Re: [BUG] 'git rebase -i ---rebase-merges' can leave MERGE_MSG behind

[Phillip Wood <phillip.wood123@gmail.com>]

On 31/07/2021 07:23, SZEDER Gábor wrote:
> Hi,
> 
> 'git rebase -i --rebase-merges' leaves a stray '.git/MERGE_MSG' file
> behind after it re-created an unchanged merge commit.  The test script
> below demonstrates this issue (it doesn't fail, but look at its
> verbose output to see what the issue and its consequences are).
> 
> This issue is as old as --rebase-merges, I see the same output with
> v2.18.0, the first release containing this feature.
> 
> I suspect that fixing this is merely a matter of adding a missing
> unlink(".git/MERGE_MSG") to the right place, but I won't dive into the
> sequencer machinery to figure out where that right place might be :)

Looking at do_merge() in sequencer.c .git/MERGE_MSG is written before it 
checks if it can fast-forward. If it does fast-forward then 'git commit' 
is not run and .git/MERGE_MSG is not removed. I think the best way to 
fix it would be to move the fast-forward code so it comes before the 
code that writes .git/MERGE_MSG. That way we'll not write the message if 
we can fast-forward.

Best Wishes

Phillip

> 
>    ---  >8  ---
> 
> #!/bin/sh
> 
> test_description='test'
> 
> . ./test-lib.sh
> . "$TEST_DIRECTORY"/lib-rebase.sh
> 
> test_expect_success '--rebase-merges leaves MERGE_MSG behind' '
> 	# A-B-M-D-E-F
> 	#  \ /
> 	#   C
> 	test_commit A &&
> 	test_commit B &&
> 	git checkout -b branch HEAD^ &&
> 	test_commit C &&
> 	git checkout master &&
> 	git merge --log branch &&
> 	test_commit D &&
> 	test_commit E &&
> 	test_commit F &&
> 
> 	# Rewrite a commit after the merge:
> 	write_script todo-editor <<-\EOF &&
> 	sed -i -e "/^pick .* E$/ s/^pick/edit/" "$1"
> 	EOF
> 
> 	# No MERGE_MSG present before starting the rebase, good:
> 	test_path_is_missing .git/MERGE_MSG &&
> 
> 	# Start rebasing before the merge, so the sequencer has to
> 	# re-create an identical merge commit.
> 	GIT_EDITOR=./todo-editor git rebase -i --rebase-merges A &&
> 
> 	# Just to make sure that the history so far is unchanged:
> 	test_cmp_rev E HEAD &&
> 
> 	# BUG: now there is a stray MERGE_MSG file:
> 	cat .git/MERGE_MSG &&
> 
> 	# And it interferes with the next "git commit", because its
> 	# content is included in the commit message template:
> 	echo foo >>E.t &&
> 	GIT_EDITOR=cat git commit -a
> '
> 
> test_done
>