ScopedCFRef helper to manage CF objects

[hrantzsch]

Add a ScopedCFRef wrapper for CF objects that ensures that CFRelease is called when it leaves the scope.

[codecov]

Codecov Report

Attention: 42 lines in your changes are missing coverage. Please review.

Comparison is base (ec80ca8) 48.51% compared to head (6708591) 50.26%.

Files	Patch %	Lines
src/keychain_mac.cpp	30.00%	7 Missing and 35 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #36      +/-   ##
==========================================
+ Coverage   48.51%   50.26%   +1.75%     
==========================================
  Files           3        3              
  Lines         202      187      -15     
  Branches       77       76       -1     
==========================================
- Hits           98       94       -4     
+ Misses         45       30      -15     
- Partials       59       63       +4     

Flag	Coverage Δ
unittests	50.26% <30.00%> (+1.75%)	⬆️

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[reneme]

I mainly looked at the ScopedCFRef<>. Not so much its usage. I'm not super familiar with the Apple APIs, unfortunately. Nevertheless, one word of warning: If some API takes ownership of a CFRef you're passing in, you might want to have something like:

T release() {
    auto out = _ref;
    _ref = nullptr;
    return out;
}

Otherwise, macOS might free something internally that your ScopedCFRef<> freed already.

Particularly createQuery() could be prone to that, as it creates a CFMutableDictionaryRef that is appended with some objects that are auto-released in the same scope. The CFMutableDictionaryRef is eventually returned and therefore lives longer than its members. Maybe CFDictionaryAddValue() creates a deep copy, though, no idea. Just putting this out here.