midx: don't reuse corrupt MIDXs when writing

midx: don't reuse corrupt MIDXs when writing

When writing a new multi-pack index, Git tries to reuse as much of the
data from an existing MIDX as possible, like object offsets. This is
done to avoid re-opening a bunch of *.idx files unnecessarily, but can
lead to problems if the data we are reusing is corrupt.

That's because we'll blindly reuse data from an existing MIDX without
checking its trailing checksum for validity. So if there is memory
corruption while writing a MIDX, or disk corruption in the intervening
period between writing and reuse, we'll blindly propagate those bad
values forward.

Suppose we experience a memory corruption while writing a MIDX such that
we write an incorrect object offset (or alternatively, the disk corrupts
the data after being written, but before being reused). Then when we go
to write a new MIDX, we'll reuse the bad object offset without checking
its validity. This means that the MIDX we just wrote is broken, but its
trailing checksum is in-tact, since we never bothered to look at the
values before writing.

In the above, a "git multi-pack-index verify" would have caught the
problem before writing, but writing a new MIDX wouldn't have noticed
anything wrong, blindly carrying forward the corrupt offset.

Individual pack indexes check their validity by verifying the crc32
attached to each entry when carrying data forward during a repack.
We could solve this problem for MIDXs in the same way, but individual
crc32's don't make much sense, since their entries are so small.
Likewise, checking the whole file on every read may be prohibitively
expensive if a repository has a lot of objects, packs, or both.

But we can check the trailing checksum when reusing an existing MIDX
when writing a new one. And a corrupt MIDX need not stop us from writing
a new one, since we can just avoid reusing the existing one at all and
pretend as if we are writing a new MIDX from scratch.

Suggested-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>