[Python-checkins] fix thread safety of `io.StringIO.truncate` (#133732)

[kumaraditya303]

https://github.com/python/cpython/commit/2cd24ebfe9a14bd52cb4d411c126b6a2dac65ae0
commit: 2cd24ebfe9a14bd52cb4d411c126b6a2dac65ae0
branch: main
author: Kumar Aditya <kumaradi...@python.org>
committer: kumaraditya303 <kumaradi...@python.org>
date: 2025-05-09T07:59:17Z
summary:

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

files:
M Modules/_io/clinic/stringio.c.h
M Modules/_io/stringio.c

diff --git a/Modules/_io/clinic/stringio.c.h b/Modules/_io/clinic/stringio.c.h
index 8e8cd8df9ab8a1..83165e5f7ad08b 100644
--- a/Modules/_io/clinic/stringio.c.h
+++ b/Modules/_io/clinic/stringio.c.h
@@ -149,13 +149,13 @@ PyDoc_STRVAR(_io_StringIO_truncate__doc__,
     {"truncate", _PyCFunction_CAST(_io_StringIO_truncate), METH_FASTCALL, 
_io_StringIO_truncate__doc__},
 
 static PyObject *
-_io_StringIO_truncate_impl(stringio *self, Py_ssize_t size);
+_io_StringIO_truncate_impl(stringio *self, PyObject *pos);
 
 static PyObject *
 _io_StringIO_truncate(PyObject *self, PyObject *const *args, Py_ssize_t nargs)
 {
     PyObject *return_value = NULL;
-    Py_ssize_t size = ((stringio *)self)->pos;
+    PyObject *pos = Py_None;
 
     if (!_PyArg_CheckPositional("truncate", nargs, 0, 1)) {
         goto exit;
@@ -163,12 +163,10 @@ _io_StringIO_truncate(PyObject *self, PyObject *const 
*args, Py_ssize_t nargs)
     if (nargs < 1) {
         goto skip_optional;
     }
-    if (!_Py_convert_optional_to_ssize_t(args[0], &size)) {
-        goto exit;
-    }
+    pos = args[0];
 skip_optional:
     Py_BEGIN_CRITICAL_SECTION(self);
-    return_value = _io_StringIO_truncate_impl((stringio *)self, size);
+    return_value = _io_StringIO_truncate_impl((stringio *)self, pos);
     Py_END_CRITICAL_SECTION();
 
 exit:
@@ -552,4 +550,4 @@ _io_StringIO_newlines_get(PyObject *self, void 
*Py_UNUSED(context))
 
     return return_value;
 }
-/*[clinic end generated code: output=5bfaaab7f41ee6b5 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=bccc25ef8e6ce9ef input=a9049054013a1b77]*/
diff --git a/Modules/_io/stringio.c b/Modules/_io/stringio.c
index 9d1bfa3ea05cea..56913fafefba8b 100644
--- a/Modules/_io/stringio.c
+++ b/Modules/_io/stringio.c
@@ -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) {

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