Re: [PATCH] test_bitmap_hashes(): handle repository without bitmaps

[Taylor Blau <me@ttaylorr.com>]

On Fri, Nov 05, 2021 at 11:52:16AM -0700, Junio C Hamano wrote:
> Jeff King <peff@peff.net> writes:
>
> > If prepare_bitmap_git() returns NULL (one easy-to-trigger cause being
> > that the repository does not have bitmaps at all), then we'll segfault
> > accessing bitmap_git->hashes:
> >
> >   $ t/helper/test-tool bitmap dump-hashes
> >   Segmentation fault
> >
> > We should treat this the same as a repository with bitmaps but no
> > name-hashes, and quietly produce an empty output. The later call to
> > free_bitmap_index() in the cleanup label is OK, as it treats a NULL
> > pointer as a noop.
> >
> > This isn't a big deal in practice, as this function is intended for and
> > used only by test-tool. It's probably worth fixing to avoid confusion,
> > but not worth adding coverage for this to the test suite.
> >
> > Signed-off-by: Jeff King <peff@peff.net>
> > ---
> > This is new in the v2.34.0 cycle, but it's so low impact it doesn't
> > matter much if we ship with the bug. OTOH, it's pretty low-risk since it
> > is only run by the test suite.
>
> ;-)

Yes, this looks obviously correct to me. Thanks for spotting and fixing
this, Peff.

I'd be happy to see it in the 2.34 cycle, too, but I agree that it would
be OK if it didn't make the cut (and certainly if it makes it easier for
Junio to handle the rest of the release cycle, then I'm in favor of
leaving it out).

> I wonder how you found it.  Diagnosing a repository that did not
> seem healthy?  What I am getting at is if we want a new option to
> make a plumbing command, other than the test-tool, that calls this
> function, as the latter is usually not deployed in the field.

I would not be surprised if this was discovered via Coverity, or by
manual inspection. Peff and I have been merging a slew of releases from
your tree into GitHub's fork and so have been reading code in the more
recently changed areas.

On the test-tool vs. plumbing thing: I think there are some compelling
reasons in either direction. There's no *good* home for these in our
current set of plumbing tools. E.g., the closest example we have is `git
rev-list --test-bitmap <rev>`, which is kind of ugly. When we needed
these new inspection tools for some of the newer bitmap-related tests,
adding them via the test-helper suite was a conscious choice to not
build on the ugliness of `--test-bitmap`.

But on occasion these test-tool things are useful to have "in the
field", as you say. It's rare enough that I usually just clone a copy of
our fork as needed and build it when I do find myself reaching for
test-helpers.

Thanks,
Taylor