refs: allow reference location in refstorage config

The 'extensions.refStorage' config is used to specify the reference
backend for a given repository. Both the 'files' and 'reftable' backends
utilize the $GIT_DIR as the reference folder by default in
`get_main_ref_store()`.

Since the reference backends are pluggable, this means that they should
work with out-of-tree reference directories too. Extend the 'refStorage'
config to also support taking an URI input, where users can specify the
reference backend and the location.

Add the required changes to obtain and propagate this value to the
individual backends. A follow up commit will add the required changes on
the backends to parse this value.

Helped-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: KarthikNayak

builtin/clone.c

@@ -1442,7 +1442,8 @@ int cmd_clone(int argc,
 	hash_algo = hash_algo_by_ptr(transport_get_hash_algo(transport));
 	initialize_repository_version(hash_algo, the_repository->ref_storage_format, 1);
 	repo_set_hash_algo(the_repository, hash_algo);
-	create_reference_database(the_repository->ref_storage_format, NULL, 1);
+	create_reference_database(the_repository->ref_storage_format,
+				  the_repository->ref_storage_payload, NULL, 1);
 
 	/*
 	 * Before fetching from the remote, download and install bundle

repository.c

@@ -193,9 +193,12 @@ void repo_set_compat_hash_algo(struct repository *repo, int algo)
 }
 
 void repo_set_ref_storage_format(struct repository *repo,
-				 enum ref_storage_format format)
+				 enum ref_storage_format format,
+				 const char *payload)
 {
 	repo->ref_storage_format = format;
+	free(repo->ref_storage_payload);
+	repo->ref_storage_payload = xstrdup_or_null(payload);
 }
 
 /*
@@ -277,7 +280,8 @@ int repo_init(struct repository *repo,
 
 	repo_set_hash_algo(repo, format.hash_algo);
 	repo_set_compat_hash_algo(repo, format.compat_hash_algo);
-	repo_set_ref_storage_format(repo, format.ref_storage_format);
+	repo_set_ref_storage_format(repo, format.ref_storage_format,
+				    format.ref_storage_payload);
 	repo->repository_format_worktree_config = format.worktree_config;
 	repo->repository_format_relative_worktrees = format.relative_worktrees;
 	repo->repository_format_precious_objects = format.precious_objects;
@@ -369,6 +373,7 @@ void repo_clear(struct repository *repo)
 	FREE_AND_NULL(repo->index_file);
 	FREE_AND_NULL(repo->worktree);
 	FREE_AND_NULL(repo->submodule_prefix);
+	FREE_AND_NULL(repo->ref_storage_payload);
 
 	odb_free(repo->objects);
 	repo->objects = NULL;

repository.h

@@ -150,6 +150,8 @@ struct repository {
 
 	/* Repository's reference storage format, as serialized on disk. */
 	enum ref_storage_format ref_storage_format;
+	/* Reference storage information as needed for the backend. */
+	char *ref_storage_payload;
 
 	/* A unique-id for tracing purposes. */
 	int trace2_repo_id;
@@ -204,7 +206,8 @@ void repo_set_worktree(struct repository *repo, const char *path);
 void repo_set_hash_algo(struct repository *repo, int algo);
 void repo_set_compat_hash_algo(struct repository *repo, int compat_algo);
 void repo_set_ref_storage_format(struct repository *repo,
-				 enum ref_storage_format format);
+				 enum ref_storage_format format,
+				 const char *payload);
 void initialize_repository(struct repository *repo);
 RESULT_MUST_BE_USED
 int repo_init(struct repository *r, const char *gitdir, const char *worktree);

setup.c

@@ -632,6 +632,21 @@ static enum extension_result handle_extension_v0(const char *var,
 		return EXTENSION_UNKNOWN;
 }
 
+static void parse_reference_uri(const char *value, char **format,
+				char **payload)
+{
+	char *schema_end;
+
+	schema_end = strstr(value, "://");
+	if (!schema_end) {
+		*format = xstrdup(value);
+		*payload = NULL;
+	} else {
+		*format = xstrndup(value, schema_end - value);
+		*payload = xstrdup_or_null(schema_end + 3);
+	}
+}
+
 /*
  * Record any new extensions in this function.
  */
@@ -674,10 +689,17 @@ static enum extension_result handle_extension(const char *var,
 		return EXTENSION_OK;
 	} else if (!strcmp(ext, "refstorage")) {
 		unsigned int format;
+		char *format_str;
 
 		if (!value)
 			return config_error_nonbool(var);
-		format = ref_storage_format_by_name(value);
+
+		parse_reference_uri(value, &format_str,
+				    &data->ref_storage_payload);
+
+		format = ref_storage_format_by_name(format_str);
+		free(format_str);
+
 		if (format == REF_STORAGE_FORMAT_UNKNOWN)
 			return error(_("invalid value for '%s': '%s'"),
 				     "extensions.refstorage", value);
@@ -850,6 +872,7 @@ void clear_repository_format(struct repository_format *format)
 	string_list_clear(&format->v1_only_extensions, 0);
 	free(format->work_tree);
 	free(format->partial_clone);
+	free(format->ref_storage_payload);
 	init_repository_format(format);
 }
 
@@ -1942,7 +1965,8 @@ const char *setup_git_directory_gently(int *nongit_ok)
 			repo_set_compat_hash_algo(the_repository,
 						  repo_fmt.compat_hash_algo);
 			repo_set_ref_storage_format(the_repository,
-						    repo_fmt.ref_storage_format);
+						    repo_fmt.ref_storage_format,
+						    repo_fmt.ref_storage_payload);
 			the_repository->repository_format_worktree_config =
 				repo_fmt.worktree_config;
 			the_repository->repository_format_relative_worktrees =
@@ -2042,7 +2066,8 @@ void check_repository_format(struct repository_format *fmt)
 	repo_set_hash_algo(the_repository, fmt->hash_algo);
 	repo_set_compat_hash_algo(the_repository, fmt->compat_hash_algo);
 	repo_set_ref_storage_format(the_repository,
-				    fmt->ref_storage_format);
+				    fmt->ref_storage_format,
+				    fmt->ref_storage_payload);
 	the_repository->repository_format_worktree_config =
 		fmt->worktree_config;
 	the_repository->repository_format_relative_worktrees =
@@ -2360,13 +2385,15 @@ static int is_reinit(void)
 }
 
 void create_reference_database(enum ref_storage_format ref_storage_format,
+			       const char *ref_storage_payload,
 			       const char *initial_branch, int quiet)
 {
 	struct strbuf err = STRBUF_INIT;
 	char *to_free = NULL;
 	int reinit = is_reinit();
 
-	repo_set_ref_storage_format(the_repository, ref_storage_format);
+	repo_set_ref_storage_format(the_repository, ref_storage_format,
+				    ref_storage_payload);
 	if (ref_store_create_on_disk(get_main_ref_store(the_repository), 0, &err))
 		die("failed to set up refs db: %s", err.buf);
 
@@ -2645,7 +2672,8 @@ static void repository_format_configure(struct repository_format *repo_fmt,
 	} else {
 		repo_fmt->ref_storage_format = REF_STORAGE_FORMAT_DEFAULT;
 	}
-	repo_set_ref_storage_format(the_repository, repo_fmt->ref_storage_format);
+	repo_set_ref_storage_format(the_repository, repo_fmt->ref_storage_format,
+				    repo_fmt->ref_storage_payload);
 }
 
 int init_db(const char *git_dir, const char *real_git_dir,
@@ -2702,6 +2730,7 @@ int init_db(const char *git_dir, const char *real_git_dir,
 
 	if (!(flags & INIT_DB_SKIP_REFDB))
 		create_reference_database(repo_fmt.ref_storage_format,
+					  repo_fmt.ref_storage_payload,
 					  initial_branch, flags & INIT_DB_QUIET);
 	create_object_directory();
 

setup.h

@@ -171,6 +171,7 @@ struct repository_format {
 	int hash_algo;
 	int compat_hash_algo;
 	enum ref_storage_format ref_storage_format;
+	char *ref_storage_payload;
 	int sparse_index;
 	char *work_tree;
 	struct string_list unknown_extensions;
@@ -241,6 +242,7 @@ void initialize_repository_version(int hash_algo,
 				   enum ref_storage_format ref_storage_format,
 				   int reinit);
 void create_reference_database(enum ref_storage_format ref_storage_format,
+			       const char *ref_storage_payload,
 			       const char *initial_branch, int quiet);
 
 /*