context_cancel_example.go

type runner struct {}

func (r *runner) run(ctx context.Context) error {  
  g, gctx := errgroup.WithContext(ctx)
  // loads of goroutines
  g.Go(func() error { 
    // something something, then 
    return context.Cause(gctx)
  })
  return g.Wait()
}

func runStuff(ctx context.Context) error {
  r := &runner{}
  ctx, cancel := context.CancelWithCause(ctx)
  errChan := make(chan error)
  errGuardError := errors.New("this is the error i want")
  go func () { errChan <- r.run(ctx) }()
  go func () { 
    time.Sleep(time.Second)
    cancel(errGuardError)
  }()
  err := <-errChan
  if errors.Is(err, errGuardError) { 
    // ????
    err = nil
  }
  return err
}