[lldb] Fix double free in python bindings error handling.

[jgorbe]

Summary

If we have a %typemap(freearg) that frees the argument, we shouldn't
free it manually on an error path before calling SWIG_fail.
SWIG_fail will already free the memory in this case, and doing it
manually results in a double free.

[bulbazord]

Thanks for taking care of that!

[JDevlieghere]

LGTM. I think it would be worth adding that information to the top of the file to prevent similar mistakes in the future.

[jgorbe]

In D147007#4225762, @JDevlieghere wrote:

LGTM. I think it would be worth adding that information to the top of the file to prevent similar mistakes in the future.

Good idea, thanks. I added a note at the top before committing.

[mib]

LGTM! Thanks @jgorbe !

lldb/bindings/python/python-typemaps.swig

-/* Typemap definitions, to allow SWIG to properly handle 'char**' data types. */
+/* Typemap definitions, to allow SWIG to properly handle 'char**' data types.
+
+NOTE: If there's logic to free memory in a %typemap(freearg), it will also be
+run if you call SWIG_fail on an error path. Don't manually free() an argument
+AND call SWIG_fail at the same time, because it will result in a double free.
+
+*/
 
 %inline %{
 
 #include "../bindings/python/python-typemaps.h"
 
 %}
 
 %typemap(in) char ** {
   /* Check if is a list  */
   if (PythonList::Check($input)) {
     PythonList list(PyRefType::Borrowed, $input);
     int size = list.GetSize();
     int i = 0;
     $1 = (char **)malloc((size + 1) * sizeof(char *));
     for (i = 0; i < size; i++) {
       PythonString py_str = list.GetItemAtIndex(i).AsType<PythonString>();
       if (!py_str.IsAllocated()) {
         PyErr_SetString(PyExc_TypeError, "list must contain strings");
-        free($1);
         SWIG_fail;
       }
 
       $1[i] = const_cast<char *>(py_str.GetString().data());
     }
     $1[i] = 0;
   } else if ($input == Py_None) {
     $1 = NULL;
if (PyList_Check($input)) {
     int size = PyList_Size($input);
     int i = 0;
     $2 = size;
     $1 = ($1_type)malloc(size * sizeof($*1_type));
     for (i = 0; i < size; i++) {
       PyObject *o = PyList_GetItem($input, i);
       if (!SetNumberFromPyObject($1[i], o)) {
         PyErr_SetString(PyExc_TypeError, "list must contain numbers");
-        free($1);
         SWIG_fail;
       }
 
       if (PyErr_Occurred()) {
-        free($1);
         SWIG_fail;
       }
     }
   } else if ($input == Py_None) {
     $1 = NULL;
     $2 = 0;
   } else {
     PyErr_SetString(PyExc_TypeError, "not a list");