===== sync.WaitGroup =====

==== What is it? ====
`sync.WaitGroup` is a counter-based synchronization primitive used to wait for a set of goroutines to finish.

==== What is it used for? ====
  * Start N goroutines and wait until all of them are done.
  * Coordinate parallel tasks (fan-out) and join results (fan-in).

==== When to use ====
  * You want to block until all spawned goroutines complete.
  * You do NOT need cancellation/timeouts (if you do, combine with `context`).

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

import (
    "fmt"
    "sync"
)

func main() {
    var wg sync.WaitGroup

    for i := 0; i < 3; i++ {
        wg.Add(1)
        go func(id int) {
            defer wg.Done()
            fmt.Println("worker:", id)
        }(i)
    }

    wg.Wait()
    fmt.Println("all done")
}
</code>

==== Notes / pitfalls ====
  * Call `Add(n)` before starting goroutines (common best practice).
  * Each goroutine must call `Done()` exactly once.
  * Do NOT copy a `WaitGroup` after first use.
  * Avoid calling `Add()` concurrently with `Wait()` unless you really know what you’re doing.

==== Related ====
  * [[go:concurrency:goroutine|Goroutine]]
  * [[go:concurrency:context|context.Context]]
  * [[go:concurrency:errgroup|errgroup]]

==== Hard words (English) ====
  * **counter** /ˈkaʊntər/: bộ đếm
  * **coordinate** /koʊˈɔːrdɪneɪt/: điều phối
  * **fan-out** /ˈfæn aʊt/: tỏa ra nhiều nhánh xử lý
  * **fan-in** /ˈfæn ɪn/: gom nhiều nhánh về một
  * **cancellation** /ˌkænsəˈleɪʃən/: huỷ