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

[Derrick Stolee <stolee@gmail.com>]

On 12/20/2021 2:52 PM, Taylor Blau wrote:
> On Mon, Dec 20, 2021 at 01:51:22PM -0500, Derrick Stolee wrote:

>> 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).

I just want to be careful of the language here, with respect to how
chunks are listed as required or optional in
Documentation/technical/pack-format.txt. "Required" means every MIDX
needs one or is invalid. "Optional" means that all versions of Git
should ignore the chunk if it does not recognize the ID.

So here, the new RIDX chunk will be ignored by older versions and
will be written only when we care about bitmaps.

(Nothing I say here is in conflict with what you said, but I
anticipate confusion with your use of the word "mandatory".)

>> 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.

Sounds good.

Thanks,
-Stolee