submodule: explicit local branch creation in module_clone

The previous code only checked out branches in cmd_add.  This commit
moves the branch-checkout logic into module_clone, where it can be
shared by cmd_add and cmd_update.  I also update the initial checkout
command to use 'reset' to preserve branches setup during module_clone.

With this change, folks cloning submodules for the first time via:

  $ git submodule update ...

will get a local branch instead of a detached HEAD, unless they are
using the default checkout-mode updates.  This is a change from the
previous situation where cmd_update always used checkout-mode logic
(regardless of the requested update mode) for updates that triggered
an initial clone, which always resulted in a detached HEAD.

This commit does not change the logic for updates after the initial
clone, which will continue to create detached HEADs for checkout-mode
updates, and integrate remote work with the local HEAD (detached or
not) in other modes.

The motivation for the change is that developers doing local work
inside the submodule are likely to select a non-checkout-mode for
updates so their local work is integrated with upstream work.
Developers who are not doing local submodule work stick with
checkout-mode updates so any apparently local work is blown away
during updates.  For example, if upstream rolls back the remote branch
or gitlinked commit to an earlier version, the checkout-mode developer
wants their old submodule checkout to be rolled back as well, instead
of getting a no-op merge/rebase with the rolled-back reference.

By using the update mode to distinguish submodule developers from
black-box submodule consumers, we can setup local branches for the
developers who will want local branches, and stick with detached HEADs
for the developers that don't care.

Testing
=======

In t7406, just-cloned checkouts now update to the gitlinked hash with
'reset', to preserve the local branch for situations where we're not
on a detached HEAD.

I also added explicit tests to t7406 for HEAD attachement after
cloning updates, showing that it depends on their update mode:

* Checkout-mode updates get detached HEADs
* Everyone else gets a local branch, matching the configured
  submodule.<name>.branch and defaulting to master.

The 'initial-setup' tag makes it easy to reset the superproject to a
known state, as several earlier tests commit to submodules and commit
the changed gitlinks to the superproject, but don't push the new
submodule commits to the upstream subprojects.  This makes it
impossible to checkout the current super master, because it references
submodule commits that don't exist in the upstream subprojects.  For a
specific example, see the tests that currently generate the
'two_new_submodule_commits' commits.

Documentation
=============

I updated the docs to describe the 'submodule update' modes in detail.
The old documentation did not distinguish between cloning and
non-cloning updates and lacked clarity on which operations would lead
to detached HEADs, and which would not.  The new documentation
addresses these issues while updating the docs to reflect the changes
introduced by this commit's explicit local branch creation in
module_clone.

I also add '--checkout' to the usage summary and group the update-mode
options into a single set.

Signed-off-by: W. Trevor King <wking@tremily.us>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 41 insertions, 17 deletions

diff --git a/git-submodule.sh b/git-submodule.sh
index 68dcbe1..626a746 100755
--- a/git-submodule.sh
+++ b/git-submodule.sh
@@ -246,6 +246,9 @@ module_name()
 # $3 = URL to clone
 # $4 = reference repository to reuse (empty for independent)
 # $5 = depth argument for shallow clones (empty for deep)
+# $6 = (remote-tracking) starting point for the local branch (empty for HEAD)
+# $7 = local branch to create (empty for a detached HEAD, unless $6 is
+#      also empty, in which case the local branch is left unchanged)
 #
 # Prior to calling, cmd_update checks that a possibly existing
 # path is not a git repository.
@@ -259,6 +262,8 @@ module_clone()
 	url=$3
 	reference="$4"
 	depth="$5"
+	start_point="$6"
+	local_branch="$7"
 	quiet=
 	if test -n "$GIT_QUIET"
 	then
@@ -312,7 +317,16 @@ module_clone()
 	echo "gitdir: $rel/$a" >"$sm_path/.git"
 
 	rel=$(echo $a | sed -e 's|[^/][^/]*|..|g')
-	(clear_local_git_env; cd "$sm_path" && GIT_WORK_TREE=. git config core.worktree "$rel/$b")
+	(
+		clear_local_git_env
+		cd "$sm_path" &&
+		GIT_WORK_TREE=. git config core.worktree "$rel/$b" &&
+		# ash fails to wordsplit ${local_branch:+-B "$local_branch"...}
+		case "$local_branch" in
+		'') git checkout -f -q ${start_point:+"$start_point"} ;;
+		?*) git checkout -f -q -B "$local_branch" ${start_point:+"$start_point"} ;;
+		esac
+	) || die "$(eval_gettext "Unable to setup cloned submodule '\$sm_path'")"
 }
 
 isnumber()
@@ -475,16 +489,15 @@ Use -f if you really want to add it." >&2
 				echo "$(eval_gettext "Reactivating local git directory for submodule '\$sm_name'.")"
 			fi
 		fi
-		module_clone "$sm_path" "$sm_name" "$realrepo" "$reference" "$depth" || exit
-		(
-			clear_local_git_env
-			cd "$sm_path" &&
-			# ash fails to wordsplit ${branch:+-b "$branch"...}
-			case "$branch" in
-			'') git checkout -f -q ;;
-			?*) git checkout -f -q -B "$branch" "origin/$branch" ;;
-			esac
-		) || die "$(eval_gettext "Unable to checkout submodule '\$sm_path'")"
+		if test -n "$branch"
+		then
+			start_point="origin/$branch"
+			local_branch="$branch"
+		else
+			start_point=""
+			local_branch=""
+		fi
+		module_clone "$sm_path" "$sm_name" "$realrepo" "$reference" "$depth" "$start_point" "$local_branch" || exit
 	fi
 	git config submodule."$sm_name".url "$realrepo"
 
@@ -803,7 +816,9 @@ cmd_update()
 		fi
 		name=$(module_name "$sm_path") || exit
 		url=$(git config submodule."$name".url)
-		branch=$(get_submodule_config "$name" branch master)
+		config_branch=$(get_submodule_config "$name" branch)
+		branch="${config_branch:-master}"
+		local_branch="$branch"
 		if ! test -z "$update"
 		then
 			update_module=$update
@@ -817,11 +832,19 @@ cmd_update()
 
 		displaypath=$(relative_path "$prefix$sm_path")
 
-		if test "$update_module" = "none"
-		then
+		case "$update_module" in
+		none)
 			echo "Skipping submodule '$displaypath'"
 			continue
-		fi
+			;;
+		checkout)
+			local_branch=""
+			;;
+		rebase | merge | !*)
+			;;
+		*)
+			die "$(eval_gettext "Invalid update mode '$update_module' for submodule '$name'")"
+		esac
 
 		if test -z "$url"
 		then
@@ -835,7 +858,8 @@ Maybe you want to use 'update --init'?")"
 
 		if ! test -d "$sm_path"/.git -o -f "$sm_path"/.git
 		then
-			module_clone "$sm_path" "$name" "$url" "$reference" "$depth" || exit
+			start_point="origin/${branch}"
+			module_clone "$sm_path" "$name" "$url" "$reference" "$depth" "$start_point" "$local_branch" || exit
 			cloned_modules="$cloned_modules;$name"
 			subsha1=
 		else
@@ -881,7 +905,7 @@ Maybe you want to use 'update --init'?")"
 			case ";$cloned_modules;" in
 			*";$name;"*)
 				# then there is no local change to integrate
-				update_module=checkout ;;
+				update_module='!git reset --hard -q'
 			esac
 
 			must_die_on_failure=