Re: [PATCH 1/3] builtin/commit-graph.c: support '--split[=<strategy>]'

> With '--split', the commit-graph machinery writes new commits in another
> incremental commit-graph which is part of the existing chain, and
> optionally decides to condense the chain into a single commit-graph.
> This is done to ensure that the aysmptotic behavior of looking up a
> commit in an incremental chain is dominated by the number of
> incrementals in that chain. It can be controlled by the '--max-commits'
> and '--size-multiple' options.
> 
> On occasion, callers may want to ensure that 'git commit-graph write
> --split' always writes an incremental, and never spends effort
> condensing the incremental chain [1]. Previously, this was possible by
> passing '--size-multiple=0', but this no longer the case following
> 63020f175f (commit-graph: prefer default size_mult when given zero,
> 2020-01-02).
> 
> Reintroduce a less-magical variant of the above with a new pair of
> arguments to '--split': '--split=no-merge' and '--split=merge-all'. When
> '--split=no-merge' is given, the commit-graph machinery will never
> condense an existing chain and will always write a new incremental.
> Conversely, if '--split=merge-all' is given, any invocation including it
> will always condense a chain if one exists.  If '--split' is given with
> no arguments, it behaves as before and defers to '--size-multiple', and
> so on.
> 
> [1]: This might occur when, for example, a server administrator running
> some program after each push may want to ensure that each job runs
> proportional in time to the size of the push, and does not "jump" when
> the commit-graph machinery decides to trigger a merge.
> 
> Signed-off-by: Taylor Blau <me@ttaylorr.com>
> ---
>  Documentation/git-commit-graph.txt | 18 +++++++++++-----
>  builtin/commit-graph.c             | 33 ++++++++++++++++++++++++++----
>  commit-graph.c                     | 19 +++++++++--------
>  commit-graph.h                     |  7 +++++++
>  t/t5324-split-commit-graph.sh      | 25 ++++++++++++++++++++++
>  5 files changed, 85 insertions(+), 17 deletions(-)
> 
> diff --git a/Documentation/git-commit-graph.txt b/Documentation/git-commit-graph.txt
> index 28d1fee505..8d61ba9f56 100644
> --- a/Documentation/git-commit-graph.txt
> +++ b/Documentation/git-commit-graph.txt
> @@ -57,11 +57,19 @@ or `--stdin-packs`.)
>  With the `--append` option, include all commits that are present in the
>  existing commit-graph file.
>  +
> -With the `--split` option, write the commit-graph as a chain of multiple
> -commit-graph files stored in `<dir>/info/commit-graphs`. The new commits
> -not already in the commit-graph are added in a new "tip" file. This file
> -is merged with the existing file if the following merge conditions are
> -met:
> +With the `--split[=<strategy>]` option, write the commit-graph as a
> +chain of multiple commit-graph files stored in
> +`<dir>/info/commit-graphs`. Commit-graph layers are merged based on the
> +strategy and other splitting options. The new commits not already in the
> +commit-graph are added in a new "tip" file. This file is merged with the
> +existing file if the following merge conditions are met:
> +* If `--split=merge-always` is specified, then a merge is always
> +conducted, and the remaining options are ignored. Conversely, if
> +`--split=no-merge` is specified, a merge is never performed, and the
> +remaining options are ignored. A bare `--split` defers to the remaining
> +options. (Note that merging a chain of commit graphs replaces the
> +existing chain with a length-1 chain where the first and only
> +incremental holds the entire graph).
>  +
>  * If `--size-multiple=<X>` is not specified, let `X` equal 2. If the new
>  tip file would have `N` commits and the previous tip has `M` commits and
> diff --git a/builtin/commit-graph.c b/builtin/commit-graph.c
> index de321c71ad..f03b46d627 100644
> --- a/builtin/commit-graph.c
> +++ b/builtin/commit-graph.c
> @@ -9,7 +9,9 @@
>  
>  static char const * const builtin_commit_graph_usage[] = {
>  	N_("git commit-graph verify [--object-dir <objdir>] [--shallow] [--[no-]progress]"),
> -	N_("git commit-graph write [--object-dir <objdir>] [--append|--split] [--reachable|--stdin-packs|--stdin-commits] [--[no-]progress] <split options>"),
> +	N_("git commit-graph write [--object-dir <objdir>] [--append] "
> +	   "[--split[=<strategy>]] [--reachable|--stdin-packs|--stdin-commits] "
> +	   "[--[no-]progress] <split options>"),
>  	NULL
>  };
>  
> @@ -19,7 +21,9 @@ static const char * const builtin_commit_graph_verify_usage[] = {
>  };
>  
>  static const char * const builtin_commit_graph_write_usage[] = {
> -	N_("git commit-graph write [--object-dir <objdir>] [--append|--split] [--reachable|--stdin-packs|--stdin-commits] [--[no-]progress] <split options>"),
> +	N_("git commit-graph write [--object-dir <objdir>] [--append] "
> +	   "[--split[=<strategy>]] [--reachable|--stdin-packs|--stdin-commits] "
> +	   "[--[no-]progress] <split options>"),
>  	NULL
>  };
>  
> @@ -101,6 +105,25 @@ static int graph_verify(int argc, const char **argv)
>  extern int read_replace_refs;
>  static struct split_commit_graph_opts split_opts;
>  
> +static int write_option_parse_split(const struct option *opt, const char *arg,
> +				    int unset)
> +{
> +	enum commit_graph_split_flags *flags = opt->value;
> +
> +	opts.split = 1;
> +	if (!arg)
> +		return 0;

> +test_expect_success '--split=merge-all always merges incrementals' '
> +	test_when_finished rm -rf a b c &&
> +	rm -rf $graphdir $infodir/commit-graph &&
> +	git reset --hard commits/10 &&
> +	git rev-list -3 HEAD~4 >a &&
> +	git rev-list -2 HEAD~2 >b &&
> +	git rev-list -2 HEAD >c &&
> +	git commit-graph write --split=no-merge --stdin-commits <a &&
> +	git commit-graph write --split=no-merge --stdin-commits <b &&
> +	test_line_count = 2 $graphdir/commit-graph-chain &&
> +	git commit-graph write --split=merge-all --stdin-commits <c &&
> +	test_line_count = 1 $graphdir/commit-graph-chain
> +'
> +
> +test_expect_success '--split=no-merge always writes an incremental' '
> +	test_when_finished rm -rf a b &&
> +	rm -rf $graphdir &&
> +	git reset --hard commits/2 &&
> +	git rev-list HEAD~1 >a &&
> +	git rev-list HEAD >b &&
> +	git commit-graph write --split --stdin-commits <a &&
> +	git commit-graph write --split=no-merge --stdin-commits <b &&
> +	test_line_count = 2 $graphdir/commit-graph-chain
> +'
> +
>  test_done