[RFC/PATCHv4] submodule update: allow custom update command

[Chris Packham <judge.packham@gmail.com>]

Users can set submodule.$name.update to '!command' which will cause
'command' to be run instead of checkout/merge/rebase.  This allows the
user some finer grained control over how the update is done. The primary
motivation for this was interoperability with stgit however being able
to intercept the submodule update process may prove useful for
integrating with or extensions to other tools.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
---
v4 adds a couple of simple tests - an equivalent of update=checkout and a test
to make sure we detect a failure reported by the update command.

 Documentation/git-submodule.txt |  4 +++-
 git-submodule.sh                |  6 ++++++
 t/t7406-submodule-update.sh     | 29 +++++++++++++++++++++++++++++
 3 files changed, 38 insertions(+), 1 deletion(-)

diff --git a/Documentation/git-submodule.txt b/Documentation/git-submodule.txt
index e576713..2f18f7d 100644
--- a/Documentation/git-submodule.txt
+++ b/Documentation/git-submodule.txt
@@ -159,7 +159,9 @@ update::
 	This will make the submodules HEAD be detached unless `--rebase` or
 	`--merge` is specified or the key `submodule.$name.update` is set to
 	`rebase`, `merge` or `none`. `none` can be overridden by specifying
-	`--checkout`.
+	`--checkout`. Setting the key `submodule.$name.update` to `!command`
+	will cause `command` to be run. `command` can be any arbitrary shell
+	command that takes a single argument, namely the sha1 to update to.
 +
 If the submodule is not yet initialized, and you just want to use the
 setting as stored in .gitmodules, you can automatically initialize the
diff --git a/git-submodule.sh b/git-submodule.sh
index eb58c8e..e7579f0 100755
--- a/git-submodule.sh
+++ b/git-submodule.sh
@@ -799,6 +799,12 @@ Maybe you want to use 'update --init'?")"
 				say_msg="$(eval_gettext "Submodule path '\$prefix\$sm_path': merged in '\$sha1'")"
 				must_die_on_failure=yes
 				;;
+			!*)
+				command="${update_module#!}"
+				die_msg="$(eval_gettext "Execution of '\$command \$sha1' failed in submodule  path '\$prefix\$sm_path'")"
+				say_msg="$(eval_gettext "Submodule path '\$prefix\$sm_path': '\$command \$sha1'")"
+				must_die_on_failure=yes
+				;;
 			*)
 				command="git checkout $subforce -q"
 				die_msg="$(eval_gettext "Unable to checkout '\$sha1' in submodule path '\$prefix\$sm_path'")"
diff --git a/t/t7406-submodule-update.sh b/t/t7406-submodule-update.sh
index a4ffea0..48d1279 100755
--- a/t/t7406-submodule-update.sh
+++ b/t/t7406-submodule-update.sh
@@ -279,6 +279,35 @@ test_expect_success 'submodule update - checkout in .git/config' '
 	)
 '
 
+test_expect_success 'submodule update - command in .git/config' '
+	(cd super &&
+	 git config submodule.submodule.update "!git checkout"
+	) &&
+	(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 update - command in .git/config catches failure' '
+	(cd super &&
+	 git config submodule.submodule.update "!false"
+	) &&
+	(cd super/submodule &&
+	  git reset --hard HEAD^
+	) &&
+	(cd super &&
+	 test_must_fail git submodule update submodule
+	)
+'
+
 test_expect_success 'submodule init picks up rebase' '
 	(cd super &&
 	 git config -f .gitmodules submodule.rebasing.update rebase &&
-- 
1.8.3.1.644.ge86575b