Re: [Bug] In `git-rev-list(1)`, using the `--objects` flag doesn't work well with the `--not` flag, as non-commit objects are not excluded

[Taylor Blau <me@ttaylorr.com>]

On Tue, Aug 15, 2023 at 12:31:37PM -0700, Junio C Hamano wrote:
> Karthik Nayak <karthik.188@gmail.com> writes:
>
> > If you notice here, the objects
> > `8baef1b4abc478178b004d62031cf7fe6db6f903`,
> > `086885f71429e3599c8c903b0e9ed491f6522879` and
> > `7a67abed5f99fdd3ee203dd137b9818d88b1bafd` are included in the output,
> > these objects are reachable from
> > `91fa9611a355db77a07f963c746d57f75af380da` and shouldn't have been
> > included since we used the `--not` flag.
>
> For performance reasons, we cannot afford to enumerate all objects
> that are reachable from --not objects and exclude them from the
> output.  So it is a balancing act to decide where to draw the line.

As a hack, you can exploit the existing bitmap traversal routine to
build up an exact mapping of what is and isn't on either side of your
reachability query.

IOW, if you run:

    git repack -ad --write-bitmap-index

and then repeat the rev-list query with `--use-bitmap-index`, you should
get exact results.

Note that this will only work if pack.useBitmapBoundaryTraversal is set
to false, since the boundary-based traversal that is behind that
configuration option is susceptible to the same one-sided error.

Thanks,
Taylor