linux-kernel - Re: Signal conflict on merging metadata-differing patches

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Message-ID: <20191118184523.GA6894@vmlxhi-102.adit-jv.com>
Date:   Mon, 18 Nov 2019 19:45:23 +0100
From:   Eugeniu Rosca <erosca@...adit-jv.com>
To:     Greg KH <gregkh@...uxfoundation.org>
CC:     Eugeniu Rosca <erosca@...adit-jv.com>, <git@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, Felipe Balbi <balbi@...nel.org>,
        Eugeniu Rosca <roscaeugeniu@...il.com>
Subject: Re: Signal conflict on merging metadata-differing patches

On Mon, Nov 18, 2019 at 06:35:17PM +0100, Greg KH wrote:
> On Mon, Nov 18, 2019 at 06:29:17PM +0100, Eugeniu Rosca wrote:
> > Dear Git community,
> > 
> > Due to high inflow of patches which Linux maintainers carry on their
> > shoulders and due to occasionally intricate relationships between
> > consecutive revisions of the same series, it may [1] happen that two
> > distinct revisions of the same patch (differing only/mostly in
> > metadata, e.g. Author's time-stamp and commit description) may end up
> > being merged on the same branch, without git to complain about that.
> 
> Why would git complain about that?

This would help those performing the merge identify and (if needed)
avoid having several slightly different patches on the same branch.

> 
> > Is there any "git merge" flag available off-the-shelf which (if used)
> > would signal such situations?
> 
> I don't understand what you are looking for here.  Two different
> versions of the patch were merged to different branches and then merged
> together, and git did the right thing with the resolution of the code.

I personally care about commit metadata (i.e. Author's/Committer's names
and timestamps, as well as commit description) as much as (and sometimes
more than) the code contents of the patch.

If I am given multiple patches which perform the same code changes, but
provide different descriptions, this _already_ generates potential work
on my plate, since I have to make sense of those differences when I
stumble upon them. Which patch do I recommend to the customer (who,
let's say, still lives on the older v4.14 LTS), if I am asked to?

Why should I (or anybody else) spend time doing research at all, if this
can be avoided by just passing an additional option to "git merge"?

It is the most basic requirement I can think of that the maintainers
select the _latest_ version of a patch series, without intertwining it
with a superseded version.

> 
> What more can it do here?

Currently Git says nothing in below merge scenarios (all of them are
conflict-less successful merges):
 - Merge two commits which perform identical code changes but have
   different metadata
 - Merge commit "A" and commits ("B", "C", "D"), the latter being
   subsets of "A"

I don't advocate for "git merge" to fail in the above scenarios. No.
I just say that Git could likely detect such scenarios and help people
like you not pushing v2 and v5 of the same patch into the main tree.

> 
> thanks,
> 
> greg k-h

-- 
Best Regards,
Eugeniu