branch: add --recurse-submodules option for branch creation

To improve the submodules UX, we would like to teach Git to handle
branches in submodules. Start this process by teaching "git branch" the
--recurse-submodules option so that "git branch --recurse-submodules
topic" will create the `topic` branch in the superproject and its
submodules.

Although this commit does not introduce breaking changes, it does not
work well with existing --recurse-submodules commands because "git
branch --recurse-submodules" writes to the submodule ref store, but most
commands only consider the superproject gitlink and ignore the submodule
ref store. For example, "git checkout --recurse-submodules" will check
out the commits in the superproject gitlinks (and put the submodules in
detached HEAD) instead of checking out the submodule branches.

Because of this, this commit introduces a new configuration value,
`submodule.propagateBranches`. The plan is for Git commands to
prioritize submodule ref store information over superproject gitlinks if
this value is true. Because "git branch --recurse-submodules" writes to
submodule ref stores, for the sake of clarity, it will not function
unless this configuration value is set.

This commit also includes changes that support working with submodules
from a superproject commit because "branch --recurse-submodules" (and
future commands) need to read .gitmodules and gitlinks from the
superproject commit, but submodules are typically read from the
filesystem's .gitmodules and the index's gitlinks. These changes are:

* add a submodules_of_tree() helper that gives the relevant
  information of an in-tree submodule (e.g. path and oid) and
  initializes the repository
* add is_tree_submodule_active() by adding a treeish_name parameter to
  is_submodule_active()
* add the "submoduleNotUpdated" advice to advise users to update the
  submodules in their trees

Incidentally, fix an incorrect usage string that combined the 'list'
usage of git branch (-l) with the 'create' usage; this string has been
incorrect since its inception, a8dfd5eac4 (Make builtin-branch.c use
parse_options., 2007-10-07).

Helped-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Glen Choo <chooglen@google.com>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

3 files changed, 47 insertions, 12 deletions

diff --git a/Documentation/config/advice.txt b/Documentation/config/advice.txt
index 063eec2511..adee26fbbb 100644
--- a/Documentation/config/advice.txt
+++ b/Documentation/config/advice.txt
@@ -116,6 +116,9 @@ advice.*::
 	submoduleAlternateErrorStrategyDie::
 		Advice shown when a submodule.alternateErrorStrategy option
 		configured to "die" causes a fatal error.
+	submodulesNotUpdated::
+		Advice shown when a user runs a submodule command that fails
+		because `git submodule update --init` was not run.
 	addIgnoredFile::
 		Advice shown if a user attempts to add an ignored file to
 		the index.
diff --git a/Documentation/config/submodule.txt b/Documentation/config/submodule.txt
index ee454f8126..6490527b45 100644
--- a/Documentation/config/submodule.txt
+++ b/Documentation/config/submodule.txt
@@ -59,18 +59,33 @@ submodule.active::
 
 submodule.recurse::
 	A boolean indicating if commands should enable the `--recurse-submodules`
-	option by default.
-	Applies to all commands that support this option
-	(`checkout`, `fetch`, `grep`, `pull`, `push`, `read-tree`, `reset`,
-	`restore` and `switch`) except `clone` and `ls-files`.
+	option by default. Defaults to false.
++
+When set to true, it can be deactivated via the
+`--no-recurse-submodules` option. Note that some Git commands
+lacking this option may call some of the above commands affected by
+`submodule.recurse`; for instance `git remote update` will call
+`git fetch` but does not have a `--no-recurse-submodules` option.
+For these commands a workaround is to temporarily change the
+configuration value by using `git -c submodule.recurse=0`.
++
+The following list shows the commands that accept
+`--recurse-submodules` and whether they are supported by this
+setting.
+
+* `checkout`, `fetch`, `grep`, `pull`, `push`, `read-tree`,
+`reset`, `restore` and `switch` are always supported.
+* `clone` and `ls-files` are not supported.
+* `branch` is supported only if `submodule.propagateBranches` is
+enabled
+
+submodule.propagateBranches::
+	[EXPERIMENTAL] A boolean that enables branching support when
+	using `--recurse-submodules` or `submodule.recurse=true`.
+	Enabling this will allow certain commands to accept
+	`--recurse-submodules` and certain commands that already accept
+	`--recurse-submodules` will now consider branches.
 	Defaults to false.
-	When set to true, it can be deactivated via the
-	`--no-recurse-submodules` option. Note that some Git commands
-	lacking this option may call some of the above commands affected by
-	`submodule.recurse`; for instance `git remote update` will call
-	`git fetch` but does not have a `--no-recurse-submodules` option.
-	For these commands a workaround is to temporarily change the
-	configuration value by using `git -c submodule.recurse=0`.
 
 submodule.fetchJobs::
 	Specifies how many submodules are fetched/cloned at the same time.
diff --git a/Documentation/git-branch.txt b/Documentation/git-branch.txt
index 731e340cbc..c8b4f9ce3c 100644
--- a/Documentation/git-branch.txt
+++ b/Documentation/git-branch.txt
@@ -16,7 +16,8 @@ SYNOPSIS
 	[--points-at <object>] [--format=<format>]
 	[(-r | --remotes) | (-a | --all)]
 	[--list] [<pattern>...]
-'git branch' [--track[=(direct|inherit)] | --no-track] [-f] <branchname> [<start-point>]
+'git branch' [--track[=(direct|inherit)] | --no-track] [-f]
+	[--recurse-submodules] <branchname> [<start-point>]
 'git branch' (--set-upstream-to=<upstream> | -u <upstream>) [<branchname>]
 'git branch' --unset-upstream [<branchname>]
 'git branch' (-m | -M) [<oldbranch>] <newbranch>
@@ -235,6 +236,22 @@ how the `branch.<name>.remote` and `branch.<name>.merge` options are used.
 	Do not set up "upstream" configuration, even if the
 	branch.autoSetupMerge configuration variable is set.
 
+--recurse-submodules::
+	THIS OPTION IS EXPERIMENTAL! Causes the current command to
+	recurse into submodules if `submodule.propagateBranches` is
+	enabled. See `submodule.propagateBranches` in
+	linkgit:git-config[1]. Currently, only branch creation is
+	supported.
++
+When used in branch creation, a new branch <branchname> will be created
+in the superproject and all of the submodules in the superproject's
+<start-point>. In submodules, the branch will point to the submodule
+commit in the superproject's <start-point> but the branch's tracking
+information will be set up based on the submodule's branches and remotes
+e.g. `git branch --recurse-submodules topic origin/main` will create the
+submodule branch "topic" that points to the submodule commit in the
+superproject's "origin/main", but tracks the submodule's "origin/main".
+
 --set-upstream::
 	As this option had confusing syntax, it is no longer supported.
 	Please use `--track` or `--set-upstream-to` instead.