hash: stop depending on the_repository in null_oid()

The `null_oid()` function returns the object ID that only consists of
zeroes. Naturally, this ID also depends on the hash algorithm used, as
the number of zeroes is different between SHA1 and SHA256. Consequently,
the function returns the hash-algorithm-specific null object ID.

This is currently done by depending on `the_hash_algo`, which implicitly
makes us depend on `the_repository`. Refactor the function to instead
pass in the hash algorithm for which we want to retrieve the null object
ID. Adapt callsites accordingly by passing in `the_repository`, thus
bubbling up the dependency on that global variable by one layer.

There are a couple of trivial exceptions for subsystems that already got
rid of `the_repository`. These subsystems instead use the repository
that is available via the calling context:

  - "builtin/grep.c"
  - "grep.c"
  - "refs/debug.c"

There are also two non-trivial exceptions:

  - "diff-no-index.c": Here we know that we may not have a repository
    initialized at all, so we cannot rely on `the_repository`. Instead,
    we adapt `diff_no_index()` to get a `struct git_hash_algo` as
    parameter. The only caller is located in "builtin/diff.c", where we
    know to call `repo_set_hash_algo()` in case we're running outside of
    a Git repository. Consequently, it is fine to continue passing
    `the_repository->hash_algo` even in this case.

  - "builtin/ls-files.c": There is an in-flight patch series that drops
    `USE_THE_REPOSITORY_VARIABLE` in this file, which causes a semantic
    conflict because we use `null_oid()` in `show_submodule()`. The
    value is passed to `repo_submodule_init()`, which may use the object
    ID to resolve a tree-ish in the superproject from which we want to
    read the submodule config. As such, the object ID should refer to an
    object in the superproject, and consequently we need to use its hash
    algorithm.

    This means that we could in theory just not bother about this edge
    case at all and just use `the_repository` in "diff-no-index.c". But
    doing so would feel misdesigned.

Remove the `USE_THE_REPOSITORY_VARIABLE` preprocessor define in
"hash.c".

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

archive.c

@@ -312,7 +312,7 @@ int write_archive_entries(struct archiver_args *args,
 	struct object_id fake_oid;
 	int i;
 
-	oidcpy(&fake_oid, null_oid());
+	oidcpy(&fake_oid, null_oid(the_hash_algo));
 
 	if (args->baselen > 0 && args->base[args->baselen - 1] == '/') {
 		size_t len = args->baselen;

blame.c

@@ -255,7 +255,7 @@ 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(the_hash_algo), 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

@@ -633,7 +633,7 @@ void create_branch(struct repository *r,
 						  0, &err);
 	if (!transaction ||
 		ref_transaction_update(transaction, ref.buf,
-					&oid, forcing ? NULL : null_oid(),
+					&oid, forcing ? NULL : null_oid(the_hash_algo),
 					NULL, NULL, flags, msg, &err) ||
 		ref_transaction_commit(transaction, &err))
 		die("%s", err.buf);

builtin/checkout.c

@@ -130,8 +130,8 @@ static int post_checkout_hook(struct commit *old_commit, struct commit *new_comm
 			      int changed)
 {
 	return run_hooks_l(the_repository, "post-checkout",
-			   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(old_commit ? &old_commit->object.oid : null_oid(the_hash_algo)),
+			   oid_to_hex(new_commit ? &new_commit->object.oid : null_oid(the_hash_algo)),
 			   changed ? "1" : "0", NULL);
 	/* "new_commit" can be NULL when checking out from the index before
 	   a commit exists. */
@@ -710,7 +710,7 @@ static int reset_tree(struct tree *tree, const struct checkout_opts *o,
 	opts.src_index = the_repository->index;
 	opts.dst_index = the_repository->index;
 	init_checkout_metadata(&opts.meta, info->refname,
-			       info->commit ? &info->commit->object.oid : null_oid(),
+			       info->commit ? &info->commit->object.oid : null_oid(the_hash_algo),
 			       NULL);
 	if (parse_tree(tree) < 0)
 		return 128;

builtin/clone.c

@@ -690,7 +690,7 @@ static int checkout(int submodule_progress, int filter_submodules,
 	if (write_locked_index(the_repository->index, &lock_file, COMMIT_LOCK))
 		die(_("unable to write new index file"));
 
-	err |= run_hooks_l(the_repository, "post-checkout", oid_to_hex(null_oid()),
+	err |= run_hooks_l(the_repository, "post-checkout", oid_to_hex(null_oid(the_hash_algo)),
 			   oid_to_hex(&oid), "1", NULL);
 
 	if (!err && (option_recurse_submodules.nr > 0)) {

builtin/describe.c

@@ -518,7 +518,7 @@ 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(the_hash_algo), oid, dst, &revs};
 
 	strvec_pushl(&args, "internal: The first arg is not parsed",
 		     "--objects", "--in-commit-order", "--reverse", "HEAD",

builtin/diff.c

@@ -104,7 +104,7 @@ static void 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(the_hash_algo),
 		     1, 0,
 		     blob[0]->path ? blob[0]->path : path,
 		     path);
@@ -498,7 +498,8 @@ int cmd_diff(int argc,
 
 	/* If this is a no-index diff, just run it and exit there. */
 	if (no_index)
-		exit(diff_no_index(&rev, no_index == DIFF_NO_INDEX_IMPLICIT,
+		exit(diff_no_index(&rev, the_repository->hash_algo,
+				   no_index == DIFF_NO_INDEX_IMPLICIT,
 				   argc, argv));
 
 

builtin/fast-export.c

@@ -869,7 +869,7 @@ 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(the_hash_algo)));
 					free(buf);
 					return;
 				}
@@ -883,7 +883,7 @@ 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(the_hash_algo)));
 	}
 	skip_prefix(name, "refs/tags/", &name);
 	printf("tag %s\n", name);
@@ -1023,7 +1023,7 @@ 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(the_hash_algo)));
 				continue;
 			}
 
@@ -1042,7 +1042,7 @@ 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(the_hash_algo)));
 					continue;
 				}
 				/* set ref to commit using oid, not mark */
@@ -1153,7 +1153,7 @@ 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(the_hash_algo)));
 	}
 }
 

builtin/fsck.c

@@ -625,7 +625,7 @@ static int fsck_loose(const struct object_id *oid, const char *path, void *data)
 	void *contents = NULL;
 	int eaten;
 	struct object_info oi = OBJECT_INFO_INIT;
-	struct object_id real_oid = *null_oid();
+	struct object_id real_oid = *null_oid(the_hash_algo);
 	int err = 0;
 
 	strbuf_reset(&cb_data->obj_type);

builtin/grep.c

@@ -453,7 +453,7 @@ static int grep_submodule(struct grep_opt *opt,
 		return 0;
 
 	subrepo = xmalloc(sizeof(*subrepo));
-	if (repo_submodule_init(subrepo, superproject, path, null_oid())) {
+	if (repo_submodule_init(subrepo, superproject, path, null_oid(opt->repo->hash_algo))) {
 		free(subrepo);
 		return 0;
 	}