[PATCH 2/2] receive-pack: support push-to-checkout hook

[Junio C Hamano <gitster@pobox.com>]

When receive.denyCurrentBranch is set to updateInstead, this hook
can be used to override the built-in "push-to-deploy" logic, which
insists that the working tree and the index must be unchanged
relative to HEAD.  The hook receives the commit with which the
tip of the current is going to be updated, and is responsible to
make any necessary changes to the working tree and to the index to
bring them to the desired state when the tip of the current branch
is updated to the new commit.

For example, the hook can simply run "git read-tree -u -m HEAD $1"
to the workflow to emulate "'git fetch' going in the reverse
direction with 'git push'" better than the push-to-deploy logic, as
the two-tree form of "read-tree -u -m" is essentially the same as
"git checkout" that switches branches while keeping the local
changes in the working tree that do not interfere with the
difference between the branches.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 builtin/receive-pack.c | 19 ++++++++++++++-
 t/t5516-fetch-push.sh  | 63 ++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 81 insertions(+), 1 deletion(-)

diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index 11800cd..fc8ec9c 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -797,6 +797,20 @@ static const char *push_to_deploy(unsigned char *sha1,
 	return NULL;
 }
 
+static const char *push_to_checkout_hook = "push-to-checkout";
+
+static const char *push_to_checkout(unsigned char *sha1,
+				    struct argv_array *env,
+				    const char *work_tree)
+{
+	argv_array_pushf(env, "GIT_WORK_TREE=%s", absolute_path(work_tree));
+	if (run_hook_le(env->argv, push_to_checkout_hook,
+			sha1_to_hex(sha1), NULL))
+		return "push-to-checkout hook declined";
+	else
+		return NULL;
+}
+
 static const char *update_worktree(unsigned char *sha1)
 {
 	const char *retval;
@@ -808,7 +822,10 @@ static const char *update_worktree(unsigned char *sha1)
 
 	argv_array_pushf(&env, "GIT_DIR=%s", absolute_path(get_git_dir()));
 
-	retval = push_to_deploy(sha1, &env, work_tree);
+	if (!find_hook(push_to_checkout_hook))
+		retval = push_to_deploy(sha1, &env, work_tree);
+	else
+		retval = push_to_checkout(sha1, &env, work_tree);
 
 	argv_array_clear(&env);
 	return retval;
diff --git a/t/t5516-fetch-push.sh b/t/t5516-fetch-push.sh
index 85c7fec..e4436c1 100755
--- a/t/t5516-fetch-push.sh
+++ b/t/t5516-fetch-push.sh
@@ -1434,4 +1434,67 @@ test_expect_success 'receive.denyCurrentBranch = updateInstead' '
 
 '
 
+test_expect_success 'updateInstead with push-to-checkout hook' '
+	rm -fr testrepo &&
+	git init testrepo &&
+	(
+		cd testrepo &&
+		git pull .. master &&
+		git reset --hard HEAD^^ &&
+		git tag initial &&
+		git config receive.denyCurrentBranch updateInstead &&
+		write_script .git/hooks/push-to-checkout <<-\EOF
+		echo >&2 updating from $(git rev-parse HEAD)
+		echo >&2 updating to "$1"
+
+		git update-index -q --refresh &&
+		git read-tree -u -m HEAD "$1" || {
+			status=$?
+			echo >&2 read-tree failed
+			exit $status
+		}
+		EOF
+	) &&
+
+	# Try pushing into a pristine
+	git push testrepo master &&
+	(
+		cd testrepo &&
+		git diff --quiet &&
+		git diff HEAD --quiet &&
+		test $(git -C .. rev-parse HEAD) = $(git rev-parse HEAD)
+	) &&
+
+	# Try pushing into a repository with conflicting change
+	(
+		cd testrepo &&
+		git reset --hard initial &&
+		echo conflicting >path2
+	) &&
+	test_must_fail git push testrepo master &&
+	(
+		cd testrepo &&
+		test $(git rev-parse initial) = $(git rev-parse HEAD) &&
+		test conflicting = "$(cat path2)" &&
+		git diff-index --quiet --cached HEAD
+	) &&
+
+	# Try pushing into a repository with unrelated change
+	(
+		cd testrepo &&
+		git reset --hard initial &&
+		echo unrelated >path1 &&
+		echo irrelevant >path5 &&
+		git add path5
+	) &&
+	git push testrepo master &&
+	(
+		cd testrepo &&
+		test "$(cat path1)" = unrelated &&
+		test "$(cat path5)" = irrelevant &&
+		test "$(git diff --name-only --cached HEAD)" = path5 &&
+		test $(git -C .. rev-parse HEAD) = $(git rev-parse HEAD)
+	)
+'
+
 test_done
-- 
2.2.0-141-gd3f4719