bpo-31764: Prevent a crash in sqlite3.Cursor.close() in case the Cursor object is uninitialized (GH-3958) (#4303)

(cherry picked from commit edb13ae)

Author: miss-islington

Lib/sqlite3/test/regression.py

@@ -190,6 +190,9 @@ def __init__(self, con):
         cur = Cursor(con)
         with self.assertRaises(sqlite.ProgrammingError):
             cur.execute("select 4+5").fetchall()
+        with self.assertRaisesRegex(sqlite.ProgrammingError,
+                                    r'^Base Cursor\.__init__ not called\.$'):
+            cur.close()
 
     def CheckStrSubclass(self):
         """

Misc/NEWS.d/next/Library/2017-10-11-22-18-04.bpo-31764.EMyIkK.rst

@@ -0,0 +1,2 @@
+Prevent a crash in ``sqlite3.Cursor.close()`` in case the ``Cursor`` object is
+uninitialized. Patch by Oren Milman.

Modules/_sqlite/cursor.c

@@ -916,6 +916,11 @@ PyObject* pysqlite_noop(pysqlite_Connection* self, PyObject* args)
 
 PyObject* pysqlite_cursor_close(pysqlite_Cursor* self, PyObject* args)
 {
+    if (!self->connection) {
+        PyErr_SetString(pysqlite_ProgrammingError,
+                        "Base Cursor.__init__ not called.");
+        return NULL;
+    }
     if (!pysqlite_check_thread(self->connection) || !pysqlite_check_connection(self->connection)) {
         return NULL;
     }