Re: [PATCH v4 12/13] commit-graph: read only from specific pack-indexes

From: Stefan Beller <sbeller@google.com>
Date: Feb 21, 2018, 22:25 UTC
Message-ID: <CAGZ79kZbdvehOQMZj3NkkSt5RoArtv0ixb8pHvFDrHTrWmVuDg@mail.gmail.com>
In-Reply-To: <1519066406-81663-13-git-send-email-dstolee@microsoft.com>

On Mon, Feb 19, 2018 at 10:53 AM, Derrick Stolee <stolee@gmail.com> wrote:

Show 29 quoted lines

>
> Teach git-commit-graph to inspect the objects only in a certain list
> of pack-indexes within the given pack directory. This allows updating
> the commit graph iteratively, since we add all commits stored in a
> previous commit graph.
>
> Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
> ---
>  Documentation/git-commit-graph.txt | 11 +++++++++++
>  builtin/commit-graph.c             | 32 +++++++++++++++++++++++++++++---
>  commit-graph.c                     | 26 ++++++++++++++++++++++++--
>  commit-graph.h                     |  4 +++-
>  packfile.c                         |  4 ++--
>  packfile.h                         |  2 ++
>  t/t5318-commit-graph.sh            | 16 ++++++++++++++++
>  7 files changed, 87 insertions(+), 8 deletions(-)
>
> diff --git a/Documentation/git-commit-graph.txt b/Documentation/git-commit-graph.txt
> index b9b4031..93d50d1 100644
> --- a/Documentation/git-commit-graph.txt
> +++ b/Documentation/git-commit-graph.txt
> @@ -42,6 +42,10 @@ With the `--delete-expired` option, delete the graph files in the pack
>  directory that are not referred to by the graph-latest file. To avoid race
>  conditions, do not delete the file previously referred to by the
>  graph-latest file if it is updated by the `--set-latest` option.
> ++
> +With the `--stdin-packs` option, generate the new commit graph by
> +walking objects only in the specified packfiles and any commits in
> +the existing graph-head.

A general question on this series: How do commit graph buildups deal with garbage collected commits? (my personal workflow is heavy on rebase, which generates lots of dangling commits, to be thrown out later)

The second half of the sentence makes it sound like once a commit is in the graph it cannot be pulled out easily again, hence the question on the impact of graphs on a long living repository which is garbage collected frequently.

AFAICT you could just run
    git commit-graph write --set-latest [--delete-expired]
as that actually looks up objects from outside the existing graph files,
such that lost objects are ignored?

> +       const char **lines = NULL;
> +       int nr_lines = 0;
> +       int alloc_lines = 0;

(nit:) I had the impression that these triplet-variables, that are used in ALLOC_GROW are allo X, X_nr and X_allow, but I might be wrong.

Show 9 quoted lines

> @@ -170,7 +178,25 @@ static int graph_write(int argc, const char **argv)
>
>         old_graph_name = get_graph_latest_contents(opts.obj_dir);
>
> -       graph_name = write_commit_graph(opts.obj_dir);
> +       if (opts.stdin_packs) {
> +               struct strbuf buf = STRBUF_INIT;
> +               nr_lines = 0;
> +               alloc_lines = 128;

alloc_lines has been initialized before, so why redo it here again? Also what is the rationale for choosing 128 as a good default? I would guess 0 is just as fine, because ALLOC_GROW makes sure that it growth fast in the first couple entries by having an additional offset. (no need to fine tune the starting allocation IMHO)

Show 6 quoted lines

> +               ALLOC_ARRAY(lines, alloc_lines);
> +
> +               while (strbuf_getline(&buf, stdin) != EOF) {
> +                       ALLOC_GROW(lines, nr_lines + 1, alloc_lines);
> +                       lines[nr_lines++] = buf.buf;
> +                       strbuf_detach(&buf, NULL);

strbuf_detach returns its previous buf.buf, such that you can combine these
two lines as
    lines[nr_lines++] = strbuf_detach(&buf, NULL);

> +               }
> +
> +               pack_indexes = lines;
> +               nr_packs = nr_lines;

Technically we do not need to strbuf_release(&buf) here, because strbuf_detach is always called, and by knowing its implementation, it is just as good.

Show 22 quoted lines

> @@ -579,7 +581,27 @@ char *write_commit_graph(const char *obj_dir)
>                 oids.alloc = 1024;
>         ALLOC_ARRAY(oids.list, oids.alloc);
>
> -       for_each_packed_object(if_packed_commit_add_to_list, &oids, 0);
> +       if (pack_indexes) {
> +               struct strbuf packname = STRBUF_INIT;
> +               int dirlen;
> +               strbuf_addf(&packname, "%s/pack/", obj_dir);
> +               dirlen = packname.len;
> +               for (i = 0; i < nr_packs; i++) {
> +                       struct packed_git *p;
> +                       strbuf_setlen(&packname, dirlen);
> +                       strbuf_addstr(&packname, pack_indexes[i]);
> +                       p = add_packed_git(packname.buf, packname.len, 1);
> +                       if (!p)
> +                               die("error adding pack %s", packname.buf);
> +                       if (open_pack_index(p))
> +                               die("error opening index for %s", packname.buf);
> +                       for_each_object_in_pack(p, if_packed_commit_add_to_list, &oids);
> +                       close_pack(p);
> +               }

strbuf_release(&packname);

> +       }
> +       else

(micro style nit)

    } else