avoid deadlock in lighthouse queryWorker (#1112)

* avoid deadlock in lighthouse queryWorker

If the lighthouse queryWorker tries to grab to call StartHandshake on
a lighthouse vpnIp, we can deadlock on the handshake_manager lock. This
change drops the handshake_manager lock before we send on the lighthouse
queryChan (which could block), and also avoids sending to the channel if
this is a lighthouse IP itself.

* need to hold lock during cacheCb

handshake_manager.go

@@ -374,13 +374,13 @@ func (hm *HandshakeManager) GetOrHandshake(vpnIp iputil.VpnIp, cacheCb func(*Han
 // StartHandshake will ensure a handshake is currently being attempted for the provided vpn ip
 func (hm *HandshakeManager) StartHandshake(vpnIp iputil.VpnIp, cacheCb func(*HandshakeHostInfo)) *HostInfo {
 	hm.Lock()
-	defer hm.Unlock()
 
 	if hh, ok := hm.vpnIps[vpnIp]; ok {
 		// We are already trying to handshake with this vpn ip
 		if cacheCb != nil {
 			cacheCb(hh)
 		}
+		hm.Unlock()
 		return hh.hostinfo
 	}
 
@@ -421,6 +421,7 @@ func (hm *HandshakeManager) StartHandshake(vpnIp iputil.VpnIp, cacheCb func(*Han
 		}
 	}
 
+	hm.Unlock()
 	hm.lightHouse.QueryServer(vpnIp)
 	return hostinfo
 }