Improve handling of different Go versions, address compile failure in Go 1.10.4+

[nathany]

What does this pull request do?

@wadey Thanks for your work on #34. This pull request builds on that work and fixes #40.

Yesterday's release of Go 1.10.4 and Go 1.11 brought another related breakage (see golang/go#24161). The specific compiler error is cannot use nil as type _Ctype_CFAllocatorRef. This requires passing C.CFAllocatorRef(0) instead of nil to a few functions that we use.

In this pull request, I also made an effort to minimize the amount of code we need to maintain across different Go versions, replacing wrap_deprecated.go with a few new (smaller) files for different Go versions.

The fix that worked for Go 1.10.4 or better didn't work as expected in Go 1.10.3 (see https://travis-ci.org/fsnotify/fsevents/jobs/420640270). Declaring the C.CFAllocatorRef(0) constant in the same file works in all 1.10.x versions, but when I extract that constant to a different file (go_1_10_after.go) it continues to work on Go 1.10.4+ but fails in 1.10.3.

To get 1.10.x versions prior to 1.10.4 working, I had to extract larger amounts of code into Go version specific files. (all of GetDeviceUUID, a new MakeCFString and CopyCFString functions). It's truly odd, but it is working now.

Please take a look and let me know if it's okay for me to merge this.

Where should the reviewer start?

go_1_10_after.go

How should this be manually tested?

go test should suffice but certainly feel free to test in a tool that uses this if you have one.

[codecov-io]

Codecov Report

Merging #41 into master will increase coverage by 1.23%.
The diff coverage is 73.68%.

@@            Coverage Diff             @@
##           master      #41      +/-   ##
==========================================
+ Coverage   71.18%   72.41%   +1.23%     
==========================================
  Files           3        4       +1     
  Lines         288      174     -114     
==========================================
- Hits          205      126      -79     
+ Misses         61       35      -26     
+ Partials       22       13       -9

Impacted Files	Coverage Δ
go_1_10_before.go	60% <60%> (ø)
go_1_10_after.go	60% <60%> (ø)
wrap.go	71.42% <88.88%> (+1.34%)	⬆️

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[xenoscopic]

Would it make more sense to use C.kCFAllocatorDefault instead of C.CFAllocatorRef(0)? The former is a bit more explicit in terms of desired behavior, especially since you're just using a constant anyway. This is what was done in rjeczalik/notify#162 to fix this issue.

On a side note: Thanks for keeping this project alive! It's super useful to be able to do native recursive watches, even if it doesn't currently fit into fsnotify's API.

[nathany]

@havoc-io Thanks for the tip. Indeed kCFAllocatorDefault would make more sense.

[nathany]

@havoc-io How does this look now? I've pushed 2 new commits based on your feedback.

[nathany]

Other than squashing some commits, are there any other changes needed before merging this?

[xenoscopic]

@nathany That looks good to me as far as the switch to kCFAllocatorDefault goes. Thanks again for this update.