Re: Alternates advertisement on GitHub

[Derrick Stolee <stolee@gmail.com>]

On 7/25/2019 11:18 PM, Taylor Blau wrote:
> Hi everybody,
> 
> Pushes to forks of git.git hosted on GitHub now advertise the tips of
> git.git as well as branches from your fork.
> 
> You may recall that Peff and I have sent a handful of patches to allow
> repositories to customize how they gather references to advertise from
> an alternate, and then to use those alternate tips as part of the
> connectivity check (in [1] and [2], respectively).

I'm glad to hear you deployed this so quickly after review!

> GitHub used to advertise '.have's on pushes to forked repositories, but
> hasn't done so since 2012. We aggregate data from all forks into a
> 'network.git', and expose the tips of each fork as:
> 
>   refs/remotes/<fork-id>/<refname>
> 
> Each fork lists the 'network.git' as its alternate, and thus the
> advertisement can get prohibitively large when there are many forks of a
> repository.
> 
> Michael Haggerty's work on packed refs makes finding references
> pertaining only to the root computationally efficient, and [1] makes it
> possible to filter down when computing the set of references to
> advertise. With [1], we can specify that computation exactly and only
> advertise branch tips from the root of a fork network.
> 
> We've been slowly rolling this out to a handful of repository networks,
> including forks of git.git hosted on GitHub. If you host your fork on
> GitHub, you shouldn't notice anything. Hopefully, pushes to your fork
> will result in smaller packfiles. In either case, nothing should break;
> if it does, please feel free to email me, or support@github.com.

I tested this by updating 'master' in derrickstolee/git to match gitster/git
and the pack was empty (ref update only). This makes fork management so much
simpler!

Thanks!
-Stolee