gbn: Ensure we always handle Pong Tick events

If we have awaited a sync after sending the previous ping, both the
pingTicker and pongTicker may have ticked when waiting to sync.
In that case, we need to ensure that the pongTicker gets prioritized,
which this commit ensures.

Author: ViktorT-11

gbn/gbn_conn.go

@@ -413,11 +413,26 @@ func (g *GoBackNConn) sendPacketsForever() error {
 			continue
 
 		case <-g.pingTicker.Ticks():
+			// If we have expected a sync after sending the previous
+			// ping, both the pingTicker and pongTicker may have
+			// ticked when waiting to sync. In that case, we can't
+			// be sure which of the signals we receive over first in
+			// the above select. We therefore need to check if the
+			// pong ticker has ticked here to ensure that it get's
+			// prioritized over the ping ticker.
+			select {
+			case <-g.pongTicker.Ticks():
+				return errKeepaliveTimeout
+			default:
+			}
 
 			// Start the pong timer.
 			g.pongTicker.Reset()
 			g.pongTicker.Resume()
 
+			// Also reset the ping timer.
+			g.pingTicker.Reset()
+
 			g.log.Tracef("Sending a PING packet")
 
 			packet = &PacketData{