diff mbox series

submodule: allow gitsubmodules to be pointed at tags

Message ID pull.1703.git.git.1718625706679.gitgitgadget@gmail.com (mailing list archive)
State New
Headers show
Series submodule: allow gitsubmodules to be pointed at tags | expand

Commit Message

dark0dave June 17, 2024, 12:01 p.m. UTC
From: dark0dave <dark0dave@mykolab.com>

Intent: Enable git submodules to point at tags,
effectively freezing them

Context: There is a large repo with 50+ submodules, the goal is
to have some submodules update to HEAD but others to remain fixed.
The update command does this by recursing through
directories. This is very useful, however if an update is run,
all the submodules are updated to HEAD.

The goals:
- Be able to create a submodule from a fixed tag (complete)
- Be able to run git submodule update, with a flag to either update
or keep the submodule pointed at the supplied tag (complete)

Example:

git submodule add changes

git submodule add --tag v34.3
https://github.com/Gibberlings3/SwordCoastStratagems.git
cat .gitmodules
[submodule "SwordCoastStratagems"]
  path = SwordCoastStratagems
  url = https://github.com/Gibberlings3/SwordCoastStratagems.git
  tag = v34.3

git submodule update changes

git submodule update  --remote -f --checkout

No changes as we are fixed to the tag

git submodule update --remote -f --checkout --no-single-tag
Submodule path 'SwordCoastStratagems': checked out
'77426a2e14dbe584af811f922a6da3a48d74c074'

Checks out to HEAD of remote

CC: Revi Ewer <revi.ewer@example.com>,
Ill Takalook <ill.takalook@example.net>

Signed-off-by: dark0dave <dark0dave@mykolab.com>
---
    submodule: allow gitsubmodules to be pointed at tags @dark0dave
    
    I want to enable users freezing their submodules to a tag.
    
    Example:
    
    
    Add
    ===
    
    git submodule add changes
    
    git submodule add --tag v34.3 https://github.com/Gibberlings3/SwordCoastStratagems.git
    cat .gitmodules
    [submodule "SwordCoastStratagems"]
      path = SwordCoastStratagems
      url = https://github.com/Gibberlings3/SwordCoastStratagems.git
      tag = v34.3
    
    
    
    Update
    ======
    
    git submodule update changes
    
    
    Single tag
    ==========
    
    git submodule update  --remote -f --checkout
    $
    
    
    No changes as we are fixed to the tag
    
    
    No Single tag
    =============
    
    git submodule update --remote -f --checkout --no-single-tag
    Submodule path 'SwordCoastStratagems': checked out '77426a2e14dbe584af811f922a6da3a48d74c074'
    
    
    Checks out to HEAD of remote

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-1703%2Fdark0dave%2Fmaster-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-1703/dark0dave/master-v1
Pull-Request: https://github.com/git/git/pull/1703

 builtin/submodule--helper.c            | 119 +++++++++++++++++++++++--
 contrib/completion/git-completion.bash |   8 +-
 git-submodule.sh                       |  59 +++++++++++-
 submodule-config.c                     |  14 +++
 submodule-config.h                     |   1 +
 5 files changed, 189 insertions(+), 12 deletions(-)


base-commit: d63586cb314731c851f28e14fc8012988467e2da

Comments

Sean Allred June 24, 2024, 1:26 p.m. UTC | #1
@dark0dave: sorry for the duplicate message; mu4e changed its default
keybindings recently and I neglected to reply to the list as well...

"dark0dave via GitGitGadget" <gitgitgadget@gmail.com> writes:
> Intent: Enable git submodules to point at tags,
> effectively freezing them
>
> Context: There is a large repo with 50+ submodules, the goal is
> to have some submodules update to HEAD but others to remain fixed.
> The update command does this by recursing through
> directories. This is very useful, however if an update is run,
> all the submodules are updated to HEAD.
>
> The goals:
> - Be able to create a submodule from a fixed tag (complete)
> - Be able to run git submodule update, with a flag to either update
> or keep the submodule pointed at the supplied tag (complete)

This is an interesting idea, so far as I understand it. I certainly
appreciate the desire to freeze your dependencies. I don't use
submodules often, but I do want to point out an inherent assumption in
your design: that tags don't move.

Tags can move. It's usually impolite, but there can be reasonable
use-cases for doing so.

With this in mind, what can go wrong if you have a submodule with a
configuration that doesn't necessarily match the tree?
Sean Allred June 24, 2024, 1:45 p.m. UTC | #2
> On 24 June 2024 13:26:48 UTC, Sean Allred <allred.sean@gmail.com> wrote:
>>This is an interesting idea, so far as I understand it. I certainly
>>appreciate the desire to freeze your dependencies. I don't use
>>submodules often, but I do want to point out an inherent assumption in
>>your design: that tags don't move.
>>
>>Tags can move. It's usually impolite, but there can be reasonable
>>use-cases for doing so.
>>
>>With this in mind, what can go wrong if you have a submodule with a
>>configuration that doesn't necessarily match the tree?

David <dark0dave@mykolab.com> writes:
> That's fine, my assumption is that if the tag moved the owner of the
> repo intends this to happen. Like a hot fix or security change.
>
> What will happen is on update the submodule will be moved to the new
> commit. Just like if a branch was updated. Its very similar code
> between the two, as this is a super light touch change.

I'm assuming that the superproject tree itself still gets the subproject
commit OID, so the commit is still stable when tags in the subproject
are moved (setting aside the possibility that the commit in the
subproject may no longer be reachable... *shudder*). But doesn't it seem
odd -- particularly for the concept of tags -- that the behavior of
updating that tree could change depending on when you do it? For
developers that add external dependencies as subprojects and choose to
use this tagging feature, they could be in for a real surprise if they
don't realize this quirk of the implementation -- and they could
consider that behavior broken.

If you're ok with that behavior, then does it make sense to generalize
it to the concept of a ref instead of just a tag name? This may do a
better job of communicating the idea that no ref is inherently
immovable. (Not saying this is a good idea, but answering that question
might further hone in on why tags are the 'right' choice here as opposed
to any named commit.)
Junio C Hamano June 24, 2024, 4:40 p.m. UTC | #3
Sean Allred <allred.sean@gmail.com> writes:

> If you're ok with that behavior, then does it make sense to
> generalize it to the concept of a ref instead of just a tag name?
> This may do a better job of communicating the idea that no ref is
> inherently immovable.

Yup, I only took a brief glance at the patch, and I didn't quite see
the point of duplicationg an almost parallel mechanism to an
existing (--single-branch, set-branch) pair of the feature, only to
replace "branch" with "tag" (the patch noise / code churn did not
seem worth it).

In other words, is there a situation where a branch cannot be used
(or using a branch is more cumbersome or awkward) when the user
wants to use a tag with this patch?  As users can just as easily
move tags as they can move branches, the verb "freeze" used in the
proposed log message probably does not reflect reality anyway, as
you already pointed out.

Thanks.
diff mbox series

Patch

diff --git a/builtin/submodule--helper.c b/builtin/submodule--helper.c
index 880ab4456ee..e7f150230e9 100644
--- a/builtin/submodule--helper.c
+++ b/builtin/submodule--helper.c
@@ -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);
diff --git a/contrib/completion/git-completion.bash b/contrib/completion/git-completion.bash
index 60a22d619a8..5e087bc443a 100644
--- a/contrib/completion/git-completion.bash
+++ b/contrib/completion/git-completion.bash
@@ -3057,6 +3057,9 @@  _git_remote ()
 	set-head,--*)
 		__gitcomp_builtin remote_set-head
 		;;
+	set-tag,--*)
+		__gitcomp_builtin remote_set-tag
+		;;
 	set-branches,--*)
 		__gitcomp_builtin remote_set-branches
 		;;
@@ -3471,7 +3474,7 @@  _git_submodule ()
 {
 	__git_has_doubledash && return
 
-	local subcommands="add status init deinit update set-branch set-url summary foreach sync absorbgitdirs"
+	local subcommands="add status init deinit update set-branch set-tag set-url summary foreach sync absorbgitdirs"
 	local subcommand="$(__git_find_on_cmdline "$subcommands")"
 	if [ -z "$subcommand" ]; then
 		case "$cur" in
@@ -3502,6 +3505,9 @@  _git_submodule ()
 			--force --rebase --merge --reference --depth --recursive --jobs
 		"
 		;;
+	set-tag,--*)
+		__gitcomp "--tag"
+		;;
 	set-branch,--*)
 		__gitcomp "--default --branch"
 		;;
diff --git a/git-submodule.sh b/git-submodule.sh
index 7f9582d9234..bbb17244d73 100755
--- a/git-submodule.sh
+++ b/git-submodule.sh
@@ -6,12 +6,13 @@ 
 
 dashless=$(basename "$0" | sed -e 's/-/ /')
 USAGE="[--quiet] [--cached]
-   or: $dashless [--quiet] add [-b <branch>] [-f|--force] [--name <name>] [--reference <repository>] [--] <repository> [<path>]
+   or: $dashless [--quiet] add [-b <branch>] [-t <tag>] [-f|--force] [--name <name>] [--reference <repository>] [--] <repository> [<path>]
    or: $dashless [--quiet] status [--cached] [--recursive] [--] [<path>...]
    or: $dashless [--quiet] init [--] [<path>...]
    or: $dashless [--quiet] deinit [-f|--force] (--all| [--] <path>...)
-   or: $dashless [--quiet] update [--init [--filter=<filter-spec>]] [--remote] [-N|--no-fetch] [-f|--force] [--checkout|--merge|--rebase] [--[no-]recommend-shallow] [--reference <repository>] [--recursive] [--[no-]single-branch] [--] [<path>...]
+   or: $dashless [--quiet] update [--init [--filter=<filter-spec>]] [--remote] [-N|--no-fetch] [-f|--force] [--checkout|--merge|--rebase] [--[no-]recommend-shallow] [--reference <repository>] [--recursive] [--[no-]single-branch] [--[no-]single-tag] [--] [<path>...]
    or: $dashless [--quiet] set-branch (--default|--branch <branch>) [--] <path>
+   or: $dashless [--quiet] set-tag (--tag <tag>) [--] <path>
    or: $dashless [--quiet] set-url [--] <path> <newurl>
    or: $dashless [--quiet] summary [--cached|--files] [--summary-limit <n>] [commit] [--] [<path>...]
    or: $dashless [--quiet] foreach [--recursive] <command>
@@ -49,6 +50,7 @@  depth=
 progress=
 dissociate=
 single_branch=
+single_tag=
 jobs=
 recommend_shallow=
 filter=
@@ -77,6 +79,11 @@  cmd_add()
 			branch=$2
 			shift
 			;;
+		-t | --tag)
+			case "$2" in '') usage ;; esac
+			tag=$2
+			shift
+			;;
 		-f | --force)
 			force=$1
 			;;
@@ -129,7 +136,7 @@  cmd_add()
 		usage
 	fi
 
-	git ${wt_prefix:+-C "$wt_prefix"} submodule--helper add ${quiet:+--quiet} ${force:+--force} ${progress:+"--progress"} ${branch:+--branch "$branch"} ${reference_path:+--reference "$reference_path"} ${dissociate:+--dissociate} ${custom_name:+--name "$custom_name"} ${depth:+"$depth"} -- "$@"
+	git ${wt_prefix:+-C "$wt_prefix"} submodule--helper add ${quiet:+--quiet} ${force:+--force} ${progress:+"--progress"} ${branch:+--branch "$branch"}  ${tag:+--tag "$tag"} ${reference_path:+--reference "$reference_path"} ${dissociate:+--dissociate} ${custom_name:+--name "$custom_name"} ${depth:+"$depth"} -- "$@"
 }
 
 #
@@ -316,6 +323,12 @@  cmd_update()
 		--no-single-branch)
 			single_branch="--no-single-branch"
 			;;
+		--single-tag)
+			single_tag="--single-tag"
+			;;
+		--no-single-tag)
+			single_tag="--no-single-tag"
+			;;
 		--filter)
 			case "$2" in '') usage ;; esac
 			filter="--filter=$2"
@@ -355,6 +368,7 @@  cmd_update()
 		${require_init:+--require-init} \
 		${dissociate:+"--dissociate"} \
 		$single_branch \
+		$single_tag \
 		$recommend_shallow \
 		$jobs \
 		$filter \
@@ -402,6 +416,43 @@  cmd_set_branch() {
 	git ${wt_prefix:+-C "$wt_prefix"} submodule--helper set-branch ${quiet:+--quiet} ${branch:+--branch "$branch"} ${default:+--default} -- "$@"
 }
 
+#
+# Configures a submodule's default tag
+#
+# $@ = requested path
+#
+cmd_set_tag() {
+	default=
+	tag=
+
+	while test $# -ne 0
+	do
+		case "$1" in
+		-q|--quiet)
+			# we don't do anything with this but we need to accept it
+			;;
+		-t|--tag)
+			case "$2" in '') usage ;; esac
+			tag=$2
+			shift
+			;;
+		--)
+			shift
+			break
+			;;
+		-*)
+			usage
+			;;
+		*)
+			break
+			;;
+		esac
+		shift
+	done
+
+	git ${wt_prefix:+-C "$wt_prefix"} submodule--helper set-tag ${quiet:+--quiet} ${tag:+--tag "$tag"} -- "$@"
+}
+
 #
 # Configures a submodule's remote url
 #
@@ -571,7 +622,7 @@  cmd_absorbgitdirs()
 while test $# != 0 && test -z "$command"
 do
 	case "$1" in
-	add | foreach | init | deinit | update | set-branch | set-url | status | summary | sync | absorbgitdirs)
+	add | foreach | init | deinit | update | set-branch | set-tag | set-url | status | summary | sync | absorbgitdirs)
 		command=$1
 		;;
 	-q|--quiet)
diff --git a/submodule-config.c b/submodule-config.c
index ec45ea67b95..a258406cced 100644
--- a/submodule-config.c
+++ b/submodule-config.c
@@ -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);
diff --git a/submodule-config.h b/submodule-config.h
index b6133af71b0..522683cc130 100644
--- a/submodule-config.h
+++ b/submodule-config.h
@@ -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;