Re: [PATCH v2] send-pack: never fetch when checking exclusions

[Derrick Stolee <stolee@gmail.com>]

On 10/11/2019 2:12 AM, Jeff King wrote:
> On Tue, Oct 08, 2019 at 11:37:39AM -0700, Jonathan Tan wrote:
> 
>> When building the packfile to be sent, send_pack() is given a list of
>> remote refs to be used as exclusions. For each ref, it first checks if
>> the ref exists locally, and if it does, passes it with a "^" prefix to
>> pack-objects. However, in a partial clone, the check may trigger a lazy
>> fetch.
>>
>> The additional commit ancestry information obtained during such fetches
>> may show that certain objects that would have been sent are already
>> known to the server, resulting in a smaller pack being sent. But this is
>> at the cost of fetching from many possibly unrelated refs, and the lazy
>> fetches do not help at all in the typical case where the client is
>> up-to-date with the upstream of the branch being pushed.
>>
>> Ensure that these lazy fetches do not occur.
> 
> That makes sense. For similar reasons, should we be using
> OBJECT_INFO_QUICK here? If the other side has a bunch of ref tips that
> we don't have, we'll end up re-scanning the pack directory over and over
> (which is _usually_ pretty quick, but can be slow if you have a lot of
> packs locally). And it's OK if we racily miss out on an exclusion due to
> somebody else repacking simultaneously.

That's a good idea. We can hint to the object store that we don't expect
misses to be due to a concurrent repack, so we don't want to reprepare
pack-files.

-Stolee