API overview

API	Summary
`@tasklet code`	Create an object that is a memoized version of `() -> code` but also acts like a `Promise` that is not settable.
`Backoff(mindelay, maxdelay) -> backoff`	Create a callable `backoff` where `backoff()` spin-wait some amount of times.
`Guard(data)`	Guard mutable `data`. Use `guarding` to obtain exclusive access to `data`.
`Once{T}(f = T)`	A concurrent object for lazily initializing an object of type `T`.
`Promise{T}()`	Create a promise of type `T`; i.e., a memory location that holds a value of type `T` that can be set once and retrieved asynchronously.
`ReadWriteGuard(data)`	Guard mutable `data`. Use `guarding` and `guarding_read` to obtain exclusive ("write") and shared ("read") access to `data`.
`ReadWriteLock()`	Create a read-write lock.
`ThreadLocalStorage{T}(factory)`	Create a thread-local storage of type `T` created by `factory()`.
`guarding(f!, guard)`	Apply `f!` to the data wrapped in `guard` while obtaining exclusive access.
`guarding_read(f, guard)`	Apply `f` to the data wrapped in `guard` while obtaining shared access.
`lock_read(rwlock)`	`lock_read(rwlock)` takes reader (shared) lock. It must be released with `unlock_read`.
`race_put_with!(thunk, promise::Promise{T}) -> value::T`	Fetch an existing `value` or set `value = thunk()`. The `thunk` is called at most once for each instance of `promise`.
`spinloop()`	A hint to the compiler, runtime, and hardware that `spinloop()` is in the middle of a spin loop. Call this in a spin loop that requires some other worker threads to make forward progress in order for the current thread to make forward progress.
`try_race_fetch(promiselike) -> Ok(value::T) or Err(NotSetError())`	Try to retrieve a `value` if it is already set. Return `Ok(value)` on success and `Err(NotSetError())` on failure.
`try_race_put!(promise::Promise{T}, value) -> Ok(value′::T) or Err(existing::T)`	Try to set a `value` in the `promise`.
`try_race_put_with!(thunk, promise::Promise{T}) -> Ok(computed::T) or Err(existing::T)`	Fetch an `existing` value or set a `computed` value (`computed = thunk()`). The `thunk` is called at most once for each instance of `promise`.
`trylock_read(rwlock) -> acquired::Bool`	Try to take reader lock. Return `true` on success.
`unlock_read(rwlock)`	Unlock the reader lock taken with a preceding invocation of `lock_read`.