Re: [PATCH] merge: allow fast-forward when merging a tracked tag

[Eric Sunshine <sunshine@sunshineco.com>]

On Thu, Feb 15, 2018 at 5:45 PM, Junio C Hamano <gitster@pobox.com> wrote:
> [...]
> Update the default (again) for "git merge" that merges a tag object
> to (1) --no-ff (i.e. create a merge commit even when side branch
> fast forwards) if the tag being merged is not at its expected place
> in refs/tags/ hierarchy and (2) --ff (i.e. allow fast-forward update
> when able) otherwise.
>
> Signed-off-by: Junio C Hamano <gitster@pobox.com>
> ---
> diff --git a/builtin/merge.c b/builtin/merge.c
> @@ -1125,6 +1126,42 @@ static struct commit_list *collect_parents(struct commit *head_commit,
> +static int merging_a_throwaway_tag(struct commit *commit)
> +{
> +       const char *tag_ref;
> +       struct object_id oid;
> +
> +       /* Are we merging a tag? */
> +       if (!merge_remote_util(commit) ||
> +           !merge_remote_util(commit)->obj ||
> +           merge_remote_util(commit)->obj->type != OBJ_TAG)
> +               return 0;
> +
> +       /*
> +        * Now we know we are merging a tag object.  Are we downstream
> +        * and following the tags from upstream?  If so, we must have
> +        * the tag object pointed at by "refs/tags/$T" where $T is the
> +        * tagname recorded in the tag object.  We want to allow such
> +        * a "just to catch up" merge to fast-forward.
> +        */
> +       tag_ref = xstrfmt("refs/tags/%s",
> +                         ((struct tag *)merge_remote_util(commit)->obj)->tag);

xstrfmt() allocates a new string...

> +       if (!read_ref(tag_ref, &oid) &&
> +           !oidcmp(&oid, &merge_remote_util(commit)->obj->oid))
> +               return 0;

...which is leaked here...

> +
> +       /*
> +        * Otherwise, we are playing an integrator's role, making a
> +        * merge with a throw-away tag from a contributor with
> +        * something like "git pull $contributor $signed_tag".
> +        * We want to forbid such a merge from fast-forwarding
> +        * by default; otherwise we would not keep the signature
> +        * anywhere.
> +        */
> +       return 1;

...and here.

> +}