Re: help for git format-patch lost diffstat

[Eric Sunshine <sunshine@sunshineco.com>]

On Sun, Apr 24, 2022 at 9:00 AM FanJun Kong <bh1scw@gmail.com> wrote:
> First, I created two commits to a demo project.
>
> when I use command:
> git format-patch --cover-letter -1
> [...]
> FanJun Kong (1):
>   First commit.
>    1.c | 4 ++++
>    1 file changed, 4 insertions(+)
>
> ok, you can see the cover letter has diffstat, just above "--".
>
> git format-patch --cover-letter -2
> [...]
> FanJun Kong (2):
>   First commit.
>   Second commit.
>
> Comparing with last command, I just want to get 2 patches,
> but this time the diffstat is missing.
>
> I am not sure if this is a bug or I miss some options.

This is expected behavior according to this code in
make_cover_letter() in builtin/log.c:

    /* We can only do diffstat with a unique reference point */
    if (origin)
        show_diffstat(rev, origin, head);

The problem is that when you format both patches of a two-commit
repository, `origin` is NULL because there is no commit preceding the
initial commit; the initial commit is the root of the history. Thus,
there is nothing prior to the first patch against which to create a
diffstat.

I have not investigated, but it may be possible to teach
show_diffstat() how to generate a diffstat against the "emptiness"
preceding the initial commit, but nobody has done so yet. Perhaps this
would be a good project for someone interested in contributing to the
project (or perhaps not -- as mentioned, I haven't investigated how
hard this would be).