Rework threading client

[danielballan]

Notes to self. Implementation in progress.

1. The user asks for one or more PVs, providing name and priority.
2. The Context:
   • Have I already got a PV instance for this? If so, return that. If not, allocate a cid and instantiate a corresponding PV object.
   • For all the new PV objects just created above, send [(name, cid), (name2, cid2)] to the Broadcaster to search.
   • Cache the cid -> PV mapping and return a list of PV objects, some or all of which may not be connected yet.
3. The Broadcaster:
   • Do I have an unexpired search result for this? If so, send (address, (cid1, cid2, ...)) to the Context. If not, send a SearchRequest using the cid provided by the Context.
   • In the recv loop, send (address, (cid1, cid2, ...)) to the Context as updates come in. (I think that all Contexts are current notified, which is a problem because they have different cids.... this could be solved using a queue per context, as we do in ChannelData.)
4. The Context:
   • Receives (address, (cid1, cid2, ...)) from the Broadcaster in its channel creation queue.
   • Looks up the cid in its cache to find the PV, which knows the priority. Checks: do I have a VirtualCircuitManager for this (address, prority) yet? If not, create a socket and a VirtualCircuitManager.
   • Creates a caproto.ClientChanel. (There is not need for a special Channel class.)
   • Tacks a reference to this caproto.ClientChannel and to the VirtualCircuitManager onto the PV.
   • Sends creation messages via VirualCircuitManager.send.
5. The VirtualCircuitManager:
   • Receives channel creation confirmation.
   • Looks up cid on the VirtualCircuitManager. Tells the PV it is connected via updating a connected attribute and possibly setting an Event, to make it possible to wait on PV connection.

[danielballan]

Closed by #161