have createSession fill the sessionInfo.cancel field, have Conn use Conn.session.cancel instead of storing its own cancellation, this should prevent any of these things from being both nil and reachable at the same time

src/yggdrasil/session.go

@@ -208,6 +208,7 @@ func (ss *sessions) createSession(theirPermKey *crypto.BoxPubKey) *sessionInfo {
 	sinfo.pingTime = now
 	sinfo.pingSend = now
 	sinfo.init = make(chan struct{})
+	sinfo.cancel = util.NewCancellation()
 	higher := false
 	for idx := range ss.core.boxPub {
 		if ss.core.boxPub[idx] > sinfo.theirPermPub[idx] {
@@ -232,6 +233,11 @@ func (ss *sessions) createSession(theirPermKey *crypto.BoxPubKey) *sessionInfo {
 	sinfo.send = make(chan []byte, 32)
 	ss.sinfos[sinfo.myHandle] = &sinfo
 	ss.byTheirPerm[sinfo.theirPermPub] = &sinfo.myHandle
+	go func() {
+		// Run cleanup when the session is canceled
+		<-sinfo.cancel.Finished()
+		sinfo.core.router.doAdmin(sinfo.close)
+	}()
 	return &sinfo
 }
 
@@ -362,7 +368,7 @@ func (ss *sessions) handlePing(ping *sessionPing) {
 			for i := range conn.nodeMask {
 				conn.nodeMask[i] = 0xFF
 			}
-			conn.session.startWorkers(conn.cancel)
+			conn.session.startWorkers()
 			ss.listener.conn <- conn
 		}
 		ss.listenerMutex.Unlock()
@@ -431,8 +437,7 @@ func (ss *sessions) reset() {
 //////////////////////////// Worker Functions Below ////////////////////////////
 ////////////////////////////////////////////////////////////////////////////////
 
-func (sinfo *sessionInfo) startWorkers(cancel util.Cancellation) {
-	sinfo.cancel = cancel
+func (sinfo *sessionInfo) startWorkers() {
 	go sinfo.recvWorker()
 	go sinfo.sendWorker()
 }