// Copyright 2021 PingCAP, Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.

package once

import (
	"sync"
	"sync/atomic"
)

type Once struct {
	done uint32
	m    sync.Mutex
}

func (o *Once) Do(f func() error) error {
	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
		}
		atomic.StoreUint32(&o.done, 1)
	}
	return nil
}