python
@@ -27,6 +27,7 @@
 import functools
 
 from test import support
+from unittest.mock import patch
 from test.test_sqlite3.test_dbapi import memory_database, managed_connect, cx_limit
 
 
@@ -469,5 +470,43 @@ def test_executescript_step_through_select(self):
             self.assertEqual(steps, values)
 
 
+class RecursiveUseOfCursors(unittest.TestCase):
+    # GH-80254: sqlite3 should not segfault for recursive use of cursors.
+    msg = "Recursive use of cursors not allowed"
+
+    def setUp(self):
+        self.con = sqlite.connect(":memory:",
+                                  detect_types=sqlite.PARSE_COLNAMES)
+        self.cur = self.con.cursor()
+        self.cur.execute("create table test(x foo)")
+        self.cur.executemany("insert into test(x) values (?)",
+                             [("foo",), ("bar",)])
+
+    def tearDown(self):
+        self.cur.close()
+        self.con.close()
+
+    def test_recursive_cursor_init(self):
+        conv = lambda x: self.cur.__init__(self.con)
+        with patch.dict(sqlite.converters, {"INIT": conv}):
+            self.cur.execute(f'select x as "x [INIT]", x from test')
+            self.assertRaisesRegex(sqlite.ProgrammingError, self.msg,
+                                   self.cur.fetchall)
+
+    def test_recursive_cursor_close(self):
+        conv = lambda x: self.cur.close()
+        with patch.dict(sqlite.converters, {"CLOSE": conv}):
+            self.cur.execute(f'select x as "x [CLOSE]", x from test')
+            self.assertRaisesRegex(sqlite.ProgrammingError, self.msg,
+                                   self.cur.fetchall)
+
+    def test_recursive_cursor_iter(self):
+        conv = lambda x, l=[]: self.cur.fetchone() if l else l.append(None)
+        with patch.dict(sqlite.converters, {"ITER": conv}):
+            self.cur.execute(f'select x as "x [ITER]", x from test')
+            self.assertRaisesRegex(sqlite.ProgrammingError, self.msg,
+                                   self.cur.fetchall)
+
+
 if __name__ == "__main__":
     unittest.main()
diff --git a/Misc/NEWS.d/next/Library/2019-06-22-11-01-45.bpo-36073.ED8mB9.rst b/Misc/NEWS.d/next/Library/2019-06-22-11-01-45.bpo-36073.ED8mB9.rst
@@ -0,0 +1,2 @@
+Raise :exc:`~sqlite3.ProgrammingError` instead of segfaulting on recursive
+usage of cursors in :mod:`sqlite3` converters. Patch by Sergey Fedoseev.
@@ -29,6 +29,17 @@
 #include "clinic/cursor.c.h"
 #undef clinic_state
 
+static inline int
+check_cursor_locked(pysqlite_Cursor *cur)
+{
+    if (cur->locked) {
+        PyErr_SetString(cur->connection->ProgrammingError,
+                        "Recursive use of cursors not allowed.");
+        return 0;
+    }
+    return 1;
+}
+
 /*[clinic input]
 module _sqlite3
 class _sqlite3.Cursor "pysqlite_Cursor *" "clinic_state()->CursorType"
@@ -70,6 +81,10 @@ pysqlite_cursor_init_impl(pysqlite_Cursor *self,
                           pysqlite_Connection *connection)
 /*[clinic end generated code: output=ac59dce49a809ca8 input=23d4265b534989fb]*/
 {
+    if (!check_cursor_locked(self)) {
+        return -1;
+    }
+
     Py_INCREF(connection);
     Py_XSETREF(self->connection, connection);
     Py_CLEAR(self->statement);
@@ -447,13 +462,9 @@ static int check_cursor(pysqlite_Cursor* cur)
         return 0;
     }
 
-    if (cur->locked) {
-        PyErr_SetString(cur->connection->state->ProgrammingError,
-                        "Recursive use of cursors not allowed.");
-        return 0;
-    }
-
-    return pysqlite_check_thread(cur->connection) && pysqlite_check_connection(cur->connection);
+    return (pysqlite_check_thread(cur->connection)
+            && pysqlite_check_connection(cur->connection)
+            && check_cursor_locked(cur));
 }
 
 static int
@@ -847,7 +858,9 @@ pysqlite_cursor_iternext(pysqlite_Cursor *self)
         return NULL;
     }
 
+    self->locked = 1;  // GH-80254: Prevent recursive use of cursors.
     PyObject *row = _pysqlite_fetch_one_row(self);
+    self->locked = 0;
     if (row == NULL) {
         return NULL;
     }
@@ -1011,6 +1024,10 @@ static PyObject *
 pysqlite_cursor_close_impl(pysqlite_Cursor *self)
 /*[clinic end generated code: output=b6055e4ec6fe63b6 input=08b36552dbb9a986]*/
 {
+    if (!check_cursor_locked(self)) {
+        return NULL;
+    }
+
     if (!self->connection) {
         PyTypeObject *tp = Py_TYPE(self);
         pysqlite_state *state = pysqlite_get_state_by_type(tp);
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		Raise :exc:`~sqlite3.ProgrammingError` instead of segfaulting on recursive
		usage of cursors in :mod:`sqlite3` converters. Patch by Sergey Fedoseev.