Gossip should not respond to GRAFT with GRAFT

As per chat with @vyzo we should not response with GRAFT when adding a peer to our mesh per its GRAFT request.

Probably should add some explicit note on that in the Gossip spec

Anton Nashatyrev, [03.12.20 13:49]
Sorry for off-topic, but would like to hear vyzo opinion on the question discussed in the neighbor channel. 

If Peer1 sends initial GRAFT to Peer2, should the Peer2 respond with GRAFT if it added Peer1 to its mesh?

Though I didn't fine any explicit statement on this in the spec, I think the Peer2 SHOULD respond with GRAFT in this case. Else the Peer1 would not be informed that it was added to the mesh of Peer2 

As the spec states: 

The GRAFT informs a peer that it has been added to the local router's mesh view for the included topic id.

vyzo, [03.12.20 13:53]
yeah no need to respond with graft

vyzo, [03.12.20 13:53]
it would be racey

Anton Nashatyrev, [03.12.20 13:58]
[In reply to vyzo]
Thanks for clarification! 
Is there any spec place which can help to clarify this?

vyzo, [03.12.20 13:59]
sure feel free to open pr!

Anton Nashatyrev, [03.12.20 14:06]
[In reply to vyzo]
Just make it clear: 

- MAY NOT respond with graft 
or 
- SHOULD NOT respond with graft

vyzo, [03.12.20 14:30]
SHOULD NOT

vyzo, [03.12.20 14:30]
if two peers graft each other concurrently they may both send graft

vyzo, [03.12.20 14:30]
and thats ok

Anton Nashatyrev, [03.12.20 14:33]
[In reply to vyzo]
Yep, though in this case you anyway would not send the second GRAFT. When you receive GRAFT the other peer would already be in your mesh and you have nothing to do

vyzo, [03.12.20 14:35]
right

Anton Nashatyrev, [03.12.20 14:42]
[In reply to vyzo]
I have actually mean the situation when you are sending GRAFT 'in response' to initial GRAFT. (that's actually not a response, but just an opposite notification).

Anton Nashatyrev, [03.12.20 14:43]
It still seems to me that sending the back GRAFT is more consistent behavior

vyzo, [03.12.20 15:20]
its not needed though

vyzo, [03.12.20 15:21]
and it would get in funny races with an immediate graft/ prune

Start with the GRAFT section of the linked GossipSub v1.0 specification and review its existing wording about mesh membership notifications. Update the specification to state that a peer SHOULD NOT send GRAFT in response to receiving GRAFT, while preserving the note about concurrent grafts, and confirm the wording is explicit.