Re: [PATCH 2/2] fetch: implement support for atomic reference updates

Patrick Steinhardt [off-list ref] writes:

Similar to `git push --atomic`, this commit thus introduces atomic
fetches. Instead of allocating one reference transaction per updated
reference, it causes us to only allocate a single transaction and commit
it as soon as all updates were received. If locking of any reference
fails, then we abort the complete transaction and don't update any
reference, which gives us an all-or-nothing fetch.

Note that this may not completely fix the first of above downsides, as
the consistent view also depends on the server-side. If the server
doesn't have a consistent view of its own references during the
reference negotiation phase, then the client would get the same
inconsistent view the server has. This is a separate problem though and,
if it actually exists, can be fixed at a later point.

With the help of the previous step, it is trivial to see that the
single transaction around store_updated_refs() would be sufficient
to achieve the atomicity.

But what about FETCH_HEAD?  Do we refrain from writing and/or appending
to it when there is any failure?

Thanks.