Go defer inside a loop doesn't run until the function returns

A classic Go gotcha: defer in a loop defers until the surrounding function returns, not until the loop iteration ends.

// BAD: files are not closed after each iteration
for _, filename := range files {
    f, _ := os.Open(filename)
    defer f.Close() // Only runs when the whole function exits!
}

If you open 1,000 files, all 1,000 stay open until the function exits. Under high load this exhausts file descriptors.

Fix 1: Wrap in an anonymous function so defer runs per iteration.

for _, filename := range files {
    func() {
        f, _ := os.Open(filename)
        defer f.Close() // Now runs at end of anonymous func
        process(f)
    }()
}

Fix 2: Just call Close() explicitly without defer.

for _, filename := range files {
    f, _ := os.Open(filename)
    process(f)
    f.Close()
}