Close file handle in cancellation handler to avoid race

[KaiOelfke]

This should fix #172 and implements the file handle closing as recommended in the Apple documentation.

[mbrandonw]

Hi @KaiOelfke, thanks so much for taking this on. Can you think of any way we can get a test in place that would have crashed without the change? Would be nice to have a record of why we made this change for our future selves.

[KaiOelfke]

@mbrandonw

In the issue I have added a comment with an unit test, but the problem is that the test is slow and non deterministic. I tried to improve the test to more reliably run into this race condition, but I didn't manage that so far. Feel free to look at the issue and the documentation by Apple for the cancellation handler to see, if you can improve the test.

I didn't commit this test as it's in my opinion too slow and flaky in the current shape.

[KaiOelfke]

@mbrandonw FYI, I've released an app update a while ago with a fork with this PR. It eliminates my biggest app crash group in the Xcode organizer. While a better test would be desirable at least there's one here in the ticket that reproduces the problem slowly and as a reference one could also mention Apple's documentation.

I'm sure there will be other apps out there crashing with this bug. Just maybe less frequently depending on how they use file storage.