Sync WaitGroup not waiting for goroutine to assign pointer - go

Question

Say I have the following code (playground):
package main
import (
"fmt"
"sync"
)
func createStr() *string {
tmp := "foo"
return &tmp
}
func main() {
var (
s *string
wg sync.WaitGroup
)
go func() {
wg.Add(1)
defer wg.Done()
s = createStr()
}()
wg.Wait()
fmt.Printf("s after: %v", s)
}
I would have expected s to not equal nil.
However, if I add a small wait, I get s != nil (playground):
package main
import (
"fmt"
"sync"
"time"
)
func createStr() *string {
tmp := "foo"
return &tmp
}
func main() {
var (
s *string
wg sync.WaitGroup
)
go func() {
wg.Add(1)
defer wg.Done()
s = createStr()
}()
wg.Wait()
time.Sleep(time.Second)
fmt.Printf("s after: %v", s)
}
This just caused a bug in a program I wrote. What is happening with sync.WaitGroup that's not causing my program to wait for s to be assigned a string pointer in my go func?

Answer 1

Your placement of Add method for sync.WaitGroup is wrong. Do not use Add inside a goroutine (anonymous goroutine here) but use it in the goroutine (main goroutine here) that's going to wait for it.
A probable situation that was happening in your code was that wg.Wait() didn't wait as the goroutine's wg.Add(1) wasn't called yet and hence s == nil. The following code fixes the issue:
Go Playground
package main
import (
"fmt"
"sync"
)
func createStr() *string {
tmp := "foo"
return &tmp
}
func main() {
var (
s *string
wg sync.WaitGroup
)
// Use wg.Add() here
wg.Add(1)
go func() {
defer wg.Done()
s = createStr()
}()
wg.Wait()
fmt.Printf("s after: %v", *s)
}