Generate a device ID if expected fields are missing

[abdelq]

Some devices are not setting the device ID properly. In the case of the cheap printer I have, it's just whitespace. Without this change I end up with usb://Unknown/Unknown?serial=Printer. Not very useful for lprint's autoadd for example...

[michaelrsweet]

I think I want to generate a device ID string in this case, but want to dig a bit more to be sure. But in concept I am OK with this change.

[abdelq]

Alright, second attempt. Feedback appreciated (maybe snprintf instead? maybe papplDeviceParseID and pass anything that is not MFG, MDL?).

Did test this with lprint devices -o verbose and got:

usb://STMicroelectronics/POS58%20Printer%20USB%20%20%20%20%20%20%20?serial=Printer
    STMicroelectronics POS58 Printer USB        (USB)
    MFG:STMicroelectronics;MDL:POS58 Printer USB       ;SN:Printer;

Trailing spaces are from the actual product name...

[michaelrsweet]

OK, I've pushed a similar change:

[master 1a28cf9] Generate device ID as needed (Issue #234)

Can you review and provide feedback?

[michaelrsweet]

Pushed a slightly different (but similar) change - basically always get the serial, manufacturer, and product strings and cache the device ID. Let me know what you think!

[abdelq]

Thanks Michael. This is more in line with the alternative implementation I was thinking of. My only comment is that setting '\0' with libusb_get_string_descriptor_ascii is redundant.

Refs:

• https://github.com/libusb/libusb/blob/f3619c4078a921c3d6bf3238ff7a8dc70d60e40e/libusb/descriptor.c#L1127
• https://github.com/libusb/libusb/blob/f3619c4078a921c3d6bf3238ff7a8dc70d60e40e/libusb/descriptor.c#L1137

[michaelrsweet]

Noted, and I pushed a change to simplify:

[master 84279c3] Clean up libusb API usage (Issue #234)