Re: [PATCH v3 2/2] fetch: write commit-graph using updated refs only

> From: Kristofer Karlsson <krka@spotify.com>
> 
> When fetch.writeCommitGraph was introduced in 50f26bd035 (fetch: add
> fetch.writeCommitGraph config setting, 2019-09-02), the stated goal
> was to stay updated with the latest commits after fetching new
> objects.  The implementation used write_commit_graph_reachable()
> because it was the only API available, but two things have changed
> since then:
> 
>  1. write_commit_graph() was added, and it accepts an explicit set of
>     commits as seeds, enabling more targeted commit-graph updates.
> 
>  2. The ref-scanning callback add_ref_to_set() became more expensive
>     in 630cd5194e (commit-graph.c: peel refs in 'add_ref_to_set',
>     2020-07-22) when it started to validate the refs against the odb
>     for correctness.  On a repository with many refs, this makes the
>     full reachable scan unnecessarily costly for a targeted fetch.
> 
> Optimize the commit-graph write by using only the newly updated refs
> as seeds instead of scanning all refs after every fetch.  To keep
> this change small, skip the optimization for multi-remote fetches
> (since that would require propagating the set of refs across process
> boundaries).
> 
> This relies on the commit-graph write being additive, keeping the
> commits that are already in the graph.  fetch already operates in
> this mode (COMMIT_GRAPH_WRITE_SPLIT) and now that becomes
> required for correctness.  Without that mode, the write would
> replace the commit-graph and lose other commits.

> After fetch_one() returns, call prepare_commit_graph() (which is
> made non-static by this commit) to determine the graph-write mode:
> 
>  - If no commit-graph exists yet, fall back to the full reachable
>    scan so the first graph creation covers all refs.
> 
>  - If a commit-graph exists and the fetch updated at least one ref,
>    write incrementally using only the new refs as seeds.
> 
>  - If a commit-graph exists but the fetch is a no-op, skip the
>    commit-graph write entirely.
> 
>  - For the multi-remote path (fetch --all), where child processes
>    do the actual fetching, fall back to the full reachable scan.
> 
> Full commit-graph coverage of all refs remains the responsibility
> of "git maintenance", "git gc" and "git commit-graph write".
> Regular Git operations may trigger "git maintenance run --auto",
> which periodically rebuilds the commit-graph from all reachable
> refs.