To perform change handoff to build systems, it would be ideal to represent diffs as virtual refs in hosted repositories.

• arc could upload changes by pushing to a virtual ref (like review) instead of by shipping JSON over Conduit.
• The repository would accept the change and redirect the write to some known internal ref (like phabricator/diff/12345) while making the client believe the write occurred cleanly. It could then lift the change out of the ref and into Differential if the ultimate destination was a diff.
• These refs would be hidden from the web UI and from git clone, so users wouldn't see them normally or have their workflows gummed up by a lot of internal glue.
• An alternate view of the repository used by build systems could see and interact with these refs.

For example, the goal is to enable workflows like these:

Create a new diff by pushing (see T5000):

$ git push origin diff
Creating a new diff:

  Diff URI: https://secure.phabricator.com/differential/diff/123/

(Also this should exit with 0 as an exit code.)

Create a new durable ref by pushing to some other ref, then get JSON back:

$ git push origin diff/json
{
  "id": 123,
  "remote-ref": "phabricator/diff/123",
  "blah1": "other information useful for arc",
  "blah2": "like we ran differential.creatediff"
}

Check out a remote and only see meaningful, human-facing refs:

$ git clone ssh://dweller@vault.phabricator.com/diffusion/X/x.git
$ cd x
$ git ls-remote origin
abcdef refs/heads/master

Add an alternate view of a remote and see "virtual" refs which are useful to a build system:

$ git remote add build-view ssh://dweller@vault.phabricator.com/diffusion/build/X/x.git
$ git ls-remote build-view
abcdef refs/heads/master
abcde1 phabricator/diff/123
abcde2 phabricator/diff/124
... <a million more entries> ...

Check out those refs:

$ git fetch build-view diff/123
(This actually works.)
$ git checkout build-view/diff/123
(This also works.)
$ run fancy builds
(This works if your code is in good shape.)

Enforce ref-based permissions checks:

$ git fetch build-view diff/456
You Shall Not Pass: You don't have permission to see this diff.

Users who can see this diff include: ...