Let’s expand on this example and show both sides of the equation: a goroutine that is waiting for a signal, and a goroutine that is sending signals. Say we have a queue of fixed length 2, and 10 items we want to push onto the queue. We want to enqueue items as soon as there is room, so we want to be notified as soon as there’s room in the queue

queue.go

package main

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

func main() {
	c := sync.NewCond(&sync.Mutex{})
	queue := make([]interface{}, 0, 10)
	removeFromQueue := func(delay time.Duration) {
		time.Sleep(delay)
		c.L.Lock()
		queue = queue[1:]
		fmt.Println("Removed from queue")
		c.L.Unlock()
		c.Signal()
	}
	for i := 0; i < 10; i++ {
		c.L.Lock()
		for len(queue) == 2 {
			c.Wait()
		}
		fmt.Println("Adding to queue")
		queue = append(queue, struct{}{})
		go removeFromQueue(1 * time.Second)
		c.L.Unlock()
	}
}