Ping remote candidate for sped when pair is selected (#3)

* Ping remote caniddate for sped when pair is selected

Ping to send piggyback dtls packet immediately after
candidate pair is nominated to send pending packets

* fix test

Author: cnderrauber

agent.go

@@ -816,6 +816,18 @@ func (a *Agent) setSelectedPair(pair *CandidatePair) {
 
 	// Notify when the selected candidate pair changes
 	a.selectedCandidatePairNotifier.EnqueueSelectedCandidatePair(pair)
+
+	// If a SPED (DTLS-in-STUN) handshake is still in flight when the pair is
+	// nominated, drive a piggybacked ping on the freshly-selected pair *now*
+	// rather than waiting for the next scheduled connectivity check. A local
+	// DTLS flight (e.g. the DTLS-client Finished on the publisher / ice-lite
+	// answerer path) that became ready just before nomination would otherwise
+	// sit un-carried until the next check tick and miss the DTLS retransmit
+	// window (~1 RTT / ~100-140ms stall). Sending it on the nominated pair
+	// immediately gets it acked within ~1 RTT. Runs on the agent task loop.
+	if a.piggybackActive() {
+		a.pingSelectedPairForPiggyback()
+	}
 }
 
 func (a *Agent) pingAllCandidates() {

selection_test.go

@@ -262,7 +262,7 @@ func newPingNoIOCand() *pingNoIOCand {
 func (d *pingNoIOCand) writeTo(b []byte, _ Candidate) (int, error) { return len(b), nil }
 
 func bareAgentForPing() *Agent {
-	return &Agent{
+	a := &Agent{
 		hostAcceptanceMinWait:  time.Hour,
 		srflxAcceptanceMinWait: time.Hour,
 		prflxAcceptanceMinWait: time.Hour,
@@ -288,6 +288,8 @@ func bareAgentForPing() *Agent {
 			candidatePairFunc: func(*CandidatePair) {},
 		}, //nolint formatting
 	}
+	a.piggyback.init()
+	return a
 }
 
 func bigStr() string { return strings.Repeat("x", 40000) }