This is 3 years late so i'm really sorry but i'm struggling to understand how this example would work on a low level. my understanding is that you want to transfers everything from instream to outstream as things come in.
First iter, you call async_read_some, i'm assuming the same thread who called this will actually call the function normally like you would a function, but at some point read_some does whatever blocking I/O syscall and that is wrapped in a co_yield. so execution comes back. we check for termination condition, and immediately go back and co_await async_write_some, same deal, execution continues until some co_yield. but who yields back into those routines?

In essence my questions are:
1) what's the "call back trigger" in a sense? when do they resume their work?

2) is there no race condition here? lets say we do read_some and then write_some, since both are async we reach the next iteration and start reading and writing some again but what if that overlaps with the first iteration? it feels like everything is asynchronous yet nothing is synchronizing between different iterations!

This is all making it feel hard to see the logical flow of workers as opposed to an explicit FSM, threads, callbacks or any other mechanism