// A WaitGroup waits for a collection of GoRoutines to finish.
// The main GoRoutine calls Add to set the number of
// GoRoutines to wait for. Then each of the GoRoutines
// runs and calls Done when finished. At the same time,
// Wait can be used to block until all GoRoutines have finished.
//
// A WaitGroup must not be copied after first use.
//
// In the terminology of the Go memory model, a call to Done
// “synchronizes before” the return of any Wait call that it unblocks.
type WaitGroup struct {
...

package main

import (
	"fmt"
	"time"
)

func printSomething(s string) {
	fmt.Println(s)
}

func main() {
	fruits := []string{
		"Apple",
		"Blackberry",
		"Cucumber",
		"Lemon",
		"Passionfruit",
		"Satsuma",
		"Solanum quitoense",
		"Ugli fruit",
		"Yuzu",
	}

	for i, v := range fruits {
		go printSomething(fmt.Sprintf("%v %v", i, v))
	}

	time.Sleep(1 * time.Second)

	printSomething("This is the second thing to be printed!")
}

8 Yuzu
4 Passionfruit
0 Apple
7 Ugli fruit
2 Cucumber
5 Satsuma
1 Blackberry
3 Lemon
6 Solanum quitoense
This is the second thing to be printed!

package main

import (
	"fmt"
	"sync"
)

func printSomething(wg *sync.WaitGroup, s string) {
	defer wg.Done()

	fmt.Println(s)
}

func main() {
	wg := sync.WaitGroup{}
	wg.Add(9)

	fruits := []string{
		"Apple",
		"Blackberry",
		"Cucumber",
		"Lemon",
		"Passionfruit",
		"Satsuma",
		"Solanum quitoense",
		"Ugli fruit",
		"Yuzu",
	}

	for i, v := range fruits {
		go printSomething(&wg, fmt.Sprintf("%v %v", i, v))
	}

	wg.Wait()

	printSomething(&wg, "This is the second thing to be printed!")
}

8 Yuzu
6 Solanum quitoense
7 Ugli fruit
0 Apple
1 Blackberry
3 Lemon
4 Passionfruit
5 Satsuma
2 Cucumber
This is the second thing to be printed!
panic: sync: negative WaitGroup counter

goroutine 1 [running]:
sync.(*WaitGroup).Add(0xc00007cde8?, 0x28?)
...

wg.Add(1)
printSomething(&wg, "This is the second thing to be printed!")

8 Yuzu
0 Apple
1 Blackberry
2 Cucumber
3 Lemon
4 Passionfruit
5 Satsuma
6 Solanum quitoense
7 Ugli fruit
This is the second thing to be printed!