Re: [RFC/PATCH v2 0/3] patch-id for merges

From: Josh Triplett <josh@joshtriplett.org>
Date: Sep 7, 2016, 22:51 UTC
Message-ID: <20160907225104.f5wi2yo4d2f26tti@x>
In-Reply-To: <20160907220101.hwwutkiagfottbdd@sigill.intra.peff.net>

On Wed, Sep 07, 2016 at 06:01:01PM -0400, Jeff King wrote:

Show 19 quoted lines

> Here's a re-roll of the series I posted at:
> 
>   http://public-inbox.org/git/20160907075346.z6wtmqnfc6bsunjb@sigill.intra.peff.net/
> 
> Basically, it drops the time for "format-patch --cherry-pick" on a
> particular case from 3 minutes down to 3 seconds, by avoiding diffs
> on merge commits. Compared to v1, it fixes the totally-broken handling
> of commit_patch_id() pointed out by Johannes.
> 
> We can drop the diffs on the merge commits because they're quite broken,
> as discussed in the commit message of patch 3 (they don't take into
> account any parent except the first). So what do we do when somebody
> asks for the patch-id of a merge commit?
> 
> This is still marked RFC, because there are really two approaches here,
> and I'm not sure which one is better for "format-patch --base". I'd like
> to get input from Xiaolong Ye (who worked on --base), and Josh Triplett
> (who has proposed some patches in that area, and is presumably using
> them).

Thanks.

I'd love to see a more resilient patch-id mechanism, to make it easier to match up patches between branches. I don't think it makes sense to talk about the patch-id of a merge commit (though it might make sense for a merge which makes additional changes not present in any of the parents). Even if someone wants to match up merge commits with merge commits, I don't think that should happen via patch-id; I think that should happen in terms of "what patches does this merge introduce", without constructing a merge-patch-id via a Merkle tree of commit patch-ids.

So, I think this patch series makes sense (modulo the comments about the commit message in patch 3). We already don't respect merge commits when doing format-patch; this seems consistent with that. If we ever make it possible for format-patch to handle merge commits, then we should also allow it to have merge commits as prerequisites.

- Josh Triplett