gh-143191: Use _PyOS_MIN_STACK_SIZE in _thread.stack_size()

Lib/test/test_thread.py

@@ -76,6 +76,14 @@ def test_stack_size(self):
         thread.stack_size(0)
         self.assertEqual(thread.stack_size(), 0, "stack_size not reset to default")
 
+        with self.assertRaises(ValueError):
+            # 123 bytes is too small
+            thread.stack_size(123)
+
+        with self.assertRaises(ValueError):
+            # size must be positive
+            thread.stack_size(-4096)
+
     @unittest.skipIf(os.name not in ("nt", "posix"), 'test meant for nt and posix')
     def test_nt_and_posix_stack_size(self):
         try:

Misc/NEWS.d/next/Library/2026-01-09-13-07-22.gh-issue-143191.PPR_vW.rst

@@ -0,0 +1,2 @@
+:func:`_thread.stack_size` now raises :exc:`ValueError` if the stack size is
+too small. Patch by Victor Stinner.

Modules/_threadmodule.c

@@ -2200,9 +2200,10 @@ thread_stack_size(PyObject *self, PyObject *args)
     if (!PyArg_ParseTuple(args, "|n:stack_size", &new_size))
         return NULL;
 
-    if (new_size < 0) {
-        PyErr_SetString(PyExc_ValueError,
-                        "size must be 0 or a positive value");
+    Py_ssize_t min_size = _PyOS_MIN_STACK_SIZE + SYSTEM_PAGE_SIZE;
+    if (new_size != 0 && new_size < min_size) {
+        PyErr_Format(PyExc_ValueError,
+                     "size must be at least %zi bytes", min_size);
         return NULL;
     }