From: Patrick Steinhardt <ps@pks.im>
Date: Mon, 22 Jun 2026 13:53:47 GMT
Subject: Re: [PATCH v4 3/3] replay: offer an option to linearize the commit topology
Message-ID: <ajk-a4a3KSJ2u7Ju@pks.im>
In-Reply-To: <20260622-toon-git-replay-drop-merges-v4-3-ff257f534319@iotcl.com>

On Mon, Jun 22, 2026 at 02:41:57PM +0200, Toon Claes wrote:
> From: Johannes Schindelin <Johannes.Schindelin@gmx.de>
> 
> One of the stated goals of git-replay(1) is to allow implementing the
> git-rebase(1) functionality on the server side.
> 
> The default mode of git-rebase(1) is to act as if `--no-rebase-merges`
> was given. This mode drops merge commits instead of replaying them, and
> linearizes the commit history into a sequence of the
> regular (single-parent) commits.
> 
> Add option `--linearize` to git-replay(1) to do the same.

git-rebase(1) essentially knows about three different modes:

  - "--no-rebase-merges", which is the default and maps to your
    "--linearize".

  - "--rebase-merges", which by default doesn't rebase cousins by using
    "--ancestry-path" internally.

  - "--rebase-merges=rebase-cousins", which doesn't pass the above
    option.

So it's not a simple boolean there, which makes me wonder whether we
should mirror the same interface so that all of git-rebase(1)'s modes
can be represented, as well.

> diff --git a/replay.c b/replay.c
> index 7921d7dba3..5539daff00 100644
> --- a/replay.c
> +++ b/replay.c
> @@ -277,12 +277,16 @@ static struct commit *pick_regular_commit(struct repository *repo,
>  					  struct commit *onto,
>  					  struct merge_options *merge_opt,
>  					  struct merge_result *result,
> +					  struct commit *replayed_base,
>  					  bool reverse,
>  					  enum replay_empty_commit_action empty)
>  {
> -	struct commit *base, *replayed_base;
> +	struct commit *base;
>  	struct tree *pickme_tree, *base_tree, *replayed_base_tree;
>  
> +	if (replayed_base && reverse)
> +		BUG("Linearizing commits is not supported when replaying in reverse");

Nit: Error messages should typically start with a lower-case letter.

> @@ -430,12 +435,25 @@ int replay_revisions(struct rev_info *revs,
>  	while ((commit = get_revision(revs))) {
>  		const struct name_decoration *decoration;
>  
> -		if (commit->parents && commit->parents->next)
> -			die(_("replaying merge commits is not supported yet!"));
> +		if (commit->parents && commit->parents->next) {
> +			if (!opts->linearize)
> +				die(_("replaying merge commits is not supported yet!"));
> +			/*
> +			 * Drop the merge commit: do not pick it and leave
> +			 * last_commit unchanged, so its children (and any ref
> +			 * pointing at it) are reparented onto the previous
> +			 * non-merge commit, which the ref-update loop below uses.
> +			 */

One could add a hint here that tells the user to pass the option. But I
guess that might be somewhat weird, as we cannot assume that we're
called by git-replay(1) here.

In any case, this here is the core of the change where we stop dying in
case "--linearize" was passed, and instead we simply skip the commit
altogether. Makes sense.

Thanks!

Patrick