WaitGroups

Use sync.WaitGroup to wait for a collection of goroutines to finish.

A sync.WaitGroup counts tasks and lets one goroutine wait until the count reaches zero. Add before launching each goroutine, call Done when it finishes, and call Wait to block.

When passing a wait group to another function, pass it by pointer. Defer Done so it runs when the worker returns.

package main
 
import (
    "fmt"
    "sync"
)
 
func main() {
    var wg sync.WaitGroup
    for i := 1; i <= 3; i++ {
        wg.Add(1)
        go func(id int) {
            defer wg.Done()
            fmt.Println("worker", id)
        }(i)
    }
    wg.Wait()
    fmt.Println("all done")
}

In production

Do not copy a WaitGroup after its first use. Add a positive count before starting a goroutine when the counter may be zero, otherwise Wait can return before the task is registered. Done runs during panic unwinding, but it does not recover the panic.

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