SSCursor: Fix connection closed check

Since the connection closes itself on an `asyncio.CancelledError`, it
cannot be used afterwards. `SSCursor.close` attempts to clear some
unfinished state first before closing using
`Connection._finish_unbuffered_query`, but that call fails if the
connection has been closed already and SSCursor itself does not
properly check that.

Author: FichteFoll

aiomysql/cursors.py

@@ -598,7 +598,8 @@ class SSCursor(Cursor):
 
     async def close(self):
         conn = self._connection
-        if conn is None:
+        if conn is None or conn.closed:
+            self._connection = None
             return
 
         if self._result is not None and self._result is conn._result:

tests/test_sscursor.py

@@ -65,6 +65,15 @@ async def test_ssursor(connection):
     await cursor.close()
 
 
+@pytest.mark.run_loop
+async def test_ssursor_conn_closed(connection):
+    conn = connection
+    cursor = await conn.cursor(SSCursor)
+
+    conn.close()
+    await cursor.close()
+
+
 @pytest.mark.run_loop
 async def test_sscursor_fetchall(connection):
     conn = connection