Addressed PR comments.

python/convert.c

@@ -319,15 +319,31 @@ bool PyUpb_Array_IsEqual(const upb_array *arr1, const upb_array *arr2,
 
 bool PyUpb_Message_IsEqual(const upb_msg *msg1, const upb_msg *msg2,
                            const upb_msgdef *m) {
-  const upb_symtab* symtab = upb_filedef_symtab(upb_msgdef_file(m));
-  size_t iter1 = UPB_MSG_BEGIN;
-  size_t iter2 = UPB_MSG_BEGIN;
-
   if (msg1 == msg2) return true;
   if (upb_msg_extcount(msg1) != upb_msg_extcount(msg2)) return false;
 
+  // Compare messages field-by-field.  This is slightly tricky, because while
+  // we can iterate over normal fields in a predictable order, the extension
+  // order is unpredictable and may be different between msg1 and msg2.
+  // So we use the following strategy:
+  //   1. Iterate over all msg1 fields (including extensions).
+  //   2. For non-extension fields, we find the corresponding field by simply
+  //      using upb_msg_next(msg2).  If the two messages have the same set of
+  //      fields, this will yield the same field.
+  //   3. For extension fields, we have to actually search for the corresponding
+  //      field, which we do with upb_msg_get(msg2, ext_f1).
+  //   4. Once iteration over msg1 is complete, we call upb_msg_next(msg2) one
+  //      final time to verify that we have visited all of msg2's regular fields
+  //      (we pass NULL for ext_dict so that iteration will *not* return
+  //      extensions).
+  //
+  // We don't need to visit all of msg2's extensions, because we verified up
+  // front that both messages have the same number of extensions.
+  const upb_symtab* symtab = upb_filedef_symtab(upb_msgdef_file(m));
   const upb_fielddef *f1, *f2;
   upb_msgval val1, val2;
+  size_t iter1 = UPB_MSG_BEGIN;
+  size_t iter2 = UPB_MSG_BEGIN;
   while (upb_msg_next(msg1, m, symtab, &f1, &val1, &iter1)) {
     if (upb_fielddef_isextension(f1)) {
       val2 = upb_msg_get(msg2, f1);