gh-99240: Fix double-free bug in Argument Clinic str_converter generated code

Lib/test/clinic.test

@@ -1740,37 +1740,36 @@ test_str_converter_encoding(PyObject *module, PyObject *const *args, Py_ssize_t
         goto exit;
     }
     return_value = test_str_converter_encoding_impl(module, a, b, c, d, d_length, e, e_length);
-
-exit:
-    /* Cleanup for a */
+    /* Post operation for a */
     if (a) {
        PyMem_FREE(a);
     }
-    /* Cleanup for b */
+    /* Post operation for b */
     if (b) {
        PyMem_FREE(b);
     }
-    /* Cleanup for c */
+    /* Post operation for c */
     if (c) {
        PyMem_FREE(c);
     }
-    /* Cleanup for d */
+    /* Post operation for d */
     if (d) {
        PyMem_FREE(d);
     }
-    /* Cleanup for e */
+    /* Post operation for e */
     if (e) {
        PyMem_FREE(e);
     }
 
+exit:
     return return_value;
 }
 
 static PyObject *
 test_str_converter_encoding_impl(PyObject *module, char *a, char *b, char *c,
                                  char *d, Py_ssize_t d_length, char *e,
                                  Py_ssize_t e_length)
-/*[clinic end generated code: output=8acb886a3843f3bc input=eb4c38e1f898f402]*/
+/*[clinic end generated code: output=5dfef501bb5f5e82 input=eb4c38e1f898f402]*/
 
 
 /*[clinic input]

Misc/NEWS.d/next/Library/2022-11-08-15-54-43.gh-issue-99240.MhYwcz.rst

@@ -0,0 +1,3 @@
+Add "post_operations" section to Argument Clinic generated code.
+Fix double-free bug in Argument Clinic str_converter generated code by
+moving memory clean up out of "exit" label.

Tools/clinic/clinic.py

@@ -348,6 +348,12 @@ def __init__(self):
         # "goto exit" if there are any.
         self.return_conversion = []
 
+        # The C statements required to do some operations
+        # after the end of parsing but before cleaning up.
+        # These operations may be, for example, memory deallocations which
+        # can only be done without any error happening during argument parsing.
+        self.post_operations = []
+
         # The C statements required to clean up after the impl call.
         self.cleanup = []
 
@@ -820,6 +826,7 @@ def parser_body(prototype, *fields, declarations=''):
                     {modifications}
                     {return_value} = {c_basename}_impl({impl_arguments});
                     {return_conversion}
+                    {post_operations}
 
                 {exit_label}
                     {cleanup}
@@ -1460,6 +1467,7 @@ def render_function(self, clinic, f):
         template_dict['impl_parameters'] = ", ".join(data.impl_parameters)
         template_dict['impl_arguments'] = ", ".join(data.impl_arguments)
         template_dict['return_conversion'] = format_escape("".join(data.return_conversion).rstrip())
+        template_dict['post_operations'] = format_escape("".join(data.post_operations).rstrip())
         template_dict['cleanup'] = format_escape("".join(data.cleanup))
         template_dict['return_value'] = data.return_value
 
@@ -1484,6 +1492,7 @@ def render_function(self, clinic, f):
                 return_conversion=template_dict['return_conversion'],
                 initializers=template_dict['initializers'],
                 modifications=template_dict['modifications'],
+                post_operations=template_dict['post_operations'],
                 cleanup=template_dict['cleanup'],
                 )
 
@@ -2725,6 +2734,10 @@ def _render_non_self(self, parameter, data):
         # parse_arguments
         self.parse_argument(data.parse_arguments)
 
+        # post_operations
+        if post_operations := self.post_operations():
+            data.post_operations.append('/* Post operation for ' + name + ' */\n' + post_operations.rstrip() + "\n")
+
         # cleanup
         cleanup = self.cleanup()
         if cleanup:
@@ -2820,6 +2833,14 @@ def modify(self):
         """
         return ""
 
+    def post_operations(self):
+        """
+        The C statements required to do some operations after the end of parsing but before cleaning up.
+        Returns a string containing this code indented at column 0.
+        If no operation is necessary, returns an empty string.
+        """
+        return ""
+
     def cleanup(self):
         """
         The C statements required to clean up after this variable.
@@ -3416,7 +3437,7 @@ def converter_init(self, *, accept={str}, encoding=None, zeroes=False):
         if NoneType in accept and self.c_default == "Py_None":
             self.c_default = "NULL"
 
-    def cleanup(self):
+    def post_operations(self):
         if self.encoding:
             name = self.name
             return "".join(["if (", name, ") {\n   PyMem_FREE(", name, ");\n}\n"])