Re: [PATCH v9 4/7] replay: yield the object ID of the final rewritten commit

>
> In a subsequent commit we'll introduce a new git-history(1) command that
> uses the replay machinery to rewrite commits. One of its supported modes
> will only want to update the "HEAD" reference, but that is not currently
> supported by the replay machinery.

> Allow implementing this use case by exposing a `final_oid` field for the
> reference updates. This field will be set to the last commit that was
> rewritten, which is sufficient information for us to implement this mode
> in git-history(1).
>
> Signed-off-by: Patrick Steinhardt <ps@pks.im>
> ---
>  replay.c |  2 ++
>  replay.h | 16 ++++++++++++++++
>  2 files changed, 18 insertions(+)
>
> diff --git a/replay.c b/replay.c
> index 313fb2a768..20560e792b 100644
> --- a/replay.c
> +++ b/replay.c
> @@ -338,6 +338,8 @@ int replay_revisions(struct repository *repo, struct rev_info *revs,
>                                            &onto->object.oid,
>                                            &last_commit->object.oid);
>
> +       out->final_oid = last_commit->object.oid;
> +
>         ret = 0;
>
>  out:
> diff --git a/replay.h b/replay.h
> index 84bc8a7a5b..f8f9889112 100644
> --- a/replay.h
> +++ b/replay.h
> @@ -46,6 +46,22 @@ struct replay_result {
>
>         /* Set to true in case the replay failed with a merge conflict. */
>         bool merge_conflict;
> +
> +       /*
> +        * The final object ID that was rewritten. Note that this field has
> +        * somewhat special semantics and may or may not be what you want:
> +        *
> +        *   - If no commits were rewritten it will remain uninitialized.
> +        *
> +        *   - If a thicket of branches is rewritten it is undefined in which
> +        *     order those branches will be rewritten, and thus the final object
> +        *     ID may point to a different commit than you'd expect.
> +        *
> +        * That being said, this field can still be useful when you know that
> +        * you only replay a single strand of commits. In that case, the final
> +        * commit will point to the tip of the rewritten strand of commits.
> +        */
> +       struct object_id final_oid;
>  };