Golang / sync.WaitGroup / data race investigation with nested structs

main.go

package main

import (
  "fmt"
  "sync"
  "time"
)

type Session struct {
  Id string
  wg *sync.WaitGroup
}

type ClientMessage struct {
  Sess *Session
}

func main() {
  doneCh := make(chan any)

  sess := Session{
    Id: "OoOoOoOo",
    wg: new(sync.WaitGroup),
  }

  sess.wg.Add(1)
  go func(sess *Session) {
    for i := 0; i < 3; i++ {
      fmt.Printf("go func(sess *Session): %v -> %d\n", sess, i)
      time.Sleep(500 * time.Millisecond)
    }
    sess.wg.Done()
  }(&sess)

  cm := ClientMessage{
    Sess: &sess,
  }

  sess.wg.Add(1)
  go func(cm *ClientMessage) {
    for i := 0; i < 10; i++ {
      fmt.Println("loop")
      fmt.Printf("go func(cm *ClientMessage): %v -> %d\n", cm.Sess, i)
      fmt.Printf("go func(cm *ClientMessage): %v -> %d\n", cm.Sess.Id, i)
      time.Sleep(1 * time.Second)
    }
    cm.Sess.wg.Done()
  }(&cm)

  cmb := ClientMessage{
    Sess: &sess,
  }

  sess.wg.Add(1)
  go func(cm *ClientMessage) {
    for i := 0; i < 10; i++ {
      fmt.Printf("go func(cm *ClientMessage): %d\n", i)
      fmt.Printf("go func(cm *ClientMessage): %v -> %d\n", cm.Sess, i)
      time.Sleep(1 * time.Second)
    }
    cm.Sess.wg.Done()
  }(&cmb)

  go func(cm *ClientMessage) {
    fmt.Println("func(cm *ClientMessage),1")
    cm.Sess.wg.Wait()
    fmt.Println("func(cm *ClientMessage),2")
    doneCh <- struct{}{}
    fmt.Println("func(cm *ClientMessage),3")
  }(&cm)

  fmt.Println("Done,1")
  <-doneCh
  fmt.Println("Done,3")
}

I found a very interesting data race when using a sync.WaitGroup implace to a nested struct with some gorountines concurrently calling .Done() while another goroutine is waiting on .Wait(). To avoid the race condition, wg must be a pointer and be explicity initialized with new(sync.WaitGroup).