Currently malloc cache is flushed long before the thread actually finishes:

thread_return_t AsanThread::ThreadStart(uptr os_id) {
...
  thread_return_t res = start_routine_(arg_);
  malloc_storage().CommitBack();
  if (flags()->use_sigaltstack) UnsetAlternateSignalStack();
...
}

void AsanThread::Destroy() {
...
  asanThreadRegistry().FinishThread(tid());
  FlushToDeadThreadStats(&stats_);
...
}

malloc_storage().CommitBack() (and UnsetAlternateSignalStack) must be called right
before asanThreadRegistry().FinishThread(tid()).
Otherwise all memory freed in between is leaked.