[#749] morph: Fix panic when closing morph client

Signed-off-by: Anton Nikiforov <an.nikiforov@yadro.com>

pkg/morph/client/client.go

@@ -73,6 +73,8 @@ type Client struct {
 
 	// channel for internal stop
 	closeChan chan struct{}
+	closed    atomic.Bool
+	wg        sync.WaitGroup
 
 	// indicates that Client is not able to
 	// establish connection to any of the

pkg/morph/client/multi.go

@@ -75,6 +75,8 @@ func (c *Client) SwitchRPC(ctx context.Context) bool {
 }
 
 func (c *Client) closeWaiter(ctx context.Context) {
+	c.wg.Add(1)
+	defer c.wg.Done()
 	select {
 	case <-ctx.Done():
 	case <-c.closeChan:

pkg/morph/client/notifications.go

@@ -16,7 +16,10 @@ func (c *Client) Close() {
 	// closing should be done via the channel
 	// to prevent switching to another RPC node
 	// in the notification loop
-	close(c.closeChan)
+	if c.closed.CompareAndSwap(false, true) {
+		close(c.closeChan)
+	}
+	c.wg.Wait()
 }
 
 // ReceiveExecutionNotifications performs subscription for notifications