Re: [PATCH 1/2] fetch-pack: use commit-graph when computing cutoff

[Taylor Blau <me@ttaylorr.com>]

(+cc Stolee, in case anything I'm saying here is wrong)

On Fri, Jan 28, 2022 at 11:17:03AM +0100, Patrick Steinhardt wrote:
> One thing to keep in mind though is that the commit-graph corrects
> committer dates:
>
>     * A commit with at least one parent has corrected committer date
>       equal to the maximum of its commiter date and one more than the
>       largest corrected committer date among its parents.

This snippet refers to how correct committer dates are computed, not how
the commit dates themselves are stored.

Indeed, the corrected committer date is used to compute the corrected
commit date offset, which is the "v2" generation number scheme (as
opposed to topological levels, which make up "v1").

But that is entirely separate from the committer dates stored by the
commit-graph file, which are faithful representations of the exact
committer date attached to each commit.

Looking at the very last few lines of the main loop in
write_graph_chunk_data() (where the committer dates are stored):

    if (sizeof((*list)->date) > 4)
      packedDate[0] = htonl(((*list)->date >> 32) & 0x3);
    else
      packedDate[0] = 0;

    packedDate[0] |= htonl(*topo_level_slab_at(ctx->topo_levels, *list) << 2);
    packedDate[1] = htonl((*list)->date);
    hashwrite(f, packedDate, 8);

the low-order 34 bits are used to store the commit's `->date` field, and
the remaining high-order 30 bits are used to store the generation
number. (You can look in `fill_commit_graph_info()` to see that we only
use those 34 bits to write back the date field).

So I think this paragraph (and the ones related to it) about this being
an approximation and that being OK since this is a heuristic can all go
away.

Thanks,
Taylor