fix: avoid panic in txsubmission (#784)

We avoid the potential for panic due to send on closed channel by not
closing the channels. We instead use a select block with DoneChan to
detect shutdown

protocol/txsubmission/server.go

@@ -76,12 +76,6 @@ func (s *Server) Start() {
 				"connection_id", s.callbackContext.ConnectionId.String(),
 			)
 		s.Protocol.Start()
-		// Start goroutine to cleanup resources on protocol shutdown
-		go func() {
-			<-s.Protocol.DoneChan()
-			close(s.requestTxIdsResultChan)
-			close(s.requestTxsResultChan)
-		}()
 	})
 }
 
@@ -103,13 +97,14 @@ func (s *Server) RequestTxIds(
 		return nil, err
 	}
 	// Wait for result
-	txIds, ok := <-s.requestTxIdsResultChan
-	if !ok {
+	select {
+	case <-s.DoneChan():
 		return nil, protocol.ProtocolShuttingDownError
+	case txIds := <-s.requestTxIdsResultChan:
+		// Update ack count for next call
+		s.ackCount = len(txIds)
+		return txIds, nil
 	}
-	// Update ack count for next call
-	s.ackCount = len(txIds)
-	return txIds, nil
 }
 
 // RequestTxs requests the content of the requested TX identifiers from the remote node's mempool
@@ -127,11 +122,12 @@ func (s *Server) RequestTxs(txIds []TxId) ([]TxBody, error) {
 		return nil, err
 	}
 	// Wait for result
-	txs, ok := <-s.requestTxsResultChan
-	if !ok {
+	select {
+	case <-s.DoneChan():
 		return nil, protocol.ProtocolShuttingDownError
+	case txs := <-s.requestTxsResultChan:
+		return txs, nil
 	}
-	return txs, nil
 }
 
 func (s *Server) messageHandler(msg protocol.Message) error {
@@ -163,12 +159,6 @@ func (s *Server) handleReplyTxIds(msg protocol.Message) error {
 			"role", "server",
 			"connection_id", s.callbackContext.ConnectionId.String(),
 		)
-	// Check for shutdown
-	select {
-	case <-s.Protocol.DoneChan():
-		return protocol.ProtocolShuttingDownError
-	default:
-	}
 	msgReplyTxIds := msg.(*MsgReplyTxIds)
 	s.requestTxIdsResultChan <- msgReplyTxIds.TxIds
 	return nil
@@ -182,12 +172,6 @@ func (s *Server) handleReplyTxs(msg protocol.Message) error {
 			"role", "server",
 			"connection_id", s.callbackContext.ConnectionId.String(),
 		)
-	// Check for shutdown
-	select {
-	case <-s.Protocol.DoneChan():
-		return protocol.ProtocolShuttingDownError
-	default:
-	}
 	msgReplyTxs := msg.(*MsgReplyTxs)
 	s.requestTxsResultChan <- msgReplyTxs.Txs
 	return nil