Defer in a loop does not execute until the function returns

Placing defer inside a for loop defers the call until the enclosing function returns, not until the end of each loop iteration. Files or locks acquired in the loop body are held for the entire function lifetime.

Wrap the loop body in an anonymous function so defer fires at the end of each iteration:

// WRONG — all files stay open until openAll() returns
func openAll(paths []string) {
    for _, p := range paths {
        f, _ := os.Open(p)
        defer f.Close() // deferred to function exit, not iteration end
        process(f)
    }
}

// RIGHT — file closed after each iteration
func openAll(paths []string) {
    for _, p := range paths {
        func() {
            f, _ := os.Open(p)
            defer f.Close()
            process(f)
        }()
    }
}

defer pushes a call onto the current function's deferred call stack, not the loop's. The stack is flushed in LIFO order when the function returns or panics.