git-submodule: add support for --rebase.

'git submodule update --rebase' rebases your local branch on top of what
would have been checked out to a detached HEAD otherwise.

In some cases, detaching the HEAD when updating a submodule complicates
the workflow to commit to this submodule (checkout master, rebase, then
commit).  For submodules that require frequent updates but infrequent
(if any) commits, a rebase can be executed directly by the git-submodule
command, ensuring that the submodules stay on their respective branches.

git-config key: submodule.$name.rebase (bool)

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-submodule.txt

@@ -12,7 +12,7 @@ SYNOPSIS
 'git submodule' [--quiet] add [-b branch] [--] <repository> <path>
 'git submodule' [--quiet] status [--cached] [--] [<path>...]
 'git submodule' [--quiet] init [--] [<path>...]
-'git submodule' [--quiet] update [--init] [-N|--no-fetch] [--] [<path>...]
+'git submodule' [--quiet] update [--init] [-N|--no-fetch] [--rebase] [--] [<path>...]
 'git submodule' [--quiet] summary [--summary-limit <n>] [commit] [--] [<path>...]
 'git submodule' [--quiet] foreach <command>
 'git submodule' [--quiet] sync [--] [<path>...]
@@ -113,7 +113,8 @@ init::
 update::
 	Update the registered submodules, i.e. clone missing submodules and
 	checkout the commit specified in the index of the containing repository.
-	This will make the submodules HEAD be detached.
+	This will make the submodules HEAD be detached unless '--rebase' is
+	specified or the key `submodule.$name.rebase` is set to `true`.
 +
 If the submodule is not yet initialized, and you just want to use the
 setting as stored in .gitmodules, you can automatically initialize the
@@ -177,6 +178,15 @@ OPTIONS
 	This option is only valid for the update command.
 	Don't fetch new objects from the remote site.
 
+--rebase::
+	This option is only valid for the update command.
+	Rebase the current branch onto the commit recorded in the
+	superproject. If this option is given, the submodule's HEAD will not
+	be detached. If a a merge failure prevents this process, you will have
+	to resolve these failures with linkgit:git-rebase[1].
+	If the key `submodule.$name.rebase` is set to `true`, this option is
+	implicit.
+
 <path>...::
 	Paths to submodule(s). When specified this will restrict the command
 	to only operate on the submodules found at the specified paths.

Documentation/gitmodules.txt

@@ -30,6 +30,9 @@ submodule.<name>.path::
 submodule.<name>.url::
 	Defines an url from where the submodule repository can be cloned.
 
+submodule.<name>.rebase::
+	Defines that the submodule should be rebased by default.
+
 
 EXAMPLES
 --------

git-submodule.sh

@@ -17,6 +17,7 @@ branch=
 quiet=
 cached=
 nofetch=
+rebase=
 
 #
 # print stuff on stdout unless -q was specified
@@ -294,6 +295,11 @@ cmd_init()
 		git config submodule."$name".url "$url" ||
 		die "Failed to register url for submodule path '$path'"
 
+		test true != "$(git config -f .gitmodules --bool \
+			submodule."$name".rebase)" ||
+		git config submodule."$name".rebase true ||
+		die "Failed to register submodule path '$path' as rebasing"
+
 		say "Submodule '$name' ($url) registered for path '$path'"
 	done
 }
@@ -321,6 +327,10 @@ cmd_update()
 			shift
 			nofetch=1
 			;;
+		-r|--rebase)
+			shift
+			rebase=true
+			;;
 		--)
 			shift
 			break
@@ -339,6 +349,7 @@ cmd_update()
 	do
 		name=$(module_name "$path") || exit
 		url=$(git config submodule."$name".url)
+		rebase_module=$(git config --bool submodule."$name".rebase)
 		if test -z "$url"
 		then
 			# Only mention uninitialized submodules when its
@@ -359,6 +370,11 @@ cmd_update()
 			die "Unable to find current revision in submodule path '$path'"
 		fi
 
+		if test true = "$rebase"
+		then
+			rebase_module=true
+		fi
+
 		if test "$subsha1" != "$sha1"
 		then
 			force=
@@ -374,11 +390,20 @@ cmd_update()
 				die "Unable to fetch in submodule path '$path'"
 			fi
 
-			(unset GIT_DIR; cd "$path" &&
-				  git-checkout $force -q "$sha1") ||
-			die "Unable to checkout '$sha1' in submodule path '$path'"
+			if test true = "$rebase_module"
+			then
+				command="git-rebase"
+				action="rebase"
+				msg="rebased onto"
+			else
+				command="git-checkout $force -q"
+				action="checkout"
+				msg="checked out"
+			fi
 
-			say "Submodule path '$path': checked out '$sha1'"
+			(unset GIT_DIR; cd "$path" && $command "$sha1") ||
+			die "Unable to $action '$sha1' in submodule path '$path'"
+			say "Submodule path '$path': $msg '$sha1'"
 		fi
 	done
 }

t/t7406-submodule-update.sh

@@ -0,0 +1,140 @@
+#!/bin/sh
+#
+# Copyright (c) 2009 Red Hat, Inc.
+#
+
+test_description='Test updating submodules
+
+This test verifies that "git submodule update" detaches the HEAD of the
+submodule and "git submodule update --rebase" does not detach the HEAD.
+'
+
+. ./test-lib.sh
+
+
+compare_head()
+{
+    sha_master=`git-rev-list --max-count=1 master`
+    sha_head=`git-rev-list --max-count=1 HEAD`
+
+    test "$sha_master" = "$sha_head"
+}
+
+
+test_expect_success 'setup a submodule tree' '
+	echo file > file &&
+	git add file &&
+	test_tick &&
+	git commit -m upstream
+	git clone . super &&
+	git clone super submodule &&
+	(cd super &&
+	 git submodule add ../submodule submodule &&
+	 test_tick &&
+	 git commit -m "submodule" &&
+	 git submodule init submodule
+	) &&
+	(cd submodule &&
+	echo "line2" > file &&
+	git add file &&
+	git commit -m "Commit 2"
+	) &&
+	(cd super &&
+	 (cd submodule &&
+	  git pull --rebase origin
+	 ) &&
+	 git add submodule &&
+	 git commit -m "submodule update"
+	)
+'
+
+test_expect_success 'submodule update detaching the HEAD ' '
+	(cd super/submodule &&
+	 git reset --hard HEAD~1
+	) &&
+	(cd super &&
+	 (cd submodule &&
+	  compare_head
+	 ) &&
+	 git submodule update submodule &&
+	 cd submodule &&
+	 ! compare_head
+	)
+'
+
+test_expect_success 'submodule update --rebase staying on master' '
+	(cd super/submodule &&
+	  git checkout master
+	) &&
+	(cd super &&
+	 (cd submodule &&
+	  compare_head
+	 ) &&
+	 git submodule update --rebase submodule &&
+	 cd submodule &&
+	 compare_head
+	)
+'
+
+test_expect_success 'submodule update - rebase true in .git/config' '
+	(cd super &&
+	 git config submodule.submodule.rebase true
+	) &&
+	(cd super/submodule &&
+	  git reset --hard HEAD~1
+	) &&
+	(cd super &&
+	 (cd submodule &&
+	  compare_head
+	 ) &&
+	 git submodule update submodule &&
+	 cd submodule &&
+	 compare_head
+	)
+'
+
+test_expect_success 'submodule update - rebase false in .git/config but --rebase given' '
+	(cd super &&
+	 git config submodule.submodule.rebase false
+	) &&
+	(cd super/submodule &&
+	  git reset --hard HEAD~1
+	) &&
+	(cd super &&
+	 (cd submodule &&
+	  compare_head
+	 ) &&
+	 git submodule update --rebase submodule &&
+	 cd submodule &&
+	 compare_head
+	)
+'
+
+test_expect_success 'submodule update - rebase false in .git/config' '
+	(cd super &&
+	 git config submodule.submodule.rebase false
+	) &&
+	(cd super/submodule &&
+	  git reset --hard HEAD^
+	) &&
+	(cd super &&
+	 (cd submodule &&
+	  compare_head
+	 ) &&
+	 git submodule update submodule &&
+	 cd submodule &&
+	 ! compare_head
+	)
+'
+
+test_expect_success 'submodule init picks up rebase' '
+	(cd super &&
+	 git config submodule.rebasing.url git://non-existing/git &&
+	 git config submodule.rebasing.path does-not-matter &&
+	 git config submodule.rebasing.rebase true &&
+	 git submodule init rebasing &&
+	 test true = $(git config --bool submodule.rebasing.rebase)
+	)
+'
+
+test_done