Prevent race condition for lookups

[lipka]

There is a possible scenario for a race condition when doing lookups in the existing code from multiple threads.

1. T1 tries to do a lookup, acquires lock, but doesn't find anything
2. T2 tries to do a lookup, fails to acquire lock, waits while T1 releases lock
3. T1 releases lock and leaves the critical section without a lookup
4. T2 acquires the lock, tries to do a lookup, finds nothing, leaves critical section without a lookup
5. T1 finally creates a disposable bag
6. T2 creates a disposable bag too, overwrites disposable bag from T1

The suggested solution does both the lookup and assignment in the same critical section.

(Sorry about the force unwrapped optional!)

[ashfurrow]

This is fantastic @lipka 💯 Thanks for the pull request! I'll merge, add a changelog entry, and submit a new release to CocoaPods Trunk 🎉

[ashfurrow]

I agree about the sad necessity of the implicitly unwrapped optional – could make the doLocked function return a generic or something, but using an implicitly unwrapped optional is close enough 😸

I've pushed a new release up to trunk: https://raw.githubusercontent.com/CocoaPods/Specs/89f45b897166a93e5ecbb427dffdf875eef85ad9/Specs/NSObject+Rx/1.1.0/NSObject+Rx.podspec.json

Also, I added you as a collaborator on the repo – may ask for your input on some pull requests if that's ok.

Thanks again!