This avoids a race condition where a receiving thread could
potentially try to handle an incoming message (as far as the C
implementation is concerned, we're done subscribing) before the
handler is populated (and then trying to run a handler at address
0x0).
This avoids a race condition where a receiving thread could potentially try to handle an incoming message (as far as the C implementation is concerned, we're done subscribing) before the handler is populated (and then trying to run a handler at address 0x0).