Provide an interface to create devices from a file descriptor

[v1993]

This functionality is provided in libusb via libusb_wrap_sys_device. It probably should be a method on the context object.

This is required to use libgusb with the XDG USB Desktop Portal.

[hughsie]

Sounds fine to me, thanks.

[v1993]

Some things to consider:

• libusb_wrap_sys_device specifies that it does not take ownership of the supplied file descriptor and it must be kept open until the device is closed (and then closed manually). It's probably a good idea to offer an option to close the fd on destruction, like Gio objects tend to allow.
• • However, the argument is only a file descriptor on unix-like systems. How should this be dealt with?
• What you get is a libusb_device_handle*; from there, we want to call libusb_get_device and possibly grab an extra reference on it. This also means that the device starts out already open.
• Closing such a device handle destroys the device object. This has some interesting implications - first, the destructor should unreference the device before closing the handle (assuming we've grabbed the extra reference). Second, manually closing such devices has to be disallowed - probably should return a NOT_SUPPORTED error. Technically, it's possible to allow that and just leave a non-functioning device object, but that's a lot of work for next to no gain.
• Finally, on a related but separate note, it would be useful to be able to create a device discovery free context: pass LIBUSB_OPTION_NO_DEVICE_DISCOVERY to libusb_init_context and do not set up hotplugging.

[hughsie]

@v1993 i think it's fine to return a GError with NOT_SUPPORTED if it's not UNIX-like. I agree with all your other suggestions -- but I don't have much time to actually action them I'm afraid. I'd be happy to review a pull request tho.

[v1993]

I'll try to look into this later when I get to it, shouldn't be too hard - unfortunately, the USB desktop portal is still too new to be used in practice; looks like only the GNOME portal backend implements it for now, which is not acceptable when USB connectivity is a core feature of your program.

Regarding how to deal with the handle not being an FD outside of Unix - I think the best solution would be to return NOT_SUPPORTED from set_close_on_unref and not close by default, i.e. a wrapped device can be created on any system where libusb allows it but only on Unix-like auto-closing can be enabled.