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

[Jeff King <peff@peff.net>]

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.

-Peff