Subject: Re: [PATCH] cherry-pick: do not error on non-merge commits when '-m 1' is specified

From: Sergey Organov <[email-masked]>
Date: Tue, 19 Mar 2019 14:29:14 +0300

Hi Junio,

[It's been a while since this discussion, and recently I've got some
thoughts and questions about "first-parent" issues in general, below.]

Junio C Hamano <[email-masked]> writes:

> Sergey Organov <[email-masked]> writes:
>
>> When cherry-picking multiple commits, it's impossible to have both
>> merge- and non-merge commits on the same command-line. Not specifying
>> '-m 1' results in cherry-pick refusing to handle merge commits, while
>> specifying '-m 1' fails on non-merge commits.
>
> Allowing "-m1" even when picking a single parent commit, because
> the 1-st parent is defined for such a commit, makes sense, espeially
> when running a cherry-pick on a range, exactly for the above reason.
> It is slightly less so when cherry-picking a single commit, but not
> by a large margin.
>
> I think the original reasoning for requiring "-m $n" not present,
> especially because cherry-pick was originally for replaying only a
> single commit, was because it would lead somebody to propose that
> the command should behave as if -m1 is always given (and when trying
> to cherry-pick a merge relative to its second parent, give -m2 to
> override it), which in turn encourage the 'first-parent is special'
> world-view from the tool-side.  IOW, "The worldview to treat the
> first-parent chain specially is correct, because Git has many
> features to work with that worldview conveniently" was something we
> wanted to avoid; rather "Such and such workflows benefit from
> treating the first-parent chain specially, so let's add features to
> do so" was we wanted to do, and of course, back then cherry-pick
> that allows mixture of merges and single-parent commits to be
> picked, which would have made the need to do something like this
> patch does felt greater, did not exist.
>
> Now, it appears, at least to me, that the world pretty much accepted
> that the first-parent worldview is often very convenient and worth
> supporting by the tool, so the next logical step might be to set
> opts->mainline to 1 by default (and allow an explicit "-m $n" from
> the command line to override it).  But that should happen after this
> patch lands---it is logically a separate step, I would think.

I think that "first-parent is special" is the way to go indeed for
porcelain, as it does make many thing easier and more convenient[*].

Is there a road-map already outlined in that direction, I wonder?

OTOH, for plumbing, it's rather keeping the original pure-DAG
"symmetrical merges" approach that seems to be the right thing to do.

[*] One example that immediately comes to mind is "git log -p" for a
merge commit. I doesn't currently (as of v2.10) show the first-parent
diff, for whatever reason. "git log -p -m --first-parent" is needed to
get the answer to most "obvious" question: what (merge) commit did to my
mainline? "git show" has its own issues.

-- Sergey