trio.wrap_file() undocumented behavior wrt to sync file object

[Nikratio]

Hello,

Looking at https://trio.readthedocs.io/en/stable/reference-io.html#trio.wrap_file, it is not clear to me if:

• Closing the returned async file object will also close the sync file object that I passed in
• Whether this will happen automatically when the async file object is garbage collected
• If I can continue to use the sync file object at the same time.

It'd be great if that could be clarified!

[A5rocks]

Looking at the code (so actually testing these behaviors may show I'm wrong), I believe this is yes, no, yes.

I think the 1st and 3rd are somewhat implied by the docs talking about this being a wrapper (... it basically just sends sync ops to a thread pool) but we could definitely be more explicit about the implications of that. And also more explicit about where we diverge from standard behavior, like not having a destructor.

[nightcityblade]

Hi, I'd like to work on this. I'll submit a PR shortly.