Goroutine Refers to Enclosing Function Variable

Why This Happens

When a goroutine captures a local variable from its enclosing function, and the function returns before the goroutine finishes, the variable could be modified or reclaimed. While Go's escape analysis usually handles this, it leads to confusing bugs when the variable changes unexpectedly.

The Problem

func startWorkers() {
    for i := 0; i < 5; i++ {
        go func() {
            fmt.Println(i) // always prints 5
        }()
    }
}

The Fix

func startWorkers() {
    for i := 0; i < 5; i++ {
        go func(n int) {
            fmt.Println(n) // prints 0, 1, 2, 3, 4
        }(i)
    }
}

Step-by-Step Fix

1. 1

   Identify captured variables

   Look for goroutines that reference variables from the enclosing scope.

2. 2

   Pass as parameter

   Pass the variable as an argument to the goroutine function to create an independent copy.

3. 3

   Or wait for completion

   Use sync.WaitGroup to ensure the goroutine completes before the enclosing function returns.