Rust Atomics and Locks

This is my coding along with this wonderful book. In general, I'll try to implement a chapter, update that implementation with the book's implementation, and record my learnings in the README.

Chapter 4 - Spin Lock

Unfortunately, I read this chapter before attempting an implementation. Luckily, I was still given the opportunity to learn a lot about Send/Sync! Most of the details are summarized in this issue but I'll summarize here.

I originally thought that Send was for transferring ownership of a value to another thread. So I thought this was specifically relating to sending a T. However:

• I didn't notice the fact that, if you send &mut T, the receiving thread can use std::mem::swap (or similar) to get an owned T. So Send is definitely required for sending &mut T.
• Since Send was for T and I hadn't really thought about &mut T, I assumed that you needed Sync to send non-T. That's untrue - Sync is only required if you need concurrent access to &T from multiple threads. The reason why &T: Send implies Sync is because &T is copy - if you're allowed to send &T to one thread then you're allowed to make copies and send to many threads. However, if you're in a situation where you have exclusive access (e.g. Mutex, spin lock), then you don't need Sync.
• You need Send even if you only access &T. If, for example, we remove the DerefMut implementation on our SpinLockGuard, we'd still need to require T: Send to impl Sync for SpinLock. When we talk about Send and "sending values to another thread" that doesn't mean passing ownership - that means doing anything with T, &T, or &mut T on a thread that the original T wasn't initialized on. Some more details in this wonderful StackOverflow answer.

Chapter 5 - Channels

As with Chapter 4, I'd unfortunately already skimmed this chapter before attempting an implementation. Luckily, I did such a bad job at reading that I got plenty of stuff wrong which leaves room for learning!

• Preventing sending the Receiver is a much easier way to avoid deadlocks (though I'll need to investigate std/crossbeam to see how this is handled in the wild).
• By dropping unsent messages in the Drop implementation of Inner we dodge some racy logic involving an extra waiting variable and avoid some kludgy Arc::strong_counting.
• Implementing Sync on Inner and then letting autoimplementations bubble up to the public types is better. This is also useful in case you make Inner public later (as we would if we were avoiding the inner allocation).
• AtomicBool::get_mut() is a neat API for when you have &mut self.