Fix 100% CPU usage when using libusb

[radfordi]

Replace broken dispatcher with a thread to fix 100% CPU usage

The dispatcher can't tell the difference between "timeout" and "empty", so it ends up infinte looping. Using a thread directly is much simpler and more correct.

Fixes #5783. Fixes #6062.

[radfordi]

@dorodnic, the broken dispatcher issue is not libusb or Mac specific. I see the issue with every program when the dispatcher's _queue is empty.

[radfordi]

I went to clean up the fact that we are creating a usb_context every time we list devices, but before I could fix it, I noticed that @bfulkers-i had already fixed this as part of #5492!

[radfordi]

I plan to rebase #5492 on top of this once this gets merged.

[dorodnic]

Hi @radfordi
Thanks for the PR.
I'm trying to understand the fix, and am a bit confused.
This was consuming 100% CPU -

_event_handler = std::make_shared<active_object<>>([this](dispatcher::cancellable_timer cancellable_timer)	
            {	
                if(_kill_handler_thread)	
                    return;	
                auto sts = libusb_handle_events_completed(_ctx, &_kill_handler_thread);	
            });

However, this does not -

_event_handler = std::thread([this]() {
                    while (!_kill_handler_thread)
                        libusb_handle_events_completed(_ctx, &_kill_handler_thread);
                });

Why? Both are sniping a thread doing essentially the same thing...

It's unfortunate that active_object<> was not used correctly in the first place - the cancellable_timer object is supposed to provide uninterruptible sleep, not consuming CPU but also allowing the thread to be interrupted instantly.

I'll look deeper into it and merge the PR

[radfordi]

Why? Both are sniping a thread doing essentially the same thing...

When _kill_handler_thread == 1, then in the new version the thread exits, while in the old version the handler just returns quickly. So what happens when there are no devices open? In the new version the thread doesn't exist, and in the old version the dispatcher loops calling the function as fast as it can with no waiting in between.

[dorodnic]

Ok thank you
I'm testing the patch today and so far looks good to me 👍