Crate parking

Thread parking and unparking.

This is a copy of the park()/unpark() mechanism from the standard library.

What is parking

Conceptually, each Parker has a token which is initially not present:

• The Parker::park() method blocks the current thread unless or until the token is available, at which point it automatically consumes the token. It may also return spuriously, without consuming the token.

• The Parker::park_timeout() method works the same as Parker::park(), but blocks until a timeout is reached.

• The Parker::park_deadline() method works the same as Parker::park(), but blocks until a deadline is reached.

• The Parker::unpark() and Unparker::unpark() methods atomically make the token available if it wasn't already. Because the token is initially absent, Unparker::unpark() followed by Parker::park() will result in the second call returning immediately.

Analogy with channels

Another way of thinking about Parker is as a bounded channel with capacity of 1.

Then, Parker::park() is equivalent to blocking on a receive operation, and Unparker::unpark() is equivalent to a non-blocking send operation.

Structs

Parker	Parks a thread.
Unparker	Unparks a thread.