[RFC PATCH 3/7] rebase -i: Stop on root commits with empty log messages

[Fabian Ruch <bafain@gmail.com>]

When `rebase` is executed with `--root` but no `--onto` is specified,
`rebase` creates a sentinel commit which is replaced with the root
commit in three steps. This combination of options results never in a
fast-forward.

 1. The sentinel commit is forced into the authorship of the root
    commit.

 2. The changes introduced by the root commit are applied to the index
    but not committed. If this step fails for whatever reason, all
    commit information will be there and the user can safely run
    `git-commit --amend` after resolving the problems.

 3. The new root commit is created by squashing the changes into the
    sentinel commit which already carries the authorship of the
    cherry-picked root commit.

The command line used to create the commit in the third step specifies
effectless and erroneous options. Remove those.

 - `--allow-empty-message` is erroneous: If the root's commit message is
   empty, the rebase shall fail like for any other commit that is on the
   to-do list and has an empty commit message.

   Fix the bug that git-rebase does not fail when the initial commit has
   an empty log message but is replayed using `--root` is specified.
   Add test.

 - `-C` is effectless: The commit being amended, which is the sentinel
   commit, already carries the authorship and log message of the
   cherry-picked root commit. The committer email and commit date fields
   are reset either way.

After all, if step two fails, `rebase --continue` won't include these
flags in the git-commit command line either.

Signed-off-by: Fabian Ruch <bafain@gmail.com>
---
 git-rebase--interactive.sh |  4 ++--
 t/t3412-rebase-root.sh     | 39 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 41 insertions(+), 2 deletions(-)

diff --git a/git-rebase--interactive.sh b/git-rebase--interactive.sh
index fffdfa5..f09eeae 100644
--- a/git-rebase--interactive.sh
+++ b/git-rebase--interactive.sh
@@ -539,8 +539,8 @@ do_pick () {
 		git commit --allow-empty --allow-empty-message --amend \
 			   --no-post-rewrite -n -q -C $1 &&
 			pick_one -n $1 &&
-			git commit --allow-empty --allow-empty-message \
-				   --amend --no-post-rewrite -n -q -C $1 \
+			git commit --allow-empty --amend \
+				   --no-post-rewrite -n -q \
 				   ${gpg_sign_opt:+"$gpg_sign_opt"} ||
 			die_with_patch $1 "Could not apply $1... $2"
 	else
diff --git a/t/t3412-rebase-root.sh b/t/t3412-rebase-root.sh
index 0b52105..3608db4 100755
--- a/t/t3412-rebase-root.sh
+++ b/t/t3412-rebase-root.sh
@@ -278,4 +278,43 @@ test_expect_success 'rebase -i -p --root with conflict (second part)' '
 	test_cmp expect-conflict-p out
 '
 
+test_expect_success 'stop rebase --root on empty root log message' '
+	# create a root commit with a non-empty tree so that rebase does
+	# not fail because of an empty commit, and an empty log message
+	echo root-commit >file &&
+	git add file &&
+	tree=$(git write-tree) &&
+	root=$(git commit-tree $tree </dev/null) &&
+	git checkout -b no-message-root-commit $root &&
+	# do not ff because otherwise neither the patch nor the message
+	# are looked at and checked for emptiness
+	test_must_fail env EDITOR=true git rebase -i --force-rebase --root &&
+	echo root-commit >file.expected &&
+	test_cmp file file.expected &&
+	git rebase --abort
+'
+
+test_expect_success 'stop rebase --root on empty child log message' '
+	# create a root commit with a non-empty tree and provide a log
+	# message so that rebase does not fail until the root commit is
+	# successfully replayed
+	echo root-commit >file &&
+	git add file &&
+	tree=$(git write-tree) &&
+	root=$(git commit-tree $tree -m root-commit) &&
+	git checkout -b no-message-child-commit $root &&
+	# create a child commit with a non-empty patch so that rebase
+	# does not fail because of an empty commit, but an empty log
+	# message
+	echo child-commit >file &&
+	git add file &&
+	git commit --allow-empty-message --no-edit &&
+	# do not ff because otherwise neither the patch nor the message
+	# are looked at and checked for emptiness
+	test_must_fail env EDITOR=true git rebase -i --force-rebase --root &&
+	echo child-commit >file.expected &&
+	test_cmp file file.expected &&
+	git rebase --abort
+'
+
 test_done
-- 
2.0.0