golang_sync_Once_TryDo

package internal

import (
    "sync"
    "sync/atomic"
)

// Once is an object that will perform exactly one action unless the action fails.
type Once struct {
    // 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/x86),
    // and fewer instructions (to calculate offset) on other architectures.
    done uint32
    m    sync.Mutex
}

// TryDo calls the function f if and only if TryDo is being called for the
// first time for this instance of Once. In other words, given
//  var once Once
// if once.TryDo(f) is called multiple times, only the first call will invoke f,
// unless the invocation results in an error A new instance of
// Once is required for each function to execute.
//
// TryDo 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.TryDo(func() error { return config.init(filename) })
//
// Because no call to TryDo returns until the one call to f returns, if f causes
// TryDo to be called, it will deadlock.
//
// If f panics, TryDo considers it to have failed.
//
func (o *Once) TryDo(f func() error) error {
    // Note: Here is an incorrect implementation of Do:
    //
    //  if atomic.CompareAndSwapUint32(&o.done, 0, 1) {
    //      f()
    //  }
    //
    // TryDo 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.

    if atomic.LoadUint32(&o.done) == 0 {
        // Outlined slow-path to allow inlining of the fast-path.
        return o.doSlow(f)
    }

    return nil
}

func (o *Once) doSlow(f func() error) error {
    o.m.Lock()
    defer o.m.Unlock()
    if o.done == 0 {
        if err := f(); err != nil {
            return err
        }
        defer atomic.StoreUint32(&o.done, 1)
    }
    return nil
}