Subject: Re: [PATCH v2] replay: drop commits that become empty

From: Junio C Hamano <[email-masked]>
Date: Tue, 16 Dec 2025 08:50:34 +0900

Phillip Wood <[email-masked]> writes:

> From: Phillip Wood <[email-masked]>
>
> If the changes in a commit being replayed are already in the branch
> that the commits are being replayed onto then "git replay" creates an
> empty commit. This is confusing because the commit message no longer
> matches the contents of the commit. Drop the commit instead. Commits
> that start off empty are not dropped. This matches the behavior of
> "git rebase --reapply-cherry-pick --empty=drop" and "git cherry-pick
> --empty-drop".

OK.  Maybe it is just me but "onto then" -> "onto," would flow the
sentence better?

> If a branch points to a commit that is dropped it will be updated to
> point to the last commit that was not dropped. This can been seen

If one thinks about it, it is the only natural behaviour to use the
last surviving commit to point the branch at.  Thanks for spelling
it out so clearly.

BTW, "can been seen" -> "can be seen" (will amend locally).

> in the new test where "topic1" is updated to point to the rebased
> "C" as "F" is dropped because it is already upstream. While this is
> a breaking change "git replay" is marked as experimental to allow
> improvements like this that change the behavior.

Again maybe it is just me, but I'd prefer to see a comma after "a
breaking change" to flow the sentence better.

> Helped-by: Elijah Newren <[email-masked]>
> Signed-off-by: Phillip Wood <[email-masked]>
> ---
> ...
> diff --git a/Documentation/git-replay.adoc b/Documentation/git-replay.adoc
> index dcb26e8a8e8..96a3a557bf3 100644
> --- a/Documentation/git-replay.adoc
> +++ b/Documentation/git-replay.adoc
> @@ -59,7 +59,9 @@ The default mode can be configured via the `replay.refAction` configuration vari
>  	be passed, but in `--advance <branch>` mode, they should have
>  	a single tip, so that it's clear where <branch> should point
>  	to. See "Specifying Ranges" in linkgit:git-rev-parse[1] and the
> -	"Commit Limiting" options below.
> +	"Commit Limiting" options below. Any commits in the range whose
> +	changes are already present in the branch the commits are being
> +	replayed onto will be dropped.

OK.

> diff --git a/replay.c b/replay.c
> index 13983dbc566..2864c213993 100644
> --- a/replay.c
> +++ b/replay.c
> @@ -88,12 +88,12 @@ struct commit *replay_pick_regular_commit(struct repository *repo,
>  					  struct merge_result *result)
>  {
>  	struct commit *base, *replayed_base;
> -	struct tree *pickme_tree, *base_tree;
> +	struct tree *pickme_tree, *base_tree, *replayed_base_tree;
>  
>  	base = pickme->parents->item;
>  	replayed_base = mapped_commit(replayed_commits, base, onto);
>  
> -	result->tree = repo_get_commit_tree(repo, replayed_base);
> +	replayed_base_tree = repo_get_commit_tree(repo, replayed_base);
>  	pickme_tree = repo_get_commit_tree(repo, pickme);
>  	base_tree = repo_get_commit_tree(repo, base);
>  
> @@ -103,13 +103,17 @@ struct commit *replay_pick_regular_commit(struct repository *repo,
>  
>  	merge_incore_nonrecursive(merge_opt,
>  				  base_tree,
> -				  result->tree,
> +				  replayed_base_tree,
>  				  pickme_tree,
>  				  result);
>  
>  	free((char*)merge_opt->ancestor);
>  	merge_opt->ancestor = NULL;
>  	if (!result->clean)
>  		return NULL;
> +	/* Drop commits that become empty */
> +	if (oideq(&replayed_base_tree->object.oid, &result->tree->object.oid) &&
> +	    !oideq(&pickme_tree->object.oid, &base_tree->object.oid))
> +		return replayed_base;
>  	return replay_create_commit(repo, result->tree, pickme, replayed_base);
>  }

OK, that is straight-forward.  Instead of overriding the
result->tree upfront, we try the same using a temporary
replayed_base_tree, and that allows us to see if the resulting tree
computed by merge_incore matches.  Only when it made a non-empty
change, we proceed to create a new commit.

> diff --git a/t/t3650-replay-basics.sh b/t/t3650-replay-basics.sh
> index cf3aacf3551..9d4b0dd1a77 100755
> --- a/t/t3650-replay-basics.sh
> +++ b/t/t3650-replay-basics.sh
> @@ -25,6 +25,8 @@ test_expect_success 'setup' '
>  	git switch -c topic3 &&
>  	test_commit G &&
>  	test_commit H &&
> +	git switch -c empty &&
> +	git commit --allow-empty --only -m empty &&

The use of "--only" here is a bit curious.  As there is no change
between the index and the commit our "empty" branch points at,
wouldn't it be unnecessary?  The option, together with --allow-empty,
would only matter if you did

	git switch -c empty &&
	modify blah &&
	git add blah &&
	git commit --allow-empty --only -m empty

because without --only, the changes to blah will be taken.