Added an API call to check the connection status of a device handle

[Quantx]

Aside from hot-plug events there is no good way to quickly check if a device handle is still attached to the system. I believe the only way to currently check if a device handle is still attached in a non-blocking manner is by attempting to claim an already claimed interface. This new API call adds a convenient method by which to check this.

int libusb_check_connected(libusb_device_handle * dev_handle)

[seanm]

This kind of API is prone to TOCTOU problems, so I'm not sure it's advisable to add. With multithreading and hotplug, the answer it gives can be incorrect an instant later.

[Quantx]

The TOCTOU issue is addressed by the fact that all other API calls will also report LIBUSB_ERROR_NO_DEVICE if the device was disconnected between a request to libusb_check_connected() and the subsequent API call. The reason I want to see this API call added is because there is currently no way to synchronously check whether a device handle is still connected without also causing some other side effect. While you can create a mechanism with hot-plug events to achieve a similar result, I think that is a lot of work to acquire a piece of information that could easily be returned to the user by this API call. Furthermore, on platforms where hot-plug events are not supported, there is currently no dedicated API call to detect whether or not a device is still connected.

[seanm]

Yeah, sounds reasonable enough.

Could you squash your commits?

[Quantx]

I've squashed it

[mcuee]

It may be good to add this to one of the existing examples as well to make it easy to test the new function.

[Quantx]

I've added a call to libusb_check_connected() inside one of the basic open/close tests. Let me know if that is sufficient or if you think there's a better test case I should use instead.

[Quantx]

If there are no further comments or concerns regarding this PR can it please be merged?

[mcuee]

If there are no further comments or concerns regarding this PR can it please be merged?

We will need to have the review and approval of the current active developers, basically @tormodvolden and/or @hjelmn.