submodule: Add --force option for git submodule update

By default git submodule update runs a simple checkout on submodules that
are not up-to-date. If the submodules contains modified or untracked
files, the command may exit sanely with an error:

  $ git submodule update
  error: Your local changes to the following files would be overwritten by
  checkout:
	  file
  Please, commit your changes or stash them before you can switch branches.
  Aborting
  Unable to checkout '1b69c6e55606b48d3284a3a9efe4b58bfb7e8c9e' in
  submodule path 'test1'

In order to reset a whole git submodule tree, a user has to run first 'git
submodule foreach --recursive git checkout -f' and then run 'git submodule
update'.

This patch adds a --force option for the update command (only used for
submodules without --rebase or --merge options). It passes the --force
option to git checkout which will throw away the local changes.

Signed-off-by: Nicolas Morey-Chaisemartin <nmorey@kalray.eu>
Acked-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-submodule.txt

@@ -185,8 +185,10 @@ OPTIONS
 
 -f::
 --force::
-	This option is only valid for the add command.
+	This option is only valid for add and update commands.
-	Allow adding an otherwise ignored submodule path.
+	When running add, allow adding an otherwise ignored submodule path.
+	When running update, throw away local changes in submodules when
+	switching to a different commit.
 
 --cached::
 	This option is only valid for status and summary commands.  These

git-submodule.sh

@@ -8,7 +8,7 @@ dashless=$(basename "$0" | sed -e 's/-/ /')
 USAGE="[--quiet] add [-b branch] [-f|--force] [--reference <repository>] [--] <repository> [<path>]
    or: $dashless [--quiet] status [--cached] [--recursive] [--] [<path>...]
    or: $dashless [--quiet] init [--] [<path>...]
-   or: $dashless [--quiet] update [--init] [-N|--no-fetch] [--rebase] [--reference <repository>] [--merge] [--recursive] [--] [<path>...]
+   or: $dashless [--quiet] update [--init] [-N|--no-fetch] [-f|--force] [--rebase] [--reference <repository>] [--merge] [--recursive] [--] [<path>...]
    or: $dashless [--quiet] summary [--cached|--files] [--summary-limit <n>] [commit] [--] [<path>...]
    or: $dashless [--quiet] foreach [--recursive] <command>
    or: $dashless [--quiet] sync [--] [<path>...]"
@@ -385,6 +385,9 @@ cmd_update()
 		-N|--no-fetch)
 			nofetch=1
 			;;
+		-f|--force)
+			force=$1
+			;;
 		-r|--rebase)
 			update="rebase"
 			;;
@@ -458,10 +461,11 @@ cmd_update()
 
 		if test "$subsha1" != "$sha1"
 		then
-			force=
+			subforce=$force
-			if test -z "$subsha1"
+			# If we don't already have a -f flag and the submodule has never been checked out
+			if test -z "$subsha1" -a -z "$force"
 			then
-				force="-f"
+				subforce="-f"
 			fi
 
 			if test -z "$nofetch"
@@ -490,7 +494,7 @@ cmd_update()
 				msg="merged in"
 				;;
 			*)
-				command="git checkout $force -q"
+				command="git checkout $subforce -q"
 				action="checkout"
 				msg="checked out"
 				;;

t/t7406-submodule-update.sh

@@ -74,6 +74,29 @@ test_expect_success 'submodule update detaching the HEAD ' '
 	)
 '
 
+test_expect_success 'submodule update should fail due to local changes' '
+	(cd super/submodule &&
+	 git reset --hard HEAD~1 &&
+	 echo "local change" > file
+	) &&
+	(cd super &&
+	 (cd submodule &&
+	  compare_head
+	 ) &&
+	 test_must_fail git submodule update submodule
+	)
+'
+test_expect_success 'submodule update should throw away changes with --force ' '
+	(cd super &&
+	 (cd submodule &&
+	  compare_head
+	 ) &&
+	 git submodule update --force submodule &&
+	 cd submodule &&
+	 ! compare_head
+	)
+'
+
 test_expect_success 'submodule update --rebase staying on master' '
 	(cd super/submodule &&
 	  git checkout master