magicsock: invalidate endpoint on trust timeout (#19415)

Endpoint's best address was cleared on trustBestAddrUntil expiry
only if it was a udprelay connection. This generalizes invalidation
to also cover direct UDP.

Trust deadline is checked in two cases:

On disco ping timeout from the endpoint's best address.
Traffic goes DERP-only, heartbeats to the old address stop.
The discovery pings are still in flight, handled by the following.

On disco ping success from an alternative. BestAddr switches to the
working path, trust refreshed, eager discovery stops. The still
in flight pongs are handled by betterAddr().

Updates #19407


Change-Id: Ic41ed18edb4a6e4350a2d49271ba01566a6a6964

Signed-off-by: Alex Valiushko <alexvaliushko@tailscale.com>

wgengine/magicsock/endpoint.go

@@ -1183,8 +1183,7 @@ func (de *endpoint) discoPingTimeout(txid stun.TxID) {
 		return
 	}
 	bestUntrusted := mono.Now().After(de.trustBestAddrUntil)
-	if sp.to == de.bestAddr.epAddr && sp.to.vni.IsSet() && bestUntrusted {
-		// TODO(jwhited): consider applying this to direct UDP paths as well
+	if sp.to == de.bestAddr.epAddr && bestUntrusted {
 		de.clearBestAddrLocked()
 	}
 	if debugDisco() || !de.bestAddr.ap.IsValid() || bestUntrusted {
@@ -1778,12 +1777,8 @@ func (de *endpoint) handlePongConnLocked(m *disco.Pong, di *discoInfo, src epAdd
 			latency: latency,
 			wireMTU: pingSizeToPktLen(sp.size, sp.to),
 		}
-		// TODO(jwhited): consider checking de.trustBestAddrUntil as well. If
-		//  de.bestAddr is untrusted we may want to clear it, otherwise we could
-		//  get stuck with a forever untrusted bestAddr that blackholes, since
-		//  we don't clear direct UDP paths on disco ping timeout (see
-		//  discoPingTimeout).
-		if betterAddr(thisPong, de.bestAddr) {
+		bestUntrusted := now.After(de.trustBestAddrUntil)
+		if betterAddr(thisPong, de.bestAddr) || bestUntrusted {
 			de.c.logf("magicsock: disco: node %v %v now using %v mtu=%v tx=%x", de.publicKey.ShortString(), de.discoShort(), sp.to, thisPong.wireMTU, m.TxID[:6])
 			de.debugUpdates.Add(EndpointChange{
 				When: time.Now(),