rebase (interactive-backend): make --keep-empty the default

Different rebase backends have different treatment for commits which
start empty (i.e. have no changes relative to their parent), and the
--keep-empty option was added at some point to allow adjusting behavior.
The handling of commits which start empty is actually quite similar to
commit b00bf1c9a8dd (git-rebase: make --allow-empty-message the default,
2018-06-27), which pointed out that the behavior for various backends is
often more happenstance than design.  The specific change made in that
commit is actually quite relevant as well and much of the logic there
directly applies here.

It makes a lot of sense in 'git commit' to error out on the creation of
empty commits, unless an override flag is provided.  However, once
someone determines that there is a rare case that merits using the
manual override to create such a commit, it is somewhere between
annoying and harmful to have to take extra steps to keep such
intentional commits around.  Granted, empty commits are quite rare,
which is why handling of them doesn't get considered much and folks tend
to defer to existing (accidental) behavior and assume there was a reason
for it, leading them to just add flags (--keep-empty in this case) that
allow them to override the bad defaults.  Fix the interactive backend so
that --keep-empty is the default, much like we did with
--allow-empty-message.  The am backend should also be fixed to have
--keep-empty semantics for commits that start empty, but that is not
included in this patch other than a testcase documenting the failure.

Note that there was one test in t3421 which appears to have been written
expecting --keep-empty to not be the default as correct behavior.  This
test was introduced in commit 00b8be5a4d38 ("add tests for rebasing of
empty commits", 2013-06-06), which was part of a series focusing on
rebase topology and which had an interesting original cover letter at
https://lore.kernel.org/git/1347949878-12578-1-git-send-email-martinvonz@gmail.com/
which noted
    Your input especially appreciated on whether you agree with the
    intent of the test cases.
and then went into a long example about how one of the many tests added
had several questions about whether it was correct.  As such, I believe
most the tests in that series were about testing rebase topology with as
many different flags as possible and were not trying to state in general
how those flags should behave otherwise.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 70 insertions, 0 deletions

diff --git a/t/t3424-rebase-empty.sh b/t/t3424-rebase-empty.sh
new file mode 100755
index 0000000000..3b716e980e
--- /dev/null
+++ b/t/t3424-rebase-empty.sh
@@ -0,0 +1,70 @@
+#!/bin/sh
+
+test_description='git rebase of commits that start or become empty'
+
+. ./test-lib.sh
+
+test_expect_success 'setup test repository' '
+	test_write_lines 1 2 3 4 5 6 7 8 9 10 >numbers &&
+	test_write_lines A B C D E F G H I J >letters &&
+	git add numbers letters &&
+	git commit -m A &&
+
+	git branch upstream &&
+	git branch localmods &&
+
+	git checkout upstream &&
+	test_write_lines A B C D E >letters &&
+	git add letters &&
+	git commit -m B &&
+
+	test_write_lines 1 2 3 4 five 6 7 8 9 ten >numbers &&
+	git add numbers &&
+	git commit -m C &&
+
+	git checkout localmods &&
+	test_write_lines 1 2 3 4 five 6 7 8 9 10 >numbers &&
+	git add numbers &&
+	git commit -m C2 &&
+
+	git commit --allow-empty -m D &&
+
+	test_write_lines A B C D E >letters &&
+	git add letters &&
+	git commit -m "Five letters ought to be enough for anybody"
+'
+
+test_expect_failure 'rebase (am-backend) with a variety of empty commits' '
+	test_when_finished "git rebase --abort" &&
+	git checkout -B testing localmods &&
+	# rebase (--am) should not drop commits that start empty
+	git rebase upstream &&
+
+	test_write_lines D C B A >expect &&
+	git log --format=%s >actual &&
+	test_cmp expect actual
+'
+
+test_expect_failure 'rebase --merge with a variety of empty commits' '
+	test_when_finished "git rebase --abort" &&
+	git checkout -B testing localmods &&
+	# rebase --merge should not halt on the commit that becomes empty
+	git rebase --merge upstream &&
+
+	test_write_lines D C B A >expect &&
+	git log --format=%s >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'rebase --interactive with a variety of empty commits' '
+	git checkout -B testing localmods &&
+	test_must_fail git rebase --interactive upstream &&
+
+	git rebase --skip &&
+
+	test_write_lines D C B A >expect &&
+	git log --format=%s >actual &&
+	test_cmp expect actual
+'
+
+test_done