Johannes Schindelin <johannes.schinde...@gmx.de> writes:

> +     /*
> +      * If HEAD is not identical to the parent of the original merge commit,
> +      * we cannot fast-forward.
> +      */
> +     can_fast_forward = commit && commit->parents &&
> +             !oidcmp(&commit->parents->item->object.oid,
> +                     &head_commit->object.oid);
> +

I think this expression and assignment should better be done much
later.  Are you going to update commit, commit->parents, etc. that
are involved in the computation in the meantime???

>       strbuf_addf(&ref_name, "refs/rewritten/%.*s", merge_arg_len, arg);
>       merge_commit = lookup_commit_reference_by_name(ref_name.buf);
>       if (!merge_commit) {
> @@ -2164,6 +2172,17 @@ static int do_merge(struct commit *commit, const char 
> *arg, int arg_len,
>               rollback_lock_file(&lock);
>               return -1;
>       }
> +
> +     if (can_fast_forward && commit->parents->next &&
> +         !commit->parents->next->next &&
> +         !oidcmp(&commit->parents->next->item->object.oid,
> +                 &merge_commit->object.oid)) {

... Namely, here.  Because the earlier one is computing "are we
replaying exactly the same commit on top of exactly the same
state?", which is merely one half of "can we fast-forward", and
storing it in a variable whose name is over-promising way before it
becomes necessary.  The other half of "can we fast-forward?" logic
is the remainder of the if() condition we see above.  IOW, when
fully spelled, this code can fast-forward when we are replaying a
commit on top of exactly the same first-parent and the commit being
replayed is a single parent merge.

We may even want to get rid of can_fast_forward variable.

> +             strbuf_release(&ref_name);
> +             rollback_lock_file(&lock);
> +             return fast_forward_to(&commit->object.oid,
> +                                    &head_commit->object.oid, 0, opts);
> +     }
> +
>       write_message(oid_to_hex(&merge_commit->object.oid), GIT_SHA1_HEXSZ,
>                     git_path_merge_head(), 0);
>       write_message("no-ff", 5, git_path_merge_mode(), 0);

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