test: cover BaseSession reentry guard

Author: Epochex

tests/client/test_session.py

@@ -127,6 +127,26 @@ async def test_client_session_requires_context_manager():
             await session.initialize()
 
 
+@pytest.mark.anyio
+async def test_client_session_reentry_raises_runtime_error():
+    client_to_server_send, client_to_server_receive = anyio.create_memory_object_stream[SessionMessage](1)
+    server_to_client_send, server_to_client_receive = anyio.create_memory_object_stream[SessionMessage](1)
+
+    async with (
+        client_to_server_send,
+        client_to_server_receive,
+        server_to_client_send,
+        server_to_client_receive,
+    ):
+        session = ClientSession(server_to_client_receive, client_to_server_send)
+        await session.__aenter__()
+        try:
+            with pytest.raises(RuntimeError, match="already running"):
+                await session.__aenter__()
+        finally:
+            await session.__aexit__(None, None, None)
+
+
 @pytest.mark.anyio
 async def test_client_session_custom_client_info():
     client_to_server_send, client_to_server_receive = anyio.create_memory_object_stream[SessionMessage](1)