Messages can be lost under concurrency

[NicolasLM]

Because the read-modify-write sequence is not atomic, it is possible to lose messages when multiple tasks try to send a message concurrently.

messages = cache.get(user_key) or []
messages.append((message, level))
cache.set(user_key, messages)

Unfortunately Django cache framework does not support atomic append to list so there is no easy solution to this problem.