How does sync.WaitGroup work and what are the most common mistakes?

// CORRECT usage pattern
var wg sync.WaitGroup

urls := []string{"http://a.com", "http://b.com", "http://c.com"}
for _, url := range urls {
    wg.Add(1)               // Add BEFORE launching — not inside the goroutine
    go func(u string) {
        defer wg.Done()     // defer ensures Done fires even on panic
        fetchURL(u)
    }(url)                  // Pass url as argument (avoids closure trap)
}
wg.Wait()                   // blocks until counter reaches zero

// MISTAKE 1: Add inside the goroutine → race with Wait
// go func() {
//     wg.Add(1)   // may execute AFTER wg.Wait() — already zero!
//     defer wg.Done()
// }()
// wg.Wait()  // may return before any goroutine calls Add

// MISTAKE 2: Passing WaitGroup by value
// processFiles(wg)   // WRONG: copies WaitGroup — broken
// processFiles(&wg)  // CORRECT: pass pointer

// MISTAKE 3: Calling Done() more times than Add() → panic
// wg.Done() // if counter is already zero, panics

// PATTERN: Add all at once when count is known
wg.Add(len(urls))
for _, url := range urls {
    go func(u string) {
        defer wg.Done()
        fetchURL(u)
    }(url)
}
wg.Wait()