So fun fact about message-passing plugin APIs, everything becomes a parallel programming problem. My lexer (the part of an interpreter that recognizes fundamental syntax elements) spawns a callback thread with request and reply channels, and then messages a plugin which is able to either talk to the callback thread or message the original thread with a successfully parsed token or an abort.

It has just occurred to me that plugins are under no obligation to sequence their requests to the callback thread, which means that having one channel for requests and one for responses no longer suffices; the requests need to each contain their response channel.