Re: [PATCH v2 0/8] midx: prevent bitmap corruption when permuting pack order

>> 3. Should the chunk be made mandatory?
>>
>> Unfortunately, the chunk format did not follow the index format's
>> example of making lowercase chunk IDs required. Instead, the chunks
>> that are necessary for v1 are necessary forever and all other chunks
>> are deemed optional. Changing this would require something more
>> drastic like updating the version number or giving some grace period
>> where released versions start treating lowercase chunk IDs as required
>> before creating a new "required" chunk.
>>
>> This does mean that if there is a version incompatibility, the RIDX
>> chunk will just be ignored by the older version of Git.
>>
>> In terms of making this a safe format upgrade, I think Taylor has
>> achieved that.
> 
> Thanks. And yeah, the chunk should (and is) mandatory when writing a
> MIDX bitmap. But if we ran `git multi-pack-index write` without
> `--bitmap`, then we would be free to not write the RIDX chunk (and
> indeed that is what we do).

>> The only thing I can think is that server operators might want to
>> deploy this version with GIT_TEST_MIDX_WRITE_REV=1 for a while, so
>> any need to downgrade would not suffer a performance penalty for a
>> missing .rev file. If that is a planned way to safely deploy this
>> change, then it might be worth adding a test that we safely delete
>> a .rev file after writing both a .rev file and a RIDX chunk. (The
>> RIDX chunk will be preferred, so maybe the previous .rev file hits
>> some logic that would skip its deletion?)
> 
> That logic (that we delete auxiliary files--including the .rev file--not
> matching the checksum of the MIDX we just wrote) is unchanged. So I
> think we should be good there since we have existing coverage.