add regression test

Author: dfalbel

tests/test_start_kernel.py

@@ -50,6 +50,36 @@ def test_ipython_start_kernel_userns():
         assert EXPECTED in text
 
 
+def test_start_kernel_background_thread():
+
+    cmd = dedent(
+        """
+        import threading
+        from ipykernel.kernelapp import launch_new_instance
+
+        def launch():
+            launch_new_instance()
+        
+        thread = threading.Thread(target=launch)
+        thread.start()
+        thread.join()
+        """
+    )
+
+    with setup_kernel(cmd) as client:
+        client.execute("a = 1")
+        msg = client.get_shell_msg(timeout=TIMEOUT)
+        content = msg["content"]
+        assert content["status"] == "ok"
+        
+        client.inspect("a")
+        msg = client.get_shell_msg(timeout=TIMEOUT)
+        content = msg["content"]
+        assert content["found"]
+        text = content["data"]["text/plain"]
+        assert "1" in text
+    
+
 @pytest.mark.flaky(max_runs=3)
 def test_ipython_start_kernel_no_userns():
     # Issue #4188 - user_ns should be passed to shell as None, not {}