Copyright 2009 The Go Authors. All rights reserved. Use of this source code is governed by a BSD-style license that can be found in the LICENSE file.

package sync

import (
	
)
Once is an object that will perform exactly one action. A Once must not be copied after first use.
done indicates whether the action has been performed. It is first in the struct because it is used in the hot path. The hot path is inlined at every call site. Placing done first allows more compact instructions on some architectures (amd64/386), and fewer instructions (to calculate offset) on other architectures.
	done uint32
	m    Mutex
}
Do calls the function f if and only if Do is being called for the first time for this instance of Once. In other words, given var once Once if once.Do(f) is called multiple times, only the first call will invoke f, even if f has a different value in each invocation. A new instance of Once is required for each function to execute. Do is intended for initialization that must be run exactly once. Since f is niladic, it may be necessary to use a function literal to capture the arguments to a function to be invoked by Do: config.once.Do(func() { config.init(filename) }) Because no call to Do returns until the one call to f returns, if f causes Do to be called, it will deadlock. If f panics, Do considers it to have returned; future calls of Do return without calling f.
Note: Here is an incorrect implementation of Do: if atomic.CompareAndSwapUint32(&o.done, 0, 1) { f() } Do guarantees that when it returns, f has finished. This implementation would not implement that guarantee: given two simultaneous calls, the winner of the cas would call f, and the second would return immediately, without waiting for the first's call to f to complete. This is why the slow path falls back to a mutex, and why the atomic.StoreUint32 must be delayed until after f returns.
Outlined slow-path to allow inlining of the fast-path.
		.doSlow()
	}
}

func ( *Once) ( func()) {
	.m.Lock()
	defer .m.Unlock()
	if .done == 0 {
		defer atomic.StoreUint32(&.done, 1)
		()
	}