From: Derrick Stolee <stolee@gmail.com>
Date: Fri, 05 Oct 2018 12:05:55 GMT
Subject: Re: [RFC PATCH] We should add a "git gc --auto" after "git clone" due to commit graph
Message-ID: <dcb8f115-ce3c-64fa-50cc-dd03569c0164@gmail.com>
In-Reply-To: <87in2hgzin.fsf@evledraar.gmail.com>

On 10/4/2018 5:42 PM, Ævar Arnfjörð Bjarmason wrote:
> I don't have time to polish this up for submission now, but here's a WIP
> patch that implements this, highlights:
>
>   * There's a gc.clone.autoDetach=false default setting which overrides
>     gc.autoDetach if 'git gc --auto' is run via git-clone (we just pass a
>     --cloning option to indicate this).

I'll repeat that it could make sense to do the same thing on clone _and_ 
fetch. Perhaps a "--post-fetch" flag would be good here to communicate 
that we just downloaded a pack from a remote.

>   * A clone of say git.git with gc.writeCommitGraph=true looks like:
>
>     [...]
>     Receiving objects: 100% (255262/255262), 100.49 MiB | 17.78 MiB/s, done.
>     Resolving deltas: 100% (188947/188947), done.
>     Computing commit graph generation numbers: 100% (55210/55210), done.

This looks like good UX. Thanks for the progress here!

>   * The 'git gc --auto' command also knows to (only) run the commit-graph
>     (and space is left for future optimization steps) if general GC isn't
>     needed, but we need "optimization":
>
>     $ rm .git/objects/info/commit-graph; ~/g/git/git --exec-path=$PWD -c gc.writeCommitGraph=true -c gc.autoDetach=false gc --auto;
>     Annotating commits in commit graph: 341229, done.
>     Computing commit graph generation numbers: 100% (165969/165969), done.
>     $

Will this also trigger a full commit-graph rewrite on every 'git commit' 
command? Or is there some way we can compute the staleness of the 
commit-graph in order to only update if we get too far ahead? 
Previously, this was solved by relying on the auto-GC threshold.

>   * The patch to gc.c looks less scary with -w, most of it is indenting
>     the existing pack-refs etc. with a "!auto_gc || should_gc" condition.
>
>   * I added a commit_graph_exists() exists function and only care if I
>     get ENOENT for the purposes of this gc mode. This would need to be
>     tweaked for the incremental mode Derrick talks about, but if we just
>     set "should_optimize" that'll also work as far as gc --auto is
>     concerned (e.g. on fetch, am etc.)

The incremental mode would operate the same as split-index, which means 
we will still look for .git/objects/info/commit-graph. That file may 
point us to more files.

> +int commit_graph_exists(const char *graph_file)
> +{
> +	struct stat st;
> +	if (stat(graph_file, &st)) {
> +		if (errno == ENOENT)
> +			return 0;
> +		else
> +			return -1;
> +	}
> +	return 1;
> +}
> +

This method serves a very similar purpose to 
generation_numbers_enabled(), except your method only cares about the 
file existing. It ignores information like `core.commitGraph`, which 
should keep us from doing anything with the commit-graph file if false.

Nothing about your method is specific to the commit-graph file, since 
you provide a filename as a parameter. It could easily be "int 
file_exists(const char *filename)".

Thanks,

-Stolee