Re: [PATCH 3/5] rebase: factor out merge_base calculation

[Elijah Newren <newren@gmail.com>]

On Mon, Aug 15, 2022 at 8:14 AM Phillip Wood via GitGitGadget
<gitgitgadget@gmail.com> wrote:
>
> From: Phillip Wood <phillip.wood@dunelm.org.uk>
>
> Separate out calculating the merge base between onto and head from the
> check for whether we can fast-forward or not. This means we can skip
> the fast-forward checks when the rebase is forced and avoid
> calculating the merge-base twice when --keep-base is given.
>
> Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
> ---
> Note the unnecessary braces around "if (keep_base)" are added here
> reduce the churn on the next commit.
> ---
>  builtin/rebase.c | 35 +++++++++++++++++++++++------------
>  1 file changed, 23 insertions(+), 12 deletions(-)
>
> diff --git a/builtin/rebase.c b/builtin/rebase.c
> index 6cf9c95f4e1..86ea731ca3a 100644
> --- a/builtin/rebase.c
> +++ b/builtin/rebase.c
> @@ -871,13 +871,9 @@ static int can_fast_forward(struct commit *onto, struct commit *upstream,
>         struct commit_list *merge_bases = NULL;
>         int res = 0;
>
> -       merge_bases = get_merge_bases(onto, head);
> -       if (!merge_bases || merge_bases->next) {
> -               oidcpy(merge_base, null_oid());
> +       if (is_null_oid(merge_base))
>                 goto done;
> -       }
>
> -       oidcpy(merge_base, &merge_bases->item->object.oid);
>         if (!oideq(merge_base, &onto->object.oid))
>                 goto done;
>
> @@ -902,6 +898,20 @@ done:
>         return res && is_linear_history(onto, head);
>  }
>
> +static void fill_merge_base(struct rebase_options *options,
> +                           struct object_id *merge_base)
> +{
> +       struct commit_list *merge_bases = NULL;
> +
> +       merge_bases = get_merge_bases(options->onto, options->orig_head);
> +       if (!merge_bases || merge_bases->next)
> +               oidcpy(merge_base, null_oid());
> +       else
> +               oidcpy(merge_base, &merge_bases->item->object.oid);
> +
> +       free_commit_list(merge_bases);
> +}
> +
>  static int parse_opt_am(const struct option *opt, const char *arg, int unset)
>  {
>         struct rebase_options *opts = opt->value;
> @@ -1668,7 +1678,11 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
>                         die(_("Does not point to a valid commit '%s'"),
>                                 options.onto_name);
>         }
> -
> +       if (keep_base) {
> +               oidcpy(&merge_base, &options.onto->object.oid);
> +       } else {
> +               fill_merge_base(&options, &merge_base);
> +       }
>         if (options.fork_point > 0)
>                 options.restrict_revision =
>                         get_fork_point(options.upstream_name, options.orig_head);
> @@ -1697,13 +1711,10 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
>          * Check if we are already based on onto with linear history,
>          * in which case we could fast-forward without replacing the commits
>          * with new commits recreated by replaying their changes.
> -        *
> -        * Note that can_fast_forward() initializes merge_base, so we have to
> -        * call it before checking allow_preemptive_ff.
>          */
> -       if (can_fast_forward(options.onto, options.upstream, options.restrict_revision,
> -                   options.orig_head, &merge_base) &&
> -           allow_preemptive_ff) {
> +       if (allow_preemptive_ff &&
> +           can_fast_forward(options.onto, options.upstream, options.restrict_revision,
> +                            options.orig_head, &merge_base)) {

I didn't catch anything new in my review of this patch, but I just
really wanted to say how happy this final hunk makes me.  I hated that
can_fast_forward() had to be called first; thanks for fixing that.


>                 int flag;
>
>                 if (!(options.flags & REBASE_FORCE)) {
> --
> gitgitgadget