Extending eio from the outside

[xeniarose]

i'm interested in implementing support for serial ports on eio, but after spending a lot of time trying to get this started and exploring the eio codebase it seems like the current state of eio makes this unreasonably difficult

for example:

• given a path, it would be nice to be able to manually call openat on linux and posix platforms with the O_NOCTTY flag. however, at least with Eio_linux the platform doesn't make the Dir_fd resource handler public so there is basically no way to call Eio_linux.Low_level.openat from an arbitrary Path.t -- the only workaround i found was to convert the Path into its native string and then use that, but i don't really like that approach
• assuming we are somehow able to manually call openat, obtain the fd, execute the setup ioctls, it would then be nice to be able to wrap this in a Flow, but there doesn't seem to be a way to do that either

what's the recommended approach here? would a hypothetical tty/serial subsystem be better served being part of eio directly? or could eio just expose a bit more of the platform-specific internals to outside libraries?

[talex5]

It's not very obvious, but you can use import_socket_stream to wrap an FD as a flow.

Exposing Eio_linux.get_dir_fd_opt seems reasonable to me. Possibly we should always pass O_NOCTTY when opening things (there's a Rust issue for this at rust-lang/rust#24307 but it got closed with no clear conclusion).