Discard results without converting them into Python objects. (#601)

Fixes #560.

Author: methane

src/MySQLdb/_mysql.c

@@ -1484,6 +1484,26 @@ _mysql_ResultObject_fetch_row(
     return NULL;
 }
 
+static const char _mysql_ResultObject_discard__doc__[] =
+"discard() -- Discard remaining rows in the resultset.";
+
+static PyObject *
+_mysql_ResultObject_discard(
+    _mysql_ResultObject *self,
+    PyObject *noargs)
+{
+    check_result_connection(self);
+
+    MYSQL_ROW row;
+    while (NULL != (row = mysql_fetch_row(self->result))) {
+        // do nothing
+    }
+    if (mysql_errno(self->conn)) {
+        return _mysql_Exception(self->conn);
+    }
+    Py_RETURN_NONE;
+}
+
 static char _mysql_ConnectionObject_change_user__doc__[] =
 "Changes the user and causes the database specified by db to\n\
 become the default (current) database on the connection\n\
@@ -2081,6 +2101,43 @@ _mysql_ConnectionObject_use_result(
     return result;
 }
 
+static const char _mysql_ConnectionObject_discard_result__doc__[] =
+"Discard current result set.\n\n"
+"This function can be called instead of use_result() or store_result(). Non-standard.";
+
+static PyObject *
+_mysql_ConnectionObject_discard_result(
+    _mysql_ConnectionObject *self,
+    PyObject *noargs)
+{
+    check_connection(self);
+    MYSQL *conn = &(self->connection);
+
+    Py_BEGIN_ALLOW_THREADS;
+
+    MYSQL_RES *res = mysql_use_result(conn);
+    if (res == NULL) {
+        Py_BLOCK_THREADS;
+        if (mysql_errno(conn) != 0) {
+            // fprintf(stderr, "mysql_use_result failed: %s\n", mysql_error(conn));
+            return _mysql_Exception(self);
+        }
+        Py_RETURN_NONE;
+    }
+
+    MYSQL_ROW row;
+    while (NULL != (row = mysql_fetch_row(res))) {
+        // do nothing.
+    }
+    mysql_free_result(res);
+    Py_END_ALLOW_THREADS;
+    if (mysql_errno(conn)) {
+        // fprintf(stderr, "mysql_free_result failed: %s\n", mysql_error(conn));
+        return _mysql_Exception(self);
+    }
+    Py_RETURN_NONE;
+}
+
 static void
 _mysql_ConnectionObject_dealloc(
     _mysql_ConnectionObject *self)
@@ -2376,6 +2433,12 @@ static PyMethodDef _mysql_ConnectionObject_methods[] = {
         METH_NOARGS,
         _mysql_ConnectionObject_use_result__doc__
     },
+    {
+        "discard_result",
+        (PyCFunction)_mysql_ConnectionObject_discard_result,
+        METH_NOARGS,
+        _mysql_ConnectionObject_discard_result__doc__
+    },
     {NULL,              NULL} /* sentinel */
 };
 
@@ -2437,6 +2500,12 @@ static PyMethodDef _mysql_ResultObject_methods[] = {
         METH_VARARGS | METH_KEYWORDS,
         _mysql_ResultObject_fetch_row__doc__
     },
+    {
+        "discard",
+        (PyCFunction)_mysql_ResultObject_discard,
+        METH_NOARGS,
+        _mysql_ResultObject_discard__doc__
+    },
     {
         "field_flags",
         (PyCFunction)_mysql_ResultObject_field_flags,

src/MySQLdb/cursors.py

@@ -75,13 +75,30 @@ def __init__(self, connection):
         self.rownumber = None
         self._rows = None
 
+    def _discard(self):
+        self.description = None
+        self.description_flags = None
+        self.rowcount = -1
+        self.lastrowid = None
+        self._rows = None
+        self.rownumber = None
+
+        if self._result:
+            self._result.discard()
+            self._result = None
+
+        con = self.connection
+        if con is None:
+            return
+        while con.next_result() == 0:  # -1 means no more data.
+            con.discard_result()
+
     def close(self):
         """Close the cursor. No further queries will be possible."""
         try:
             if self.connection is None:
                 return
-            while self.nextset():
-                pass
+            self._discard()
         finally:
             self.connection = None
             self._result = None
@@ -180,8 +197,7 @@ def execute(self, query, args=None):
 
         Returns integer represents rows affected, if any
         """
-        while self.nextset():
-            pass
+        self._discard()
 
         mogrified_query = self._mogrify(query, args)
 

tests/test_cursor.py

@@ -1,4 +1,4 @@
-# import pytest
+import pytest
 import MySQLdb.cursors
 from configdb import connection_factory
 
@@ -186,3 +186,39 @@ def test_mogrify_with_dict_args():
 
     assert mogrified_query == "SELECT 1, 2"
     assert mogrified_query == cursor._executed.decode()
+
+
+# Test that cursor can be used without reading whole resultset.
+@pytest.mark.parametrize("Cursor", [MySQLdb.cursors.Cursor, MySQLdb.cursors.SSCursor])
+def test_cursor_discard_result(Cursor):
+    conn = connect()
+    cursor = conn.cursor(Cursor)
+
+    cursor.execute(
+        """\
+CREATE TABLE test_cursor_discard_result (
+    id INTEGER PRIMARY KEY AUTO_INCREMENT,
+    data VARCHAR(100)
+)"""
+    )
+    _tables.append("test_cursor_discard_result")
+
+    cursor.executemany(
+        "INSERT INTO test_cursor_discard_result (id, data) VALUES (%s, %s)",
+        [(i, f"row {i}") for i in range(1, 101)],
+    )
+
+    cursor.execute(
+        """\
+SELECT * FROM test_cursor_discard_result WHERE id <= 10;
+SELECT * FROM test_cursor_discard_result WHERE id BETWEEN 11 AND 20;
+SELECT * FROM test_cursor_discard_result WHERE id BETWEEN 21 AND 30;
+"""
+    )
+    cursor.nextset()
+    assert cursor.fetchone() == (11, "row 11")
+
+    cursor.execute(
+        "SELECT * FROM test_cursor_discard_result WHERE id BETWEEN 31 AND 40"
+    )
+    assert cursor.fetchone() == (31, "row 31")