cache_entry *create_from_disk(struct mem_pool *ce_mem_pool,
 	ce->ce_flags = flags & ~CE_NAMEMASK;
 	ce->ce_namelen = len;
 	ce->index = 0;
-	hashcpy(ce->oid.hash, ondisk->data);
+	oidread(&ce->oid, ondisk->data);
 	memcpy(ce->name, name, len);
 	ce->name[len] = '\0';
 
@@ -2236,7 +2236,7 @@ int do_read_index(struct index_state *istate, const char *path, int must_exist)
 	if (verify_hdr(hdr, mmap_size) < 0)
 		goto unmap;
 
-	hashcpy(istate->oid.hash, (const unsigned char *)hdr + mmap_size - the_hash_algo->rawsz);
+	oidread(&istate->oid, (const unsigned char *)hdr + mmap_size - the_hash_algo->rawsz);
 	istate->version = ntohl(hdr->hdr_version);
 	istate->cache_nr = ntohl(hdr->hdr_entries);
 	istate->cache_alloc = alloc_nr(istate->cache_nr);

refs.c

@@ -1107,7 +1107,7 @@ int ref_transaction_create(struct ref_transaction *transaction,
 	if (!new_oid || is_null_oid(new_oid))
 		BUG("create called without valid new_oid");
 	return ref_transaction_update(transaction, refname, new_oid,
-				      &null_oid, flags, msg, err);
+				      null_oid(), flags, msg, err);
 }
 
 int ref_transaction_delete(struct ref_transaction *transaction,
@@ -1119,7 +1119,7 @@ int ref_transaction_delete(struct ref_transaction *transaction,
 	if (old_oid && is_null_oid(old_oid))
 		BUG("delete called with old_oid set to zeros");
 	return ref_transaction_update(transaction, refname,
-				      &null_oid, old_oid,
+				      null_oid(), old_oid,
 				      flags, msg, err);
 }
 

refs/debug.c

@@ -243,7 +243,7 @@ static int debug_read_raw_ref(struct ref_store *ref_store, const char *refname,
 	struct debug_ref_store *drefs = (struct debug_ref_store *)ref_store;
 	int res = 0;
 
-	oidcpy(oid, &null_oid);
+	oidcpy(oid, null_oid());
 	errno = 0;
 	res = drefs->refs->be->read_raw_ref(drefs->refs, refname, oid, referent,
 					    type);

refs/files-backend.c

@@ -1084,7 +1084,7 @@ static void prune_ref(struct files_ref_store *refs, struct ref_to_prune *r)
 	ref_transaction_add_update(
 			transaction, r->name,
 			REF_NO_DEREF | REF_HAVE_NEW | REF_HAVE_OLD | REF_IS_PRUNING,
-			&null_oid, &r->oid, NULL);
+			null_oid(), &r->oid, NULL);
 	if (ref_transaction_commit(transaction, &err))
 		goto cleanup;
 

reset.c

@@ -128,7 +128,7 @@ reset_head_refs:
 	}
 	if (run_hook)
 		run_hook_le(NULL, "post-checkout",
-			    oid_to_hex(orig ? orig : &null_oid),
+			    oid_to_hex(orig ? orig : null_oid()),
 			    oid_to_hex(oid), "1", NULL);
 
 leave_reset_head:

sequencer.c

@@ -525,7 +525,7 @@ static int fast_forward_to(struct repository *r,
 	if (!transaction ||
 	    ref_transaction_update(transaction, "HEAD",
 				   to, unborn && !is_rebase_i(opts) ?
-				   &null_oid : from,
+				   null_oid() : from,
 				   0, sb.buf, &err) ||
 	    ref_transaction_commit(transaction, &err)) {
 		ref_transaction_free(transaction);
@@ -1132,7 +1132,7 @@ int update_head_with_reflog(const struct commit *old_head,
 	transaction = ref_transaction_begin(err);
 	if (!transaction ||
 	    ref_transaction_update(transaction, "HEAD", new_head,
-				   old_head ? &old_head->object.oid : &null_oid,
+				   old_head ? &old_head->object.oid : null_oid(),
 				   0, sb.buf, err) ||
 	    ref_transaction_commit(transaction, err)) {
 		ret = -1;

split-index.c

@@ -21,7 +21,7 @@ int read_link_extension(struct index_state *istate,
 	if (sz < the_hash_algo->rawsz)
 		return error("corrupt link extension (too short)");
 	si = init_split_index(istate);
-	hashcpy(si->base_oid.hash, data);
+	oidread(&si->base_oid, data);
 	data += the_hash_algo->rawsz;
 	sz -= the_hash_algo->rawsz;
 	if (!sz)

submodule-config.c

@@ -671,7 +671,7 @@ static int gitmodules_cb(const char *var, const char *value, void *data)
 
 	parameter.cache = repo->submodule_cache;
 	parameter.treeish_name = NULL;
-	parameter.gitmodules_oid = &null_oid;
+	parameter.gitmodules_oid = null_oid();
 	parameter.overwrite = 1;
 
 	return parse_config(var, value, &parameter);

submodule.c

@@ -113,7 +113,7 @@ int update_path_in_gitmodules(const char *oldpath, const char *newpath)
 	if (is_gitmodules_unmerged(the_repository->index))
 		die(_("Cannot change unmerged .gitmodules, resolve merge conflicts first"));
 
-	submodule = submodule_from_path(the_repository, &null_oid, oldpath);
+	submodule = submodule_from_path(the_repository, null_oid(), oldpath);
 	if (!submodule || !submodule->name) {
 		warning(_("Could not find section in .gitmodules where path=%s"), oldpath);
 		return -1;
@@ -142,7 +142,7 @@ int remove_path_from_gitmodules(const char *path)
 	if (is_gitmodules_unmerged(the_repository->index))
 		die(_("Cannot change unmerged .gitmodules, resolve merge conflicts first"));
 
-	submodule = submodule_from_path(the_repository, &null_oid, path);
+	submodule = submodule_from_path(the_repository, null_oid(), path);
 	if (!submodule || !submodule->name) {
 		warning(_("Could not find section in .gitmodules where path=%s"), path);
 		return -1;
@@ -188,7 +188,8 @@ void set_diffopt_flags_from_submodule_config(struct diff_options *diffopt,
 					     const char *path)
 {
 	const struct submodule *submodule = submodule_from_path(the_repository,
-								&null_oid, path);
+								null_oid(),
+								path);
 	if (submodule) {
 		const char *ignore;
 		char *key;
@@ -244,7 +245,7 @@ int is_submodule_active(struct repository *repo, const char *path)
 	const struct string_list *sl;
 	const struct submodule *module;
 
-	module = submodule_from_path(repo, &null_oid, path);
+	module = submodule_from_path(repo, null_oid(), path);
 
 	/* early return if there isn't a path->module mapping */
 	if (!module)
@@ -745,7 +746,7 @@ const struct submodule *submodule_from_ce(const struct cache_entry *ce)
 	if (!should_update_submodules())
 		return NULL;
 
-	return submodule_from_path(the_repository, &null_oid, ce->name);
+	return submodule_from_path(the_repository, null_oid(), ce->name);
 }
 
 static struct oid_array *submodule_commits(struct string_list *submodules,
@@ -1037,7 +1038,7 @@ int find_unpushed_submodules(struct repository *r,
 		const struct submodule *submodule;
 		const char *path = NULL;
 
-		submodule = submodule_from_name(r, &null_oid, name->string);
+		submodule = submodule_from_name(r, null_oid(), name->string);
 		if (submodule)
 			path = submodule->path;
 		else
@@ -1224,7 +1225,7 @@ static void calculate_changed_submodule_paths(struct repository *r,
 		const struct submodule *submodule;
 		const char *path = NULL;
 
-		submodule = submodule_from_name(r, &null_oid, name->string);
+		submodule = submodule_from_name(r, null_oid(), name->string);
 		if (submodule)
 			path = submodule->path;
 		else
@@ -1361,7 +1362,7 @@ static struct fetch_task *fetch_task_create(struct repository *r,
 	struct fetch_task *task = xmalloc(sizeof(*task));
 	memset(task, 0, sizeof(*task));
 
-	task->sub = submodule_from_path(r, &null_oid, path);
+	task->sub = submodule_from_path(r, null_oid(), path);
 	if (!task->sub) {
 		/*
 		 * No entry in .gitmodules? Technically not a submodule,
@@ -1917,7 +1918,7 @@ int submodule_move_head(const char *path,
 	if (old_head && !is_submodule_populated_gently(path, error_code_ptr))
 		return 0;
 
-	sub = submodule_from_path(the_repository, &null_oid, path);
+	sub = submodule_from_path(the_repository, null_oid(), path);
 
 	if (!sub)
 		BUG("could not get submodule information for '%s'", path);
@@ -2076,7 +2077,7 @@ static void relocate_single_git_dir_into_superproject(const char *path)
 
 	real_old_git_dir = real_pathdup(old_git_dir, 1);
 
-	sub = submodule_from_path(the_repository, &null_oid, path);
+	sub = submodule_from_path(the_repository, null_oid(), path);
 	if (!sub)
 		die(_("could not lookup name for submodule '%s'"), path);
 
@@ -2135,7 +2136,7 @@ void absorb_git_dir_into_superproject(const char *path,
 		* superproject did not rewrite the git file links yet,
 		* fix it now.
 		*/
-		sub = submodule_from_path(the_repository, &null_oid, path);
+		sub = submodule_from_path(the_repository, null_oid(), path);
 		if (!sub)
 			die(_("could not lookup name for submodule '%s'"), path);
 		connect_work_tree_and_git_dir(path,
@@ -2283,7 +2284,8 @@ int submodule_to_gitdir(struct strbuf *buf, const char *submodule)
 		strbuf_addstr(buf, git_dir);
 	}
 	if (!is_git_directory(buf->buf)) {
-		sub = submodule_from_path(the_repository, &null_oid, submodule);
+		sub = submodule_from_path(the_repository, null_oid(),
+					  submodule);
 		if (!sub) {
 			ret = -1;
 			goto cleanup;

t/helper/test-submodule-nested-repo-config.c

@@ -18,7 +18,7 @@ int cmd__submodule_nested_repo_config(int argc, const char **argv)
 
 	setup_git_directory();
 
-	sub = submodule_from_path(the_repository, &null_oid, argv[1]);
+	sub = submodule_from_path(the_repository, null_oid(), argv[1]);
 	if (repo_submodule_init(&subrepo, the_repository, sub)) {
 		die_usage(argv, "Submodule not found.");
 	}

tree-diff.c

@@ -161,7 +161,7 @@ static struct combine_diff_path *path_appendnew(struct combine_diff_path *last,
 	memcpy(p->path + base->len, path, pathlen);
 	p->path[len] = 0;
 	p->mode = mode;
-	oidcpy(&p->oid, oid ? oid : &null_oid);
+	oidcpy(&p->oid, oid ? oid : null_oid());
 
 	return p;
 }
@@ -243,7 +243,7 @@ static struct combine_diff_path *emit_path(struct combine_diff_path *p,
 				mode_i = tp[i].entry.mode;
 			}
 			else {
-				oid_i = &null_oid;
+				oid_i = null_oid();
 				mode_i = 0;
 			}
 

tree-walk.c

@@ -49,7 +49,7 @@ static int decode_tree_entry(struct tree_desc *desc, const char *buf, unsigned l
 	desc->entry.path = path;
 	desc->entry.mode = canon_mode(mode);
 	desc->entry.pathlen = len - 1;
-	hashcpy(desc->entry.oid.hash, (const unsigned char *)path + len);
+	oidread(&desc->entry.oid, (const unsigned char *)path + len);
 
 	return 0;
 }

wt-status.c

@@ -1690,10 +1690,10 @@ void wt_status_get_state(struct repository *r,
 	if (!sequencer_get_last_command(r, &action)) {
 		if (action == REPLAY_PICK) {
 			state->cherry_pick_in_progress = 1;
-			oidcpy(&state->cherry_pick_head_oid, &null_oid);
+			oidcpy(&state->cherry_pick_head_oid, null_oid());
 		} else {
 			state->revert_in_progress = 1;
-			oidcpy(&state->revert_head_oid, &null_oid);
+			oidcpy(&state->revert_head_oid, null_oid());
 		}
 	}
 	if (get_detached_from)

xdiff-interface.c

@@ -172,7 +172,7 @@ void read_mmblob(mmfile_t *ptr, const struct object_id *oid)
 	unsigned long size;
 	enum object_type type;
 
-	if (oideq(oid, &null_oid)) {
+	if (oideq(oid, null_oid())) {
 		ptr->ptr = xstrdup("");
 		ptr->size = 0;
 		return;