Debugging Message Arrival

Working with asynchronous grains can be interesting when certain optional asynchronous behaviour needs to be observed. Consider the following example.

Lets assume that A pushes messages to B, and B optionally pushes a messages as a result. How can we determine if B will push a message, without resorting to receive with an infinite timeout? Grains provides a Grains.debug_reply_chain(chain, payload) function to handle this. The function creates a message with a payload, which is passed along the chain grains. As soon as this special message hits the last grain in that chain, the payload is send() to the original caller of Grains.debug_reply_chain/2.