Re: [PATCH v4] builtin/commit-graph.c: support '--input=graphed'

["SZEDER Gábor" <szeder.dev@gmail.com>]

On Mon, Mar 23, 2020 at 02:12:19PM -0600, Taylor Blau wrote:
> Hi,
> 
> In response to some discussion in [1], here is another idea instead of
> '--input=none' that may make things a little clearer. Since it had been
> a long time, I reminded myself that '--input=none' means
> "--input=append, but don't look at packs as is usually the default".
> 
> In [1], Gabor suggested that we could call this '--input=exists' or
> '--input=existing', but I think that 'graphed' may be clearer, since
> it is closer to "only _graphed_ commits".
> 
> Another option would be to call this '--input=only-graphed', but I think
> that may be overly verbose for what we're going for here.
> 
> Let me know what you think.
> 
> [1]: https://lore.kernel.org/git/20200322110424.GC2224@szeder.dev/
> 
> --- 8< ---
> 
> In the previous commit, we introduced '--split=<no-merge|merge-all>',
> and alluded to the fact that '--split=merge-all' would be useful for
> callers who wish to always trigger a merge of an incremental chain.
> 
> There is a problem with the above approach, which is that there is no
> way to specify to the commit-graph builtin that a caller only wants to
> include commits already in the graph. One can specify '--input=append'
> to include all commits in the existing graphs, but the absence of
> '--input=stdin-{commits,packs}' causes the builtin to call
> 'fill_oids_from_all_packs()'.
> 
> Passing '--input=reachable' (as in 'git commit-graph write
> --split=merge-all --input=reachable --input=append') works around this
> issue by making '--input=reachable' effectively a no-op, but this can be
> prohibitively expensive in large repositories, making it an undesirable
> choice for some users.
> 
> Teach '--input=graphed' as an option to behave as if '--input=append' were
> given, but to consider no other sources in addition.
> 
> This, in conjunction with the option introduced in the previous patch
> offers the convenient way to force the commit-graph machinery to
> condense a chain of incrementals without requiring any new commits:
> 
>   $ git commit-graph write --split=merge-all --input=graphed
> 
> Signed-off-by: Taylor Blau <me@ttaylorr.com>
> ---
>  Documentation/git-commit-graph.txt |  8 +++++++-
>  builtin/commit-graph.c             | 11 ++++++++---
>  commit-graph.c                     |  6 ++++--
>  commit-graph.h                     |  3 ++-
>  t/t5324-split-commit-graph.sh      | 26 ++++++++++++++++++++++++++
>  5 files changed, 47 insertions(+), 7 deletions(-)
> 
> diff --git a/Documentation/git-commit-graph.txt b/Documentation/git-commit-graph.txt
> index 0a320cccdd..4d8fbbe8ff 100644
> --- a/Documentation/git-commit-graph.txt
> +++ b/Documentation/git-commit-graph.txt
> @@ -39,7 +39,7 @@ COMMANDS
>  --------
>  'write'::
> 
> -Write a commit-graph file based on the commits found in packfiles.
> +Write a commit-graph file based on the specified sources of input:
>  +
>  With the `--input=stdin-packs` option, generate the new commit graph by
>  walking objects only in the specified pack-indexes. (Cannot be combined
> @@ -57,6 +57,12 @@ walking commits starting at all refs. (Cannot be combined with
>  With the `--input=append` option, include all commits that are present
>  in the existing commit-graph file.
>  +
> +With the `--input=graphed` option, behave as if `--input=append` were
> +given, but do not walk other packs to find additional commits.

s/walk/scan/ would be more fitting, I think.

In an earlier version of these patches I asked for clarification about
what happens with expired commits that are still included in the
commit-graph... and I do remember that you replied to that, but,
unfortunately, not what your reply was.  And after reading this log
message and the documentation update it's still not clear to me.

> ++
> +If none of the above options are given, then generate the new
> +commit-graph by walking over all pack-indexes.

s/walking/scanning/

> ++
>  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