A clever way to limit the number of goroutine workers.

slots.go

package main

import (
	"context"
	"fmt"
	"sync"
	"time"
)

func f(ctx context.Context, xs []int) {
	var wg sync.WaitGroup
	maxGoroutines := 3
	guard := make(chan struct{}, maxGoroutines)

	for _, x := range xs {
		guard <- struct{}{} // Acquire a slot.
		wg.Add(1)
		go func(x int) {
			defer wg.Done()
			defer func() { <-guard }() // Release the slot.

			select {
			case <-ctx.Done():
				return
			default:
				fmt.Printf("[X] Sleeping: x=%d\n", x)
				time.Sleep(3 * time.Second)
			}
		}(x)
	}

	wg.Wait()
}

func main() {
	f(context.Background(), []int{0, 1, 2, 3, 4, 5, 6, 7})
}