Re: Two issues with mark_reachable_objects

[Jeff King <peff@peff.net>]

On Tue, Apr 27, 2021 at 10:41:18AM -0400, Jeff King wrote:

> I think we'd just want to run the whole mark_recent block after doing
> the bitmap traversal.
> 
> There may be some subtlety with reusing the rev_info struct again. I
> think we'd want to reset the pending objects list after calling into the
> bitmap code. It _usually_ does an actual traversal that consumes the
> list, but not necessarily. I think traverse_bitmap_commit_list()
> probably ought to be the one to do it, so it behaves more like
> traverse_commit_list(). (OTOH, I don't think it's _too_ bad if we don't;
> we'd include those already-seen objects in our traversal, but they
> should all by definition have the SEEN bit set, so we'd stop there).

Nope, I was wrong here. It's actually prepare_bitmap_walk() which would
want to clear the pending list, and it does so (it may later re-add
objects in find_objects(), but if it does so, it will definitely
traverse and consume them).

> It's possible that we could do the second mark_recent traversal also
> with bitmaps (but still separately). I can't offhand think of a reason
> that ignore_missing_links wouldn't behave well there. But since we
> expect it to be small, I'd be more comfortable just using the regular
> traversal code.

I poked at this a bit, and indeed, the bitmap code is not ready to
handle the caller passing ignore_missing_links (it performs two separate
traversals for the wanted and uninteresting objects, and manipulates
ignore_missing_links itself between the two). It would probably be easy
to change, but I think we should focus on the minimal fix for the bug
you found first.

-Peff