submodule: allow gitsubmodules to be pointed at tags

index 880ab4456ee..e7f150230e9 100644
@@ -1541,9 +1541,11 @@ struct module_clone_data {
 	unsigned int dissociate: 1;
 	unsigned int require_init: 1;
 	int single_branch;
+	int single_tag;
 };
 #define MODULE_CLONE_DATA_INIT { \
 	.single_branch = -1, \
+	.single_tag = -1, \
 }
 
 struct submodule_alternate_setup {

@@ -1754,6 +1756,10 @@ static int clone_submodule(const struct module_clone_data *clone_data,
 			strvec_push(&cp.args, clone_data->single_branch ?
 				    "--single-branch" :
 				    "--no-single-branch");
+		if (clone_data->single_tag >= 0)
+			strvec_push(&cp.args, clone_data->single_tag ?
+				    "--single-branch" :
+				    "--no-single-branch");
 
 		strvec_push(&cp.args, "--");
 		strvec_push(&cp.args, clone_data->url);

@@ -1981,6 +1987,7 @@ struct update_data {
 	int depth;
 	int max_jobs;
 	int single_branch;
+	int single_tag;
 	int recommend_shallow;
 	unsigned int require_init;
 	unsigned int force;

@@ -2004,6 +2011,7 @@ struct update_data {
 	.recommend_shallow = -1, \
 	.references = STRING_LIST_INIT_DUP, \
 	.single_branch = -1, \
+	.single_tag = -1, \
 	.max_jobs = 1, \
 }
 

@@ -2151,6 +2159,10 @@ static int prepare_to_clone_next_submodule(const struct cache_entry *ce,
 		strvec_push(&child->args, suc->update_data->single_branch ?
 					      "--single-branch" :
 					      "--no-single-branch");
+	if (suc->update_data->single_tag >= 0)
+		strvec_push(&child->args, suc->update_data->single_tag ?
+					      "--single-tag" :
+					      "--no-single-tag");
 
 cleanup:
 	free(displaypath);

@@ -2297,6 +2309,7 @@ static int fetch_in_submodule(const char *module_path, int depth, int quiet,
 	cp.dir = module_path;
 
 	strvec_push(&cp.args, "fetch");
+
 	if (quiet)
 		strvec_push(&cp.args, "--quiet");
 	if (depth)

@@ -2434,6 +2447,30 @@ static int run_update_procedure(const struct update_data *ud)
 	return run_update_command(ud, subforce);
 }
 
+static int remote_submodule_tag(const char *path, const char **tag)
+{
+	const struct submodule *sub;
+	char *key;
+	*tag = NULL;
+
+	sub = submodule_from_path(the_repository, null_oid(), path);
+	if (!sub)
+		return die_message(_("could not initialize submodule at path '%s'"),
+				   path);
+
+	key = xstrfmt("submodule.%s.tag", sub->name);
+	if (repo_config_get_string_tmp(the_repository, key, tag))
+		*tag = sub->tag;
+	free(key);
+
+	if (!*tag) {
+		/* No tag found */
+		return 1;
+	}
+
+	return 0;
+}
+
 static int remote_submodule_branch(const char *path, const char **branch)
 {
 	const struct submodule *sub;

@@ -2579,6 +2616,10 @@ static void update_data_to_args(const struct update_data *update_data,
 		strvec_push(args, update_data->single_branch ?
 				    "--single-branch" :
 				    "--no-single-branch");
+	if (update_data->single_tag >= 0)
+		strvec_push(args, update_data->single_tag ?
+				    "--single-branch" :
+				    "--no-single-branch");
 }
 
 static int update_submodule(struct update_data *update_data)

@@ -2606,16 +2647,22 @@ static int update_submodule(struct update_data *update_data)
 	if (update_data->remote) {
 		char *remote_name;
 		const char *branch;
+		const char *tag;
 		char *remote_ref;
 		int code;
 
 		code = get_default_remote_submodule(update_data->sm_path, &remote_name);
 		if (code)
 			return code;
-		code = remote_submodule_branch(update_data->sm_path, &branch);
-		if (code)
-			return code;
-		remote_ref = xstrfmt("refs/remotes/%s/%s", remote_name, branch);
+		code = remote_submodule_tag(update_data->sm_path, &tag);
+		if (!code && update_data->single_tag) {
+				remote_ref = xstrfmt("refs/tags/%s", tag);
+		} else {
+			code = remote_submodule_branch(update_data->sm_path, &branch);
+			if (code)
+				return code;
+			remote_ref = xstrfmt("refs/remotes/%s/%s", remote_name, branch);
+		}
 
 		free(remote_name);
 

@@ -2781,6 +2828,8 @@ static int module_update(int argc, const char **argv, const char *prefix)
 			   N_("disallow cloning into non-empty directory, implies --init")),
 		OPT_BOOL(0, "single-branch", &opt.single_branch,
 			 N_("clone only one branch, HEAD or --branch")),
+		OPT_BOOL(0, "single-tag", &opt.single_tag,
+			 N_("clone only one tag, --tag")),
 		OPT_PARSE_LIST_OBJECTS_FILTER(&filter_options),
 		OPT_END()
 	};

@@ -2790,7 +2839,7 @@ static int module_update(int argc, const char **argv, const char *prefix)
 		" [-N|--no-fetch] [-f|--force]"
 		" [--checkout|--merge|--rebase]"
 		" [--[no-]recommend-shallow] [--reference <repository>]"
-		" [--recursive] [--[no-]single-branch] [--] [<path>...]"),
+		" [--recursive] [--[no-]single-branch] [--[no-]single-tag] [--] [<path>...]"),
 		NULL
 	};
 

@@ -3058,6 +3107,52 @@ static int module_set_branch(int argc, const char **argv, const char *prefix)
 	return !!ret;
 }
 
+
+static int module_set_tag(int argc, const char **argv, const char *prefix)
+{
+	int ret;
+	const char *opt_tag = NULL;
+	const char *path;
+	char *config_name;
+	struct option options[] = {
+		/*
+		 * We accept the `quiet` option for uniformity across subcommands,
+		 * though there is nothing to make less verbose in this subcommand.
+		 */
+		OPT_NOOP_NOARG('q', "quiet"),
+
+		OPT_STRING('t', "tag", &opt_tag, N_("tag"),
+			N_("set the tracking tag")),
+		OPT_END()
+	};
+
+	const char *const usage[] = {
+		N_("git submodule set-tag [-q|--quiet] (-t|--tag) <tag> <path>"),
+		NULL
+	};
+	const struct submodule *sub;
+
+	argc = parse_options(argc, argv, prefix, options, usage, 0);
+
+	if (!opt_tag)
+		die(_("--tag required"));
+
+	if (argc != 1 || !(path = argv[0]))
+		usage_with_options(usage, options);
+
+	sub = submodule_from_path(the_repository, null_oid(), path);
+
+	if (!sub)
+		die(_("no submodule mapping found in .gitmodules for path '%s'"),
+		    path);
+
+	config_name = xstrfmt("submodule.%s.tag", sub->tag);
+	ret = config_set_in_gitmodules_file_gently(config_name, opt_tag);
+
+	free(config_name);
+	return !!ret;
+}
+
 static int module_create_branch(int argc, const char **argv, const char *prefix)
 {
 	enum branch_track track;

@@ -3098,6 +3193,7 @@ static int module_create_branch(int argc, const char **argv, const char *prefix)
 struct add_data {
 	const char *prefix;
 	const char *branch;
+	const char *tag;
 	const char *reference_path;
 	char *sm_path;
 	const char *sm_name;

@@ -3219,7 +3315,9 @@ static int add_submodule(const struct add_data *add_data)
 		 */
 		strvec_pushl(&cp.args, "checkout", "-f", "-q", NULL);
 
-		if (add_data->branch) {
+		if (add_data->tag) {
+			strvec_pushf(&cp.args, "%s", add_data->tag);
+		} else if (add_data->branch) {
 			strvec_pushl(&cp.args, "-B", add_data->branch, NULL);
 			strvec_pushf(&cp.args, "origin/%s", add_data->branch);
 		}

@@ -3272,7 +3370,11 @@ static void configure_added_submodule(struct add_data *add_data)
 	    config_submodule_in_gitmodules(add_data->sm_name, "url", add_data->repo))
 		die(_("Failed to register submodule '%s'"), add_data->sm_path);
 
-	if (add_data->branch) {
+	if (add_data->tag) {
+		if (config_submodule_in_gitmodules(add_data->sm_name,
+						"tag", add_data->tag))
+			die(_("Failed to register submodule '%s'"), add_data->sm_path);
+	} else if (add_data->branch) {
 		if (config_submodule_in_gitmodules(add_data->sm_name,
 						   "branch", add_data->branch))
 			die(_("Failed to register submodule '%s'"), add_data->sm_path);

@@ -3371,6 +3473,8 @@ static int module_add(int argc, const char **argv, const char *prefix)
 	struct option options[] = {
 		OPT_STRING('b', "branch", &add_data.branch, N_("branch"),
 			   N_("branch of repository to add as submodule")),
+		OPT_STRING('t', "tag", &add_data.tag, N_("tag"),
+			   N_("tag of repository to add as submodule")),
 		OPT__FORCE(&force, N_("allow adding an otherwise ignored submodule path"),
 			   PARSE_OPT_NOCOMPLETE),
 		OPT__QUIET(&quiet, N_("print only error messages")),

@@ -3506,6 +3610,7 @@ int cmd_submodule__helper(int argc, const char **argv, const char *prefix)
 		OPT_SUBCOMMAND("set-url", &fn, module_set_url),
 		OPT_SUBCOMMAND("set-branch", &fn, module_set_branch),
 		OPT_SUBCOMMAND("create-branch", &fn, module_create_branch),
+		OPT_SUBCOMMAND("set-tag", &fn, module_set_tag),
 		OPT_END()
 	};
 	argc = parse_options(argc, argv, prefix, options, usage, 0);

index ec45ea67b95..a258406cced 100644
@@ -93,6 +93,7 @@ static void free_one_config(struct submodule_entry *entry)
 	free((void *) entry->config->branch);
 	free((void *) entry->config->url);
 	free((void *) entry->config->ignore);
+	free((void *) entry->config->tag);
 	free((void *) entry->config->update_strategy.command);
 	free(entry->config);
 }

@@ -415,6 +416,7 @@ static struct submodule *lookup_or_create_by_name(struct submodule_cache *cache,
 	submodule->fetch_recurse = RECURSE_SUBMODULES_NONE;
 	submodule->ignore = NULL;
 	submodule->branch = NULL;
+	submodule->tag = NULL;
 	submodule->recommend_shallow = -1;
 
 	oidcpy(&submodule->gitmodules_oid, gitmodules_oid);

@@ -660,6 +662,8 @@ static int parse_config(const char *var, const char *value,
 	} else if (!strcmp(item.buf, "branch")) {
 		if (!value)
 			ret = config_error_nonbool(var);
+		else if (submodule->branch && submodule->tag)
+			die(_("can not specify both tag and branch '%s'"), var);
 		else if (!me->overwrite && submodule->branch)
 			warn_multiple_config(me->treeish_name, submodule->name,
 					     "branch");

@@ -667,6 +671,16 @@ static int parse_config(const char *var, const char *value,
 			free((void *)submodule->branch);
 			submodule->branch = xstrdup(value);
 		}
+	} else if (!strcmp(item.buf, "tag")) {
+		if (!value)
+			ret = config_error_nonbool(var);
+		else if (!me->overwrite && submodule->tag)
+			warn_multiple_config(me->treeish_name, submodule->name,
+					     "tag");
+		else {
+			free((void *)submodule->tag);
+			submodule->tag = xstrdup(value);
+		}
 	}
 
 	strbuf_release(&name);

index b6133af71b0..522683cc130 100644
@@ -38,6 +38,7 @@ struct submodule {
 	enum submodule_recurse_mode fetch_recurse;
 	const char *ignore;
 	const char *branch;
+	const char *tag;
 	struct submodule_update_strategy update_strategy;
 	/* the object id of the responsible .gitmodules file */
 	struct object_id gitmodules_oid;