fix thread safety of io.StringIO.truncate (python#133732)

Author: kumaraditya303

Modules/_io/clinic/stringio.c.h

@@ -444,7 +444,7 @@ stringio_iternext(PyObject *op)
 /*[clinic input]
 @critical_section
 _io.StringIO.truncate
-    pos as size: Py_ssize_t(accept={int, NoneType}, c_default="((stringio *)self)->pos") = None
+    pos: object = None
     /
 
 Truncate size to pos.
@@ -455,16 +455,26 @@ Returns the new absolute position.
 [clinic start generated code]*/
 
 static PyObject *
-_io_StringIO_truncate_impl(stringio *self, Py_ssize_t size)
-/*[clinic end generated code: output=eb3aef8e06701365 input=fa8a6c98bb2ba780]*/
+_io_StringIO_truncate_impl(stringio *self, PyObject *pos)
+/*[clinic end generated code: output=c76c43b5ecfaf4e2 input=d59fd2ee49757ae6]*/
 {
     CHECK_INITIALIZED(self);
     CHECK_CLOSED(self);
 
-    if (size < 0) {
-        PyErr_Format(PyExc_ValueError,
-                     "Negative size value %zd", size);
-        return NULL;
+    Py_ssize_t size;
+    if (pos == Py_None) {
+        size = self->pos;
+    }
+    else {
+        size = PyLong_AsLong(pos);
+        if (size == -1 && PyErr_Occurred()) {
+            return NULL;
+        }
+        if (size < 0) {
+            PyErr_Format(PyExc_ValueError,
+                         "negative pos value %zd", size);
+            return NULL;
+        }
     }
 
     if (size < self->string_size) {