Jeff King <p...@peff.net> writes:

> This patch defines the patch-id of a merge commit as
> essentially "null"; it has no patch-id. As a result,
> merges cannot match patch-ids via "--cherry-pick", and
> "format-patch --base" will not list merges in its list of
> prerequisite patch ids.

At first I wondered if such a change would make all merges look the
same, but the patch-ids.c comparison is not for ordering/sorting but
only for equality, so as long as the comparison function knows that
a comparison of anything with "null" yields "They are different", we
are OK.

> diff --git a/patch-ids.c b/patch-ids.c
> index 77e4663..8d06099 100644
> --- a/patch-ids.c
> +++ b/patch-ids.c
> @@ -7,18 +7,40 @@
>  int commit_patch_id(struct commit *commit, struct diff_options *options,
>                   unsigned char *sha1, int diff_header_only)
>  {
> -     if (commit->parents)
> +     if (commit->parents) {
> +             if (commit->parents->next)
> +                     return PATCH_ID_NONE;
>               diff_tree_sha1(commit->parents->item->object.oid.hash,
>                              commit->object.oid.hash, "", options);
> -     else
> +     } else
>               diff_root_tree_sha1(commit->object.oid.hash, "", options);
>       diffcore_std(options);
> -     return diff_flush_patch_id(options, sha1, diff_header_only);
> +     if (diff_flush_patch_id(options, sha1, diff_header_only))
> +             return PATCH_ID_ERROR;
> +     return PATCH_ID_OK;
> +}

Looks sensible.  Thanks.

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