[erlang-questions] testing side effects of asynchronous code

[Motiejus Jakštys]

Hi List,

Let's say I am testing side effects of this asynchronous function:
gateway_api:send(Message) -> reference()

It does some asynchronous processing and produces side effects. After
processing completes, gen_gwserver:ack_sent is invoked, like this:
gen_gwserver:ack_sent(Ref :: reference()) -> ok.

The question is: how can I know that the function ack_sent has been
called? Then I know that asynchronous work has been completed, so I can
start validating the side effects.

An imaginary API in a test case, which blocks until
gen_gwserver:ack_sent/1 has been called:
?call_and_sleep(
    gateway_api:send(Message),
    gen_gwserver, ack_sent, 1
)

Some options that we ruled out:
* timer:sleep(some random value)
* making internal gateway_api:ack(Reference) calls (inconvenient, since
  necessary only for testing)
* meck:passthrough looked promising, but the problem is it doesn't
  return, so it cannot be implemented using meck cleanly.

Thanks,
Motiejus Jakštys