Re: [PATCH 1/1] commit-graph.c: avoid unnecessary tag dereference when merging

From: Jeff King <peff@peff.net>
To: Taylor Blau <me@ttaylorr.com>
Cc: Derrick Stolee <stolee@gmail.com>,
	Junio C Hamano <gitster@pobox.com>,
	git@vger.kernel.org, dstolee@microsoft.com
Subject: Re: [PATCH 1/1] commit-graph.c: avoid unnecessary tag dereference when merging
Date: Tue, 24 Mar 2020 02:11:59 -0400	[thread overview]
Message-ID: <20200324061159.GC610977@coredump.intra.peff.net> (raw)
In-Reply-To: <20200322154749.GB53402@syl.local>

On Sun, Mar 22, 2020 at 09:47:49AM -0600, Taylor Blau wrote:

> > > [1] I'm actually not quite sure about correctness here. It should be
> > >     fine to generate a graph file without any given commit; readers will
> > >     just have to load that commit the old-fashioned way. But at this
> > >     phase of "commit-graph write", I think we'll already have done the
> > >     close_reachable() check. What does it mean to throw away a commit at
> > >     this stage? If we're the parent of another commit, then it will have
> > >     trouble referring to us by a uint32_t. Will the actual writing phase
> > >     barf, or will we generate an invalid graph file?
> >
> > It doesn't seem great. If I instrument Git like this to simulate an
> > object temporarily "missing" (if it were really missing the whole repo
> > would be corrupt; we're trying to see what would happen if a race causes
> > us to momentarily not see it):
> 
> This is definitely a problem on either side of this patch, which is
> demonstrated by the fact that you applied your changes without my patch
> on top (and that my patch isn't changing anything substantial in this
> area like removing the 'continue' statement).
> 
> Should we address this before moving on with my patch? I think that we
> *could*, but I'd rather go forward with what we have for now, since it's
> only improving the situation, and not introducing a new bug.

I do agree it's a problem before your patch. But I think your patch may
make it a lot more common, if only because it means we'd _actually_ be
dropping entries for objects that went away, instead of accidentally
keeping them due to re-using the graph result. So it probably is worth
trying to deal with it now, or at least thinking hard about it.

The trouble is that I'm not sure what _should_ happen. Aborting the
whole commit-graph generation seems overboard (and would be annoying for
cases where whole swaths of history became unreachable and went away;
presumably we'd be dropping _all_ of those objects, and the write phase
would be just fine). I have a feeling the correct solution is to do this
merging pass earlier, before we check close_reachable(). Or if not a
true merging pass, at least a pass to check which existing entries are
still valid. But I don't know how painful that reordering would be.

-Peff