Re: [PATCH v2 09/24] midx: infer preferred pack when not given one

On Mon, Jun 21, 2021 at 06:25:21PM -0400, Taylor Blau wrote:

In 9218c6a40c (midx: allow marking a pack as preferred, 2021-03-30), the
multi-pack index code learned how to select a pack which all duplicate
objects are selected from. That is, if an object appears in multiple
packs, select the copy in the preferred pack before breaking ties
according to the other rules like pack mtime and readdir() order.

Not specifying a preferred pack can cause serious problems with
multi-pack reachability bitmaps, because these bitmaps rely on having at
least one pack from which all duplicates are selected. Not having such a
pack causes problems with the pack reuse code (e.g., like assuming that
a base object was sent from that pack via reuse when in fact the base
was selected from a different pack).

It might be helpful to use a more descriptive name for "pack reuse code"
here, since it's kind of vague for people who have not been actively
working on bitmaps.

I don't have a short name for that chunk of code, but maybe:

  ...causes problems with the code in pack-objects to reuse packs
  verbatim (e.g., that code assumes that a delta object in a chunk of
  pack sent verbatim will have its base object sent from the same pack).

So why does not marking a pack preferred cause problems here? The reason
is roughly as follows:

  - Ties are broken (when handling duplicate objects) by sorting
    according to midx_oid_compare(), which sorts objects by OID,
    preferred-ness, pack mtime, and finally pack ID (more on that
    later).

  - The psuedo pack-order (described in
    Documentation/technical/bitmap-format.txt) is computed by
    midx_pack_order(), and sorts by pack ID and pack offset, with
    preferred packs sorting first.

I think the .rev description in pack-format.txt may be a better
reference here.

  - But! Pack IDs come from incrementing the pack count in
    add_pack_to_midx(), which is a callback to
    for_each_file_in_pack_dir(), meaning that pack IDs are assigned in
    readdir() order.

When specifying a preferred pack, all of that works fine, because
duplicate objects are correctly resolved in favor of the copy in the
preferred pack, and the preferred pack sorts first in the object order.

"Sorting first" is critical, because the bitmap code relies on finding
out which pack holds the first object in the MIDX's pseudo pack-order to
determine which pack is preferred.

But if we didn't specify a preferred pack, and the pack which comes
first in readdir() order does not also have the lowest timestamp, then
it's possible that that pack (the one that sorts first in pseudo-pack
order, which the bitmap code will treat as the preferred one) did *not*
have all duplicate objects resolved in its favor, resulting in breakage.

The fix is simple: pick a (semi-arbitrary) preferred pack when none was
specified. This forces that pack to have duplicates resolved in its
favor, and (critically) to sort first in pseudo-pack order.
Unfortunately, testing this behavior portably isn't possible, since it
depends on readdir() order which isn't guaranteed by POSIX.

This explanation is rather confusing, but I'm not sure if we can do much
better. I followed all of it, because I was there when we found the bug
that this is fixing. And of course that happened _after_ we implemented
midx bitmaps and in particular adapted the verbatim reuse stuff in
pack-objects to make use of it.

I see why you'd want to float the fix up before then, so we don't ever
have the broken state. But it's hard to understand what bug this is
fixing, because the bug does not even exist yet at this point in
the series!

I dunno. Like I said, I was able to follow it, so maybe it is
sufficient. I'm just not sure others would be able to.

+
+		if (!found)
+			warning(_("unknown preferred pack: '%s'"),
+				preferred_pack_name);
+	} else if (ctx.nr && (flags & MIDX_WRITE_REV_INDEX)) {
+		time_t oldest = ctx.info[0].p->mtime;
+		ctx.preferred_pack_idx = 0;
+
+		if (packs_to_drop && packs_to_drop->nr)
+			BUG("cannot write a MIDX bitmap during expiration");

Likewise, this BUG() feels somewhat out-of-place. At this point in the
series, we don't have bitmaps yet. :)

I can live with that, though. And I don't want to make a lot of work by
trying to re-order this patch within the series. Mostly I want to make
sure that if somebody stumbles on this commit via git-log or in a
bisection, that they can make some sense of what it's trying to do.

-Peff