py/qstr: Reset mpstate.qstr_last_chunk before raising an error.

The qstr_last_chunk is not collected by the garbage collector. This relies on the assertion that qstr_pool_t also references the qstr_last_chunk. If an exception is raised while allocating the qstr_pool_t, qstr_last_chunk has to be invalidated not to become a dangling reference at the next garbage collection. Signed-off-by: Emilie Feral <emilie.feral@numworks.com>

Author: EmilieNumworks

py/qstr.c

@@ -153,6 +153,12 @@ STATIC qstr qstr_add(const byte *q_ptr) {
  #endif
  qstr_pool_t *pool = m_new_obj_var_maybe(qstr_pool_t, const char *, new_alloc);
  if (pool == NULL) {
+ // Keep qstr_last_chunk consistent with qstr_pool_t: qstr_last_chunk is not scanned
+ // at garbage collection since it's reachable from a qstr_pool_t. And the caller of
+ // this function expects q_ptr to be stored in a qstr_pool_t so it can be reached
+ // by the collector. If qstr_pool_t allocation failed, qstr_last_chunk needs to be
+ // NULL'd. Otherwise it may become a dangling pointer at the next garbage collection.
+ MP_STATE_VM(qstr_last_chunk) = NULL;
  QSTR_EXIT();
  m_malloc_fail(new_alloc);
  }