Go WaitGroup

External

• https://pkg.go.dev/sync#WaitGroup

Internal

• The sync package
• Goroutines

Overview

A WaitGroup is a counting semaphore that waits for a set of goroutines to finish.

The main goroutine calls Add() to set the number of goroutines to wait for. Then it can block on Wait() until all other goroutines have finished. When each of the counted goroutines runs, they should call Done() when finished, to increment the counting semaphore.

import "sync"

func aFunction(wg *sync.WaitGroup, s string) {
  defer wg.Done()
  fmt.Println(s)
}

...

wg := sync.WaitGroup{}
wg.Add(2)
go aFunction(&wg, "A")
go aFunction(&wg, "B")
wg.Wait()
fmt.Println("main is done")

Result:

B
A
main is done

Note that the WaitGroup instance must be passed to the functions executed on the counted goroutines by pointer, not value, because otherwise a copy of the sync.WaitGroup struct will be made on the secondary goroutine's stack, and the Done() will have no effect on the original struct instance.