[3.13] gh-116738: Make _csv module thread-safe (GH-118344) (#125328)

gh-116738: Make `_csv` module thread-safe (GH-118344)
(cherry picked from commit a00221e)

Co-authored-by: AN Long <aisk@users.noreply.github.com>

Author: miss-islington

Modules/_csv.c

@@ -14,6 +14,7 @@ module instead.
 #endif
 
 #include "Python.h"
+#include "pycore_pyatomic_ft_wrappers.h"
 
 #include <stddef.h>               // offsetof()
 #include <stdbool.h>
@@ -34,7 +35,7 @@ typedef struct {
     PyTypeObject *dialect_type;
     PyTypeObject *reader_type;
     PyTypeObject *writer_type;
-    long field_limit;   /* max parsed field size */
+    Py_ssize_t field_limit;   /* max parsed field size */
     PyObject *str_write;
 } _csvstate;
 
@@ -702,10 +703,11 @@ parse_grow_buff(ReaderObj *self)
 static int
 parse_add_char(ReaderObj *self, _csvstate *module_state, Py_UCS4 c)
 {
-    if (self->field_len >= module_state->field_limit) {
+    Py_ssize_t field_limit = FT_ATOMIC_LOAD_SSIZE_RELAXED(module_state->field_limit);
+    if (self->field_len >= field_limit) {
         PyErr_Format(module_state->error_obj,
-                     "field larger than field limit (%ld)",
-                     module_state->field_limit);
+                     "field larger than field limit (%zd)",
+                     field_limit);
         return -1;
     }
     if (self->field_len == self->field_size && !parse_grow_buff(self))
@@ -1651,20 +1653,20 @@ _csv_field_size_limit_impl(PyObject *module, PyObject *new_limit)
 /*[clinic end generated code: output=f2799ecd908e250b input=cec70e9226406435]*/
 {
     _csvstate *module_state = get_csv_state(module);
-    long old_limit = module_state->field_limit;
+    Py_ssize_t old_limit = FT_ATOMIC_LOAD_SSIZE_RELAXED(module_state->field_limit);
     if (new_limit != NULL) {
         if (!PyLong_CheckExact(new_limit)) {
             PyErr_Format(PyExc_TypeError,
                          "limit must be an integer");
             return NULL;
         }
-        module_state->field_limit = PyLong_AsLong(new_limit);
-        if (module_state->field_limit == -1 && PyErr_Occurred()) {
-            module_state->field_limit = old_limit;
+        Py_ssize_t new_limit_value = PyLong_AsSsize_t(new_limit);
+        if (new_limit_value == -1 && PyErr_Occurred()) {
             return NULL;
         }
+        FT_ATOMIC_STORE_SSIZE_RELAXED(module_state->field_limit, new_limit_value);
     }
-    return PyLong_FromLong(old_limit);
+    return PyLong_FromSsize_t(old_limit);
 }
 
 static PyType_Slot error_slots[] = {