The way [[XMPP pairing|design/assistant/XMPP]] currently works, each
separate repository needs to use a different XMPP account. If two
repositories use the same XMPP account, then they will be combined together
when XMPP pairing takes place.

There are two different UIs for XMPP pairing. While the same protocol
is running behind the scenes, these UIs should be considered separately.

## Share with your other devices

Here, I think it makes sense to require the user to use the same XMPP
account on all their devices (otherwise it's pairing with a friend), and
automatically combine repositories of devices that use the same XMPP
account.

The UI is pretty clear about this:

	If you have multiple devices, all running git-annex, and using #
	your Jabber account #{account}, you can configure them to share #
	your files between themselves.

Doing it this way avoids needing to confirm pair requests coming from the same
XMPP account. Which means that, for example, you can have a device at home,
and one at work, and pair them by simply initiating a pair request from one
to the other. You don't have to travel between home and work to confirm 
the request.

(Also, when you have a lot of devices, this avoids a combinatorial explosion
of pair request confirmations.)

The only problem with this is that users who want multiple repositories
need to find multiple XMPP accounts. However, I'm inclined to think this is
a reasonable requirement.

## Share with a friend

Suppose that Alice wants to share with Bob. Bob is using the same XMPP
account for two separate repositories (B1 and B2), that are not
themselves paired.

When Alice chooses to share with Bob, a XMPP pair request is sent.
Both of Bob's repositories see it, and both ask him to confirm.
Bob can choose to only confirm the request in B1, and 
not in B2. This should work ok.

* The UI for this only says "Pair request received from #{name}",
  it does not indicate which repository of Alice's is being paired
  with. This could be improved. If Alice has two repositories as well,
  she and Bob will want to coordinate pairing the right ones together.
  Could be fixed by just displaying the description of Alice's repository
  to Bob.

-----

Now, suppose that Alice makes a second, distinct repository (A2),
and chooses to share it with Bob (intending to share with B2). This
sends an XMPP pair request to both of Bob's repositories.

B1 has already paired with Alice, so it assumes this
new pair request is from a different device belonging to Alice, and it
automatically ACKs the pair request. 

The result is that Alice's new A2 repository combines with B1,
which is already combined with A1. Effectively combining all of A1, A2, B1,
and B2, unexpectedly. **This is a bug**.

Some possible fixes:

1. Stop auto-accepting pair requests
   from friends we're already paired with. Require another confirmation.
2. Or, only auto-accept pair requests from friends we're already paired with
   when they come from a repository whose UUID we already know. This
   enhancment to fix #1 makes it easier to build more robust networks of
   repositories. **done**

Hmm, I don't think those fixes are sufficient. Suppose they're in place.
Then when Alice shares A2 with Bob, both his repositories will ask him to
confirm the pair request. He confirms in B2, and pairing proceeds.

But, XMPP git push is broadcast to all clients of an XMPP account.
So when B2 sends a push, A1 sees it, and happily merges away. The
repositories still combine!

So, we need another fix:

* Send UUID in XMPP git push protocol messages. Only respond to git push
  messages from a known UUID, and ignore all others. (XMPP pairing
  already sends the UUID, so it will be known.) **done**

----

* Alternatively, we could say that the problem is that Bob has two 
  distinct repositories using the same XMPP account, and try to prevent
  him from doing that in the first place. 
   
  One way to do this would be, when configuring the
  XMPP account, scan for other repositories using the same account, and
  don't let it be used unless the user confirms they want to pair them.
  But, this doesn't seem viable because if the other repository is on
  another device, which is turned off, this check wouldn't see it.

  Or there could be a warning about account reuse. Doesn't seem likely to
  be effective.

-----

> [[done]]. I've put in the fixes around pairing with friends. --[[Joey]]
