Re: [PATCH 3/4] rebase -r: don't write .git/MERGE_MSG when fast-forwarding

> From: Phillip Wood <phillip.wood@dunelm.org.uk>
> 
> When fast-forwarding we do not create a new commit so .git/MERGE_MSG
> is not removed and can end up seeding the message of a commit made
> after the rebase has finished. Avoid writing .git/MERGE_MSG when we
> are fast-forwarding by writing the file after the fast-forward
> checks.
> 
> Note that the way this change is implemented means we no longer write
> the author script when fast-forwarding either. I believe this is safe
> for the reasons below but it is a departure from what we do when
> fast-forwarding a non-merge commit. If we reword the merge then 'git
> commit --amend' will keep the authorship of the commit we're rewording
> as it ignores GIT_AUTHOR_* unless --reset-author is passed. It will
> also export the correct GIT_AUTHOR_* variables to any hooks and we
> already test the authorship of the reworded commit. If we are not
> rewording then we no longer call spilt_ident() which means we are no
> longer checking the commit author header looks sane. However this is
> what we already do when fast-forwarding non-merge commits in
> skip_unnecessary_picks() so I don't think we're breaking any promises
> by not checking the author here.

> Reported-by: SZEDER Gábor <szeder.dev@gmail.com>
> Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
> ---
>  sequencer.c     | 81 +++++++++++++++++++++++++------------------------
>  t/lib-rebase.sh | 10 ++++--
>  2 files changed, 49 insertions(+), 42 deletions(-)
> 
> diff --git a/sequencer.c b/sequencer.c
> index cc8a361cceb..c2cba5ed4b1 100644
> --- a/sequencer.c
> +++ b/sequencer.c
> @@ -983,7 +983,8 @@ static int run_git_commit(const char *defmsg,
>  
>  	cmd.git_cmd = 1;
>  
> -	if (is_rebase_i(opts) && read_env_script(&cmd.env_array)) {
> +	if (is_rebase_i(opts) && !(!defmsg && (flags & AMEND_MSG)) &&
> +	    read_env_script(&cmd.env_array)) {
>  		const char *gpg_opt = gpg_sign_opt_quoted(opts);
>  
>  		return error(_(staged_changes_advice),
> @@ -3815,6 +3816,45 @@ static int do_merge(struct repository *r,
>  		goto leave_merge;
>  	}
>  
> +	/*
> +	 * If HEAD is not identical to the first parent of the original merge
> +	 * commit, we cannot fast-forward.
> +	 */
> +	can_fast_forward = opts->allow_ff && commit && commit->parents &&
> +		oideq(&commit->parents->item->object.oid,
> +		      &head_commit->object.oid);
> +
> +	/*
> +	 * If any merge head is different from the original one, we cannot
> +	 * fast-forward.
> +	 */
> +	if (can_fast_forward) {
> +		struct commit_list *p = commit->parents->next;
> +
> +		for (j = to_merge; j && p; j = j->next, p = p->next)
> +			if (!oideq(&j->item->object.oid,
> +				   &p->item->object.oid)) {
> +				can_fast_forward = 0;
> +				break;
> +			}
> +		/*
> +		 * If the number of merge heads differs from the original merge
> +		 * commit, we cannot fast-forward.
> +		 */
> +		if (j || p)
> +			can_fast_forward = 0;
> +	}
> +
> +	if (can_fast_forward) {
> +		rollback_lock_file(&lock);
> +		ret = fast_forward_to(r, &commit->object.oid,
> +				      &head_commit->object.oid, 0, opts);
> +		if (flags & TODO_EDIT_MERGE_MSG)
> +			goto fast_forward_edit;
> +
> +		goto leave_merge;
> +	}
> +
>  	if (commit) {
>  		const char *encoding = get_commit_output_encoding();
>  		const char *message = logmsg_reencode(commit, NULL, encoding);
> @@ -3864,45 +3904,6 @@ static int do_merge(struct repository *r,
>  		}
>  	}
>  
> -	/*
> -	 * If HEAD is not identical to the first parent of the original merge
> -	 * commit, we cannot fast-forward.
> -	 */
> -	can_fast_forward = opts->allow_ff && commit && commit->parents &&
> -		oideq(&commit->parents->item->object.oid,
> -		      &head_commit->object.oid);
> -
> -	/*
> -	 * If any merge head is different from the original one, we cannot
> -	 * fast-forward.
> -	 */
> -	if (can_fast_forward) {
> -		struct commit_list *p = commit->parents->next;
> -
> -		for (j = to_merge; j && p; j = j->next, p = p->next)
> -			if (!oideq(&j->item->object.oid,
> -				   &p->item->object.oid)) {
> -				can_fast_forward = 0;
> -				break;
> -			}
> -		/*
> -		 * If the number of merge heads differs from the original merge
> -		 * commit, we cannot fast-forward.
> -		 */
> -		if (j || p)
> -			can_fast_forward = 0;
> -	}
> -
> -	if (can_fast_forward) {
> -		rollback_lock_file(&lock);
> -		ret = fast_forward_to(r, &commit->object.oid,
> -				      &head_commit->object.oid, 0, opts);
> -		if (flags & TODO_EDIT_MERGE_MSG)
> -			goto fast_forward_edit;
> -
> -		goto leave_merge;
> -	}
> -
>  	if (strategy || to_merge->next) {
>  		/* Octopus merge */
>  		struct child_process cmd = CHILD_PROCESS_INIT;
> diff --git a/t/lib-rebase.sh b/t/lib-rebase.sh
> index 99d9e7efd2d..ec6b9b107da 100644
> --- a/t/lib-rebase.sh
> +++ b/t/lib-rebase.sh
> @@ -173,10 +173,16 @@ set_reword_editor () {
>  
>  	write_script reword-editor.sh <<-EOF &&
>  	# Save the oid of the first reworded commit so we can check rebase
> -	# fast-forwards to it
> +	# fast-forwards to it. Also check that we do not write .git/MERGE_MSG
> +	# when fast-forwarding
>  	if ! test -s reword-oid
>  	then
> -		git rev-parse HEAD >reword-oid
> +		git rev-parse HEAD >reword-oid &&
> +		if test -f .git/MERGE_MSG
> +		then
> +			echo 1>&2 "error: .git/MERGE_MSG exists"
> +			exit 1
> +		fi
>  	fi &&
>  	# There should be no uncommited changes
>  	git diff --exit-code HEAD &&
> -- 
> gitgitgadget
>