[fix #9] trying to send on closed connection

client.go

@@ -63,6 +63,9 @@ func (c *Client) Err() <-chan error {
 // network errors if any write away. May block
 // until network timeout
 func (c *Client) MustSend(msg []byte) error {
+	if c.conn == nil {
+		return fmt.Errorf("%w: closed connection: failed to send message", ErrConn)
+	}
 	if err := c.setWTimeout(); err != nil {
 		return fmt.Errorf("%w: failed to set net timeout: %q", ErrConn, err)
 	}

client_test.go

@@ -203,6 +203,14 @@ func TestClientWriteToConnection(t *testing.T) {
 		err := cl.MustSend([]byte("must send\n"))
 		assert.ErrorContains(t, err, "io: read/write on closed")
 	})
+
+	t.Run("MustSend returns error when client is closed", func(t *testing.T) {
+		conn, _ := net.Pipe()
+		client := makeClient(conn)
+		client.Close()
+		err := client.MustSend([]byte("must send\n"))
+		assert.ErrorContains(t, err, "closed connection")
+	})
 }
 
 func TestClientReadPartialNetworkInput(t *testing.T) {