Re: [PATCH 3/3] builtin/commit-graph.c: support '--input=none'

["Martin Ågren" <martin.agren@gmail.com>]

On Fri, 31 Jan 2020 at 01:30, Taylor Blau <me@ttaylorr.com> wrote:
> In the previous commit, we introduced '--[no-]merge', and alluded to the
> fact that '--merge' would be useful for callers who wish to always
> trigger a merge of an incremental chain.

Hmmm. So it looks like you've already had similar thoughts as I did
about patch 1/3. At some point, you had a separate `--merge=...` option,
then later made that `--split=...`. :-) Could you say something about why
you changed your mind?

> 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()'.

(Use one of those options with an empty stdin? Anyway, let's read on.)

> 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=none' as an option to behave as if '--input=append' were
> given, but to consider no other sources in addition.

`--input=none` almost makes me wonder if it should produce an empty
commit-graph. But there wouldn't be much point in that... I guess
another way of defining this would be that it "uses no input, and
implies `--append`".

> 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=none

Right.

> --- a/Documentation/git-commit-graph.txt
> +++ b/Documentation/git-commit-graph.txt
> @@ -39,24 +39,29 @@ COMMANDS
>  --------
>  'write'::
>
> -Write a commit-graph file based on the commits found in packfiles.
> +Write a commit-graph file based on the commits specified:
> +* With the `--input=stdin-packs` option, generate the new commit graph
> +by walking objects only in the specified pack-indexes. (Cannot be
> +combined with `--input=stdin-commits` or `--input=reachable`.)
>  +
> -With the `--input=stdin-packs` option, generate the new commit graph by
> -walking objects only in the specified pack-indexes. (Cannot be combined
> -with `--input=stdin-commits` or `--input=reachable`.)
> -+
> -With the `--input=stdin-commits` option, generate the new commit graph
> +* With the `--input=stdin-commits` option, generate the new commit graph
>  by walking commits starting at the commits specified in stdin as a list
>  of OIDs in hex, one OID per line. (Cannot be combined with
>  `--input=stdin-packs` or `--input=reachable`.)
>  +
> -With the `--input=reachable` option, generate the new commit graph by
> +* With the `--input=reachable` option, generate the new commit graph by
>  walking commits starting at all refs. (Cannot be combined with
>  `--input=stdin-commits` or `--input=stdin-packs`.)
>  +
> -With the `--input=append` option, include all commits that are present
> +* With the `--input=append` option, include all commits that are present
>  in the existing commit-graph file.

Do these changes above really belong in this commit?

> +* With the `--input=none` option, behave as if `input=append` were
> +given, but do not walk other packs to find additional commits.
> +
> +If none of the above options are given, then commits found in
> +packfiles are specified.

"specified"? Plus, that also happens for `--input=append` right? (It
really seems like "append" is an odd one among all the inputs.)

>         N_("git commit-graph write [--object-dir <objdir>] [--append] "
> -          "[--split[=<strategy>]] [--input=<reachable|stdin-packs|stdin-commits>] "
> +          "[--split[=<strategy>]] "
> +          "[--input=<reachable|stdin-packs|stdin-commits|none>] "
>            "[--[no-]progress] <split options>"),

Hmm, you've left "--append" the old way.


Martin