Defer in Loop — Resource Leak

Why This Happens

The defer statement schedules a function call to run when the surrounding function returns, not when the loop iteration ends. If you open files or connections in a loop and defer their close, all resources stay open until the function exits. This can exhaust file descriptors or connections.

The Problem

func processFiles(paths []string) error {
    for _, path := range paths {
        f, err := os.Open(path)
        if err != nil {
            return err
        }
        defer f.Close() // all files stay open until function returns
        // process f
    }
    return nil
}

The Fix

func processFiles(paths []string) error {
    for _, path := range paths {
        if err := processOneFile(path); err != nil {
            return err
        }
    }
    return nil
}

func processOneFile(path string) error {
    f, err := os.Open(path)
    if err != nil {
        return err
    }
    defer f.Close() // closes at end of this function
    // process f
    return nil
}

Step-by-Step Fix

1. 1

   Identify the defer in loop

   Find the defer call inside a for loop that opens resources.

2. 2

   Extract to a function

   Move the loop body into a separate function so defer runs at the end of each iteration.

3. 3

   Verify resource cleanup

   Test that resources are properly closed after each iteration, not accumulated.