Wait for sysfs node to ready before access (API BREAK)

[nocko]

With current versions of sysfs-gpio on Raspberry Pi, the first run of the application would fail (Permission Denied on SYSFS_GPIO_VALUE_PATH); but the second time (or if the pins were already exported); the library works fine.

As it turns out, on Raspberry Pi (Raspbian Jessie); it takes some time for the sysfs nodes under the GPIO pin to have the correct permissions assigned if you're not running your application as root.

Doing a time.sleep(0.5) works, but is dirty. This branch attempts to do it the "Right Way"; BUT that means Controller.alloc_pin() returns a deferred... which breaks existing code.

On the other hand, the raspberry pi may be the most commonly used / available python+GPIO hardware target so having the library work there is good.

[derekstavis]

Thanks for the PR @nocko! Good job here! 🍻

I would suggest not breaking the API. We have two ways of doing this:

• Clone alloc_pin into a new function alloc_pin_deferred
  • Usage: Controller.alloc_pin_deferred(7)
• Accept a keyword argument like deferred=True in alloc_pin
  • Usage: Controller.alloc_pin(7, deferred=True)

This way we can publish to PyPI and keep compatibility, while allowing new implementations to wait for pin availability. What's your opinion on the more appropriate solution?