gh-99537: Use Py_SETREF() function in C code (#99657)

Fix potential race condition in code patterns:

* Replace "Py_DECREF(var); var = new;" with "Py_SETREF(var, new);"
* Replace "Py_XDECREF(var); var = new;" with "Py_XSETREF(var, new);"
* Replace "Py_CLEAR(var); var = new;" with "Py_XSETREF(var, new);"

Other changes:

* Replace "old = var; var = new; Py_DECREF(var)"
  with "Py_SETREF(var, new);"
* Replace "old = var; var = new; Py_XDECREF(var)"
  with "Py_XSETREF(var, new);"
* And remove the "old" variable.

1 files changed, 2 insertions, 4 deletions

diff --git a/Objects/stringlib/unicode_format.h b/Objects/stringlib/unicode_format.h
index e970588..ccd7c77 100644
--- a/Objects/stringlib/unicode_format.h
+++ b/Objects/stringlib/unicode_format.h
@@ -473,8 +473,7 @@ get_field_object(SubString *input, PyObject *args, PyObject *kwargs,
             goto error;
 
         /* assign to obj */
-        Py_DECREF(obj);
-        obj = tmp;
+        Py_SETREF(obj, tmp);
     }
     /* end of iterator, this is the non-error case */
     if (ok == 1)
@@ -825,8 +824,7 @@ output_markup(SubString *field_name, SubString *format_spec,
             goto done;
 
         /* do the assignment, transferring ownership: fieldobj = tmp */
-        Py_DECREF(fieldobj);
-        fieldobj = tmp;
+        Py_SETREF(fieldobj, tmp);
         tmp = NULL;
     }