Lwt_io.of_bytes + position don't work

[NathanReb]

I you run the following in utop:

let ichan = Lwt_io.of_bytes ~mode:Lwt_io.input (Lwt_bytes.of_string "abcd") in
Lwt_io.position ichan;;

you get:

- : int64 = -4L

which obviously is a bug as you can't set_position on a negative value.

I'm not familiar with Lwt's internals but had a look and it appears of_bytes behaves inconsistently with the broader make.
Here, in make, max depends on the mode while here, in of_bytes, it doesn't.

I found it by writing tests for a function that is supposed to reset the position after processing the channel. I assume it should work on regular channels as if it wasn't, someone would probably have found out by now. Still I'll try it out and let you know.

I'm happy to submit a fix if this is indeed the issue here!

[aantron]

Confirmed. The code in lwt_io.ml is definitely wrong. Not sure which is the most "right" way to fix it yet, but any reasonable fix is welcome :)

[NathanReb]

Good! I'll give it a shot!