[client] pqkem: resend the in-flight KEM offer during an offer glare

The offer exchange is installed before the Signal send, so a failed send
leaves it in flight with nothing delivered. Ignoring the responder's offer
then stalled the connection until the exchange timed out. Resending the
same offer delivers it; the responder answers a duplicate from its cache.

client/internal/peer/handshaker.go

@@ -207,17 +207,21 @@ func (h *Handshaker) handleRemoteAnswer(remoteOfferAnswer OfferAnswer) {
 // The re-offer reuses our stable ICE session id, so the peer dedups repeats.
 //
 // Returns true when the caller must not answer the peer's offer: either a re-offer went
-// out, or our own KEM offer is already in flight (an offer glare). In the glare the
-// peer's credentials reach us in its answer to our offer, and a KEM-less answer from us
-// could let the peer bring WireGuard up before it reads our offer, on no PSK at all.
-// Otherwise, as for a non-capable peer, it returns false and the caller answers normally:
-// dropping the offer then would lose the only copy of the peer's ICE and relay details.
+// out, or our own KEM offer is already in flight (an offer glare). In the glare we send
+// that same offer again instead of answering: a KEM-less answer could let the peer bring
+// WireGuard up before it reads our offer, on no PSK at all. Resending covers an earlier
+// send that failed after the exchange was installed, and the peer answers a duplicate
+// from its cache. Otherwise, as for a non-capable peer, it returns false and the caller
+// answers normally: the peer's offer is then the only copy of its ICE and relay details.
 func (h *Handshaker) pqControllerReoffer() bool {
 	if h.config.PQ == nil || !isController(h.config) {
 		return false
 	}
 	if h.config.PQ.SignalOfferInFlight(h.config.Key) {
-		h.log.Debugf("pqkem: controller received a responder offer while its own KEM offer is in flight, leaving it unanswered")
+		h.log.Debugf("pqkem: controller received a responder offer while its own KEM offer is in flight, resending that offer instead of answering")
+		if err := h.sendOffer(); err != nil {
+			h.log.Errorf("failed to resend KEM offer during an offer glare: %s", err)
+		}
 		return true
 	}
 	if !h.config.PQ.ShouldSendBootstrapOffer(h.config.Key) {

client/internal/peer/handshaker_pq_test.go

@@ -64,18 +64,19 @@ func TestHandshaker_ControllerReoffersOnResponderOffer(t *testing.T) {
 		"the controller must reply with its own offer, not an answer")
 }
 
-// TestHandshaker_ControllerIgnoresResponderOfferInGlare covers an offer glare: the
-// controller's KEM offer and the responder's KEM-less offer cross. The controller must leave
-// the responder's offer unanswered. A KEM-less answer could reach the responder before the
-// KEM offer and let it bring WireGuard up with no PSK.
-func TestHandshaker_ControllerIgnoresResponderOfferInGlare(t *testing.T) {
+// TestHandshaker_ControllerResendsOfferInGlare covers an offer glare: the controller's KEM
+// offer and the responder's KEM-less offer cross. The controller must not answer: a
+// KEM-less answer could reach the responder before the KEM offer and let it bring WireGuard
+// up with no PSK. It sends its in-flight offer again instead, in case the first send failed.
+func TestHandshaker_ControllerResendsOfferInGlare(t *testing.T) {
 	h, sent := newControllerHandshaker(t, bootstrapPQ{inFlight: true})
 	notified := false
 	h.AddICEListener(func(*OfferAnswer) { notified = true }) // called synchronously
 
 	h.handleRemoteOffer(OfferAnswer{IceCredentials: IceCredentials{UFrag: "ufrag", Pwd: "pwd"}})
 
-	assert.Empty(t, sent(), "in a glare the controller must send neither an answer nor a second offer")
+	assert.Equal(t, []sProto.Body_Type{sProto.Body_OFFER}, sent(),
+		"in a glare the controller must resend its offer, not answer")
 	assert.False(t, notified, "in a glare the controller must not bring the connection up from the responder's offer")
 }