Add AllowCallWithBlock option to ThreadSafety/DirChdir cop

[viralpraxis]

Using Dir.chdir with provided block is much more safer than block-less call since it restores PWD to initial state . I think it makes sense to add an option to allow these safe calls.

[viralpraxis]

@mikegee could you please take a look? 🙂

[mikegee]

I definitely support using a block to restore PWD. That's awesome. 👍

I don't understand why we would consider it thread-safe. Am I missing something?

[viralpraxis]

I have a gem I'm currently developing; I'm confident that block-ful chdir is safe in my case so I don't wont it to be marked as on offense, but I still want block-less calls to be detected since it is permanent global state modification leading to bugs, e.g. if I forget to restore PWD. So I think there should be an option between disabling this cop and leaving it's configuration as-is. Flag AllowCallWithBlock solves this problem. So, there will be 3 configuration variants:

1. Gem is disabled, nothing is detected
2. AllowCallWithBlock is enabled, only block-ful calls are allowed
3. Default configuration, no chdir is allowed

[mikegee]

Ok, I can support the option. As long as it is not the default configuration and there is sufficient warning that using a block does not make the code thread-safe.

I also wonder if it is possible to write app code in/around that block to guarantee thread-safety. Maybe with a global mutex? Then the cop could detect/suggest that pattern. That would be extra awesome.

[viralpraxis]

Ok, I can support the option. As long as it is not the default configuration and there is sufficient warning that using a block does not make the code thread-safe.

Great. I seem some room for cop documentation improvements, I'll address it in another PR.

I also wonder if it is possible to write app code in/around that block to guarantee thread-safety. Maybe with a global mutex? Then the cop could detect/suggest that pattern. That would be extra awesome.

That's a good point! I'll into that.

After merging this we'll be able to ship a new version. This is important since I want to finish and merge #70 before releasing 0.7. That documentation files must be accessible with a git tag so it will be picked by docs.rubocop.org'.