> I think that the code would have to be quite defensive and essentially assume the most parallel/concurrent version of IO to be used.

Exactly, but why would anyone think differently when the goal is to support both synchronous and async execution?

However, if asynchrony is done well at the lower levels of IO event handler, it should be simple to implemcent by following these principles everywhere — the "worst" that could happen is that your code runs sequentially (thus slower), but not run into races or deadlocks.