pex: dial seeds when address book needs more addresses (#3603)

If we are low on addresses for peering, we need to discover more peers. The
previous behavior would query existing peers; however, if an existing peer
does not participate in peer exchange, then our node will not discover more peers.

This change consults both existing peers as well as seeds when there is a deficit
in address book addresses. This allows for discovering peers though existing channels
as well as via seeds if existing peers do not share addresses.

CHANGELOG_PENDING.md

@@ -27,3 +27,4 @@
 - [p2p] \#3532 limit the number of attempts to connect to a peer in seed mode
   to 16 (as a result, the node will stop retrying after a 35 hours time window)
 - [consensus] \#2723, \#3451 and \#3317 Fix non-deterministic tests
+- [pex] \#3603 Dial seeds when addrbook needs more addresses (@defunctzombie)

p2p/pex/pex_reactor.go

@@ -493,9 +493,10 @@ func (r *PEXReactor) ensurePeers() {
 		}
 	}
 
-	// If we are not connected to nor dialing anybody, fallback to dialing a seed.
-	if out+in+dial+len(toDial) == 0 {
-		r.Logger.Info("No addresses to dial nor connected peers. Falling back to seeds")
+	// If we are not dialing anyone and need more addresses - dial a seed
+	// This is done in addition to asking a peer for addresses to work-around peers not participating in PEX
+	if r.book.NeedMoreAddrs() && len(toDial) == 0 {
+		r.Logger.Info("No addresses to dial. Falling back to seeds")
 		r.dialSeeds()
 	}
 }