Re: RFC: Superseded-by: follow-up trailer

[Konstantin Ryabitsev <konstantin@linuxfoundation.org>]

On Wed, Apr 14, 2021 at 09:30:57AM +0200, Peter Zijlstra wrote:
> > I'd rather folks used the following notation in the cover letter/patch
> > basement:
> > 
> >     Previous versions:
> > 
> >     Link: https://lore.kernel.org/r/foo # v2
> >     Link: https://lore.kernel.org/r/bar # v1
> > 
> > This is much more succinct, indicates the version number in the trailer, and
> > removes the need to remember how to spell "Superseded", as there seems to be
> > little consensus. :)
> 
> How would you handle the difference between the above and:
> 
> Depends on:
> 
> Link: https://lkml.kernel.org/r/foo
> 
> Which is also a semi common thing to do with larger work that comes in
> multiple dependent series.

It's the "# vX" at the end that we'd be looking for. There's no other reason
to put it there if you aren't linking to a previous revision of the series, so
it's a very unambiguous pattern. However, read below.

> That is; I supppse; a long winded way of saying that for parsing (both
> human and machine) it might be useful to have distinct tag names. Now I
> agree with you that Supersedes is a crap word to spell, but it has the
> distinction of being clear on intent.
> 
> So my 2ct go to adding Supercedes: and Depends-on:

Hmm... there is an officially sanctioned way to indicate patch
interdependencies using "prerequisite-patch-id:" footers (see
https://git-scm.com/docs/git-format-patch#_base_tree_information).

That said, I've never seen anyone actually use them, I guess for two reasons:

1. it's per-patch, not per-series
2. it requires running "git patch-id" on each patch in the dependencies

We could build upon that using "prerequisite-series-link", but I find that
unwieldy, especially if it will be followed by a long URL.

How about we borrow RPM-spec lingo, for stuff to go into the cover letter or
below ---, e.g.:

    Subject: [PATCH v3] frobdrv: improve frob latency

    Commit message here

    Signed-off-by: Dave Elloper <dave@example.net>
    ---
    Requires: https://link.to/required-patch
    Obsoletes: https://link.to/v1 # v1
    Obsoletes: https://link.to/v2 # v2
    ---
    Changes in v3:
    Style tweaks.

    [diffstat, patch, etc]

or:

    Subject: [PATCH v3 0/2] frobdrv: improve frob latency

    This series improves frob latency by 20%.

    Requires: https://link.to/required-patch
    Obsoletes: https://link.to/v1 # v1
    Obsoletes: https://link.to/v2 # v2

    [diffstat, etc]

Then I can add automation that will send "Obsoleted-by:" and maybe
"Required-by:" follow-ups to the linked series when it encounters these tags.

Does this work for others?

-K