AtomicBoolean: add compare_and_set API

Compare-and-set is a useful functionality (that's why it exists in the Java API). It can save one from race conditions, as explained below.

Use case

• Assume an atomicboolean called LOCKED, set to false.
• Multiple concurrent threads run the following code:

if !LOCKED.value
  LOCKED.value = true
  # do something ....
  LOCKED.value = false
end

In this scenario, concurrent threads can read a false in if !LOCKED.value, and enter the if block. That defeats the purpose of an atomicboolean-backed lock.

If there was a CAS API, the fixed code would look like this:

if LOCKED.compare_and_set(false, true)

Furthermore

Ruby has no "tryLock" either. So it's not immediately obvious how to create non-blocking synchronized code. Boolean CAS would offer a nice alternative which Java programmers (and others) already are familiar with.

What do you think?

Cheers - Victor

Start by locating the AtomicBoolean implementation and its current value API. Review how the existing atomic value behavior is tested, then determine where the compare_and_set entry point belongs. Done means the proposed API supports the false-to-true use case described in the issue and its behavior is covered by the project’s tests.