Device shouldn't reference context
elmarco opened this issue · 7 comments
We should follow libusb more closely there.
open() should be a method on the Context, not on Device. This would remove the need to track the context in Device.
Alternatively, make open() take a context argument.
One of the reason is that there are cases where all you have is a libusb_device*, without its context, and you may want to still wrap it with Device.
Current ugly workaround... rusb::Device::from_libusb(FakeUsbContext {}, ptr)
This reference is required for memory safety -- a Device is tied to a particular Context and must not outlive it. Context is reference-counted internally, so the reference from Device keeps the Context alive.
If you don't want to manage contexts, use GlobalContext.
That makes sense. But it could be made optional, so you could bind devices without context, like my example (unsafe anyway).
I don't see an explicit documentation about the fact that device can't outline their context:
https://libusb.sourceforge.io/api-1.0/group__libusb__dev.html#ga77eedd00d01eb7569b880e861a971c2b
Well libusb_device contains a libusb_context* and pretty much any operation on it will dereference that.
https://github.com/libusb/libusb/blob/1a906274a66dd58bf81836db1306902d4a7dc185/libusb/libusbi.h#L486
But reading source code reveals that it indeed keeps a pointer to the context, without taking a ref. Bad idea, I wonder if it could be fixed in libusb.
So I close this issues because we must store reference to context