Re: [PATCH 3/3] commit-graph: respect 'core.useBloomFilters'

From: Jeff King <peff@peff.net>
Date: Jun 30, 2020, 19:18 UTC
Message-ID: <20200630191834.GC1888406@coredump.intra.peff.net>
In-Reply-To: <4cfa086e503e19763a9d581fcb6a2ef818776dfc.1593536481.git.me@ttaylorr.com>

On Tue, Jun 30, 2020 at 01:17:48PM -0400, Taylor Blau wrote:

> Git uses the 'core.commitGraph' configuration value to control whether
> or not the commit graph is used when parsing commits or performing a
> traversal.

I think this is a good thing to have, and the patch itself makes sense to me (this is actually my first time reviewing it, despite its intended use within GitHub :) ).

If I may bikeshed for a moment:

Show 8 quoted lines

> Introduce 'core.useBloomFilters' to control whether or not Bloom filters
> are read. Note that this configuration is independent from both:
> 
>   - 'core.commitGraph', to allow flexibility in using all parts of a
>     commit-graph _except_ for its Bloom filters.
> 
>   - The '--changed-paths' option for 'git commit-graph write', to allow
>     reading and writing Bloom filters to be controlled independently.

Should we avoid exposing the user to the words "Bloom filter"?

The command-line option for writing them was genericized to "changed-paths", which I think is good. The use of Bloom filters is an implementation detail. What the user cares about is whether we can optimize queries of which paths changed in a commit.

When we introduced reachability bitmaps long ago, we made the mistake of just calling them "bitmaps". That jargon is well understood by people who work with that code, but it's confusing outside of that (even within other parts of Git) because bitmaps are just a generic data structure. You can have a bitmap of just about anything (and indeed we do use other bitmaps these days). Consistently calling them "reachability bitmaps", especially in the user facing bits, would have reduced confusion over the years.

Similarly, Bloom filters are a generic structure we might use elsewhere. I don't really care if we use the word "Bloom" internally to refer to this feature, but we'll be stuck with this config option for all time. I think it's worth picking something more clear.

It might even be worth considering whether "changed paths" needs more context (or would if we add new features in the future). On a "git commit-graph write" command-line it is perfectly clear, but would core.commitGraphChangedPaths be worth it? It's definitely more specific, but it's also way more ugly. ;)

Show 11 quoted lines

> diff --git a/t/helper/test-read-graph.c b/t/helper/test-read-graph.c
> index 6d0c962438..5f585a1725 100644
> --- a/t/helper/test-read-graph.c
> +++ b/t/helper/test-read-graph.c
> @@ -12,11 +12,12 @@ int cmd__read_graph(int argc, const char **argv)
>  	setup_git_directory();
>  	odb = the_repository->objects->odb;
>  
> +	prepare_repo_settings(the_repository);
> +
>  	graph = read_commit_graph_one(the_repository, odb);

I wondered why we would need this prepare_repo_settings() now, when it should have been needed already to cover core.commitGraph already. I strongly suspect the answer is: "test-tool read-graph" never properly respected core.commitGraph in the first place.

And now presumably it would. If true, I don't think any tests need adjusted because the only places we set it are:

  - on a "git -c" command line, which wouldn't run a test-tool helper

  - when we do set it, it is always to "true", which is the default
    anyway

Show 7 quoted lines

>  	if (!graph)
>  		return 1;
>  
> -
>  	printf("header: %08x %d %d %d %d\n",
>  		ntohl(*(uint32_t*)graph->data),
>  		*(unsigned char*)(graph->data + 4),

Oh good, I happened to be looking at this code earlier today for an unrelated reason and was bothered by this extra newline. :)

-Peff