RE: [PATCH] name-rev: use generation numbers if available

From: "Keller, Jacob E" <jacob.e.keller@intel.com>
To: Derrick Stolee <derrickstolee@github.com>,
	"git@vger.kernel.org" <git@vger.kernel.org>
Cc: Junio C Hamano <gitster@pobox.com>,
	Jacob Keller <jacob.keller@gmail.com>
Subject: RE: [PATCH] name-rev: use generation numbers if available
Date: Mon, 28 Feb 2022 20:59:16 +0000	[thread overview]
Message-ID: <CO1PR11MB508965B6F5F4A4B8D45649BFD6019@CO1PR11MB5089.namprd11.prod.outlook.com> (raw)
In-Reply-To: <117ed093-f3e0-ba24-2364-74e43a1306fe@github.com>

> -----Original Message-----
> From: Derrick Stolee <derrickstolee@github.com>
> Sent: Monday, February 28, 2022 12:24 PM
> To: Keller, Jacob E <jacob.e.keller@intel.com>; git@vger.kernel.org
> Cc: Junio C Hamano <gitster@pobox.com>; Jacob Keller
> <jacob.keller@gmail.com>
> Subject: Re: [PATCH] name-rev: use generation numbers if available
> 
> On 2/28/2022 3:20 PM, Keller, Jacob E wrote:
> > On 2/28/2022 11:50 AM, Derrick Stolee wrote:
> >> On 2/28/2022 2:07 PM, Jacob Keller wrote:
> >>> From: Jacob Keller <jacob.keller@gmail.com>
> >>> +/* Check if a commit is before the cutoff. Prioritize generation numbers
> >>> + * first, but use the commit timestamp if we lack generation data.
> >>> + */
> >>> +static int commit_is_before_cutoff(struct commit *commit)
> >>> +{
> >>> +	if (generation_cutoff < GENERATION_NUMBER_INFINITY)
> >>> +		return commit_graph_generation(commit) < generation_cutoff;
> >>> +
> >>> +	return commit->date < cutoff;
> >>> +}
> >>
> >> There are two subtle things going on here when generation_cutoff is
> >> zero:
> >>
> >> 1. In a commit-graph with topological levels _or_ generation numbers v2,
> >>    commit_graph_generation(commit) will always be positive, so we don't
> >>    need to do the lookup.
> >
> > I.e. once we have a generation_cutoff of 0 we can just completely bypass
> > the lookup, saving some time.
> >
> > I think we can do "return generation_cutoff &&
> > commit_graph_generation(commit) < generation_cutoff"
> >
> >>
> >> 2. If the commit-graph was written by an older Git version before
> >>    topological levels were implemented, then the generation of commits
> >>    in the commit-graph are all zero(!). This means that the logic here
> >>    would be incorrect for the "all" case.
> >>
> >> The fix for both cases is to return 1 if generation_cutoff is zero:
> >>
> >
> > I think you mean return 0? Because this returns true if the commit is
> > before the cutoff, but false if its not. (i.e. if its true, we should
> > stop searching this commit, but if its false we should continue searching?
> 
> Yes, sorry I had it mixed up. Your generation_cutoff && ... approach
> will work in that case.
> 

Alright. Will fix that for v2.

> >>> +test_expect_success 'name-rev --all works with non-monotonic' '
> >>
> >> This is working because of the commit-graph, right? We still have
> >> it from the previous test, so we aren't testing that this works
> >> when we only have the commit date as a cutoff.
> >>
> >
> > I can either extend this test or add a separate test which covers this.
> > The test failed before I added the commit graph line.
> >
> >>> +	(
> >>> +		cd non-monotonic &&
> >>> +
> >>> +		cat >expect <<-\EOF &&
> >>> +		E
> >>> +		D
> >>> +		D~1
> >>> +		D~2
> >>> +		A
> >>> +		EOF
> >>> +
> >>> +		git log --pretty=%H >revs &&
> >>> +		git name-rev --tags --annotate-stdin --name-only <revs >actual
> &&
> >>> +
> >>> +		test_cmp expect actual
> >>> +	)
> >>
> >> Do you want to include a test showing the "expected" behavior
> >> when there isn't a commit-graph file? You might need to delete
> >> an existing commit-graph (it will exist in the case of
> >> GIT_TEST_COMMIT_GRAPH=1).
> >>
> >
> > This test actually is intended to show that it works regardless of
> > whether we have a commit graph. (Because in --annotate-stdin mode we
> > disable the heuristic since we don't know what commits we'll see in advance)
> >
> > Is there a good way to delete the graph file?
> 
> The basic way is "rm -rf .git/info/commit-graph*" to be absolutely
> sure (there might be an incremental commit-graph which appears as
> a "commit-graphs" directory).
> 
> >> I also see that you intended to test the "--all" option, which
> >> is not included in your test. That's probably the real key to
> >> getting this test to work correctly. Deleting the graph will
> >> probably cause a failure on this test unless "--all" is added.
> >>
> >
> > Actually both --all and --annotate-stdin disable the heuristic. However,
> > I think adding a test for both makes sense.
> 
> Ah. OK. They could be assertions within the same test since the
> output is expected to be the same.
> 
> Thanks,
> -Stolee