gh-138092: Fix mmap.flush argument handling

Lib/test/test_mmap.py

@@ -1145,6 +1145,18 @@ def test_access_violations(self):
         self.assertEqual(stdout.strip(), b'')
         self.assertEqual(stderr.strip(), b'')
 
+    def test_flush_parameters(self):
+        with open(TESTFN, 'wb+') as f:
+            f.write(b'x' * PAGESIZE * 3)
+            f.flush()
+
+            m = mmap.mmap(f.fileno(), PAGESIZE * 3)
+            self.addCleanup(m.close)
+
+            m.flush()
+            m.flush(PAGESIZE)
+            m.flush(PAGESIZE, PAGESIZE)
+
 
 class LargeMmapTests(unittest.TestCase):
 

Misc/NEWS.d/next/Library/2025-08-24-02-04-32.gh-issue-138092.V4-wTO.rst

@@ -0,0 +1,2 @@
+Fixed a bug in :meth:`mmap.mmap.flush` where calling with only an offset
+parameter would fail.

Modules/mmapmodule.c

@@ -933,11 +933,15 @@ static PyObject *
 mmap_flush_method(PyObject *op, PyObject *args)
 {
     Py_ssize_t offset = 0;
+    Py_ssize_t size = -1;
     mmap_object *self = mmap_object_CAST(op);
-    Py_ssize_t size = self->size;
     CHECK_VALID(NULL);
-    if (!PyArg_ParseTuple(args, "|nn:flush", &offset, &size))
+    if (!PyArg_ParseTuple(args, "|nn:flush", &offset, &size)) {
         return NULL;
+    }
+    if (size == -1) {
+        size = self->size - offset;
+    }
     if (size < 0 || offset < 0 || self->size - offset < size) {
         PyErr_SetString(PyExc_ValueError, "flush values out of range");
         return NULL;