wgengine/magicsock: move trustBestAddrUntil forward on non-disco rx (#10274)

This is gated behind the silent disco control knob, which is still in
its infancy. Prior to this change disco pong reception was the only
event that could move trustBestAddrUntil forward, so even though we
weren't heartbeating, we would kick off discovery pings every
trustUDPAddrDuration and mirror to DERP.

Updates #540

Signed-off-by: Jordan Whited <jordan@tailscale.com>

wgengine/magicsock/endpoint.go

@@ -239,14 +239,22 @@ func (de *endpoint) initFakeUDPAddr() {
 func (de *endpoint) noteRecvActivity(ipp netip.AddrPort) {
 	now := mono.Now()
 
-	// TODO(raggi): this probably applies relatively equally well to disco
-	// managed endpoints, but that would be a less conservative change.
 	if de.isWireguardOnly {
 		de.mu.Lock()
 		de.bestAddr.AddrPort = ipp
 		de.bestAddrAt = now
 		de.trustBestAddrUntil = now.Add(5 * time.Second)
 		de.mu.Unlock()
+	} else {
+		// TODO(jwhited): subject to change as part of silent disco effort.
+		// Necessary when heartbeat is disabled for the endpoint, otherwise we
+		// kick off discovery disco pings every trustUDPAddrDuration and mirror
+		// to DERP.
+		de.mu.Lock()
+		if de.heartbeatDisabled && de.bestAddr.AddrPort == ipp {
+			de.trustBestAddrUntil = now.Add(trustUDPAddrDuration)
+		}
+		de.mu.Unlock()
 	}
 
 	elapsed := now.Sub(de.lastRecv.LoadAtomic())