How do you use a buffered channel as a semaphore to limit goroutine concurrency?

A buffered channel of capacity N acts as a counting semaphore — at most N goroutines can be in a critical section simultaneously. This is a simple, idiomatic Go pattern for throttling concurrent HTTP requests, database queries, or any resource-bounded operation.

// Semaphore: limit to 10 concurrent HTTP fetches
func fetchAll(ctx context.Context, urls []string) []string {
    const maxConcurrent = 10
    sem := make(chan struct{}, maxConcurrent)

    results := make([]string, len(urls))
    var wg sync.WaitGroup

    for i, url := range urls {
        wg.Add(1)
        go func(idx int, u string) {
            defer wg.Done()

            // Acquire a slot (blocks if 10 goroutines are already running)
            select {
            case sem <- struct{}{}:
            case <-ctx.Done(): return
            }
            defer func() { <-sem }() // release slot on exit

            results[idx] = fetch(ctx, u)
        }(i, url)
    }

    wg.Wait()
    return results
}

// Why struct{} and not bool or int?
// struct{} is zero-size — no memory allocated for the value itself

// golang.org/x/sync/semaphore: weighted semaphore for varying costs
sem := semaphore.NewWeighted(10)
if err := sem.Acquire(ctx, 1); err != nil { return err }
defer sem.Release(1)

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