fix server panic trying to send on stream as client disconnects #1111 (#1115)

transport/handler_server.go

@@ -179,12 +179,19 @@ func (a strAddr) String() string { return string(a) }
 
 // do runs fn in the ServeHTTP goroutine.
 func (ht *serverHandlerTransport) do(fn func()) error {
+	// Avoid a panic writing to closed channel. Imperfect but maybe good enough.
+	select {
+	case <-ht.closedCh:
+		return ErrConnClosing
+	default:
 		select {
 		case ht.writes <- fn:
 			return nil
 		case <-ht.closedCh:
 			return ErrConnClosing
 		}
+
+	}
 }
 
 func (ht *serverHandlerTransport) WriteStatus(s *Stream, st *status.Status) error {