===== sync.Once =====

==== What is it? ====
`sync.Once` ensures that a function is executed only once, even when called by multiple goroutines concurrently.

==== What is it used for? ====
  * One-time initialization (lazy init), e.g., create a singleton, load config once.
  * Safe "initialize-on-first-use" without manual locking.

==== When to use ====
  * You need exactly-once initialization across goroutines.
  * The initialization must happen at most once, not zero-or-more.

==== Example ====
<code go>
package main

import (
    "fmt"
    "sync"
)

var once sync.Once
var value string

func initValue() {
    value = "initialized"
}

func GetValue() string {
    once.Do(initValue)
    return value
}

func main() {
    fmt.Println(GetValue())
}
</code>

==== Notes / pitfalls ====
  * If the function passed to `Do()` panics, `Once` is considered "done" (it will not run again).
  * Use `Once` for initialization only; do not use it as a general lock.

==== Related ====
  * [[go:concurrency:mutex|sync.Mutex]]
  * [[go:concurrency:waitgroup|sync.WaitGroup]]

==== Hard words (English) ====
  * **ensure** /ɪnˈʃʊr/: đảm bảo
  * **executed** /ˈeksɪkjuːtɪd/: được thực thi
  * **initialization** /ɪˌnɪʃələˈzeɪʃən/: khởi tạo
  * **singleton** /ˈsɪŋɡəlˌtən/: đối tượng duy nhất (một instance)
  * **lazy** /ˈleɪzi/: trì hoãn (chỉ làm khi cần)
  * **panic** /ˈpænɪk/: lỗi runtime kiểu “panic”