Go ordered semaphore

experiments_test.go

package experiments_test

import (
	"fmt"
	"sync"
	"time"

	. "github.com/onsi/ginkgo"
	. "github.com/onsi/gomega"
)

var _ = Describe("Semaphore", func() {
	var semaphore chan struct{}
	var N, maxConcurrent int

	BeforeEach(func() {
		N = 1000
		maxConcurrent = 20
		semaphore = make(chan struct{}, maxConcurrent)
		//fill the semaphore
		for i := 0; i < maxConcurrent; i++ {
			semaphore <- struct{}{}
		}

	})

	It("should unblock goroutines waiting on the semaphore in FIFO order", func() {
		results := []int{}
		lock := &sync.Mutex{}
		wg := &sync.WaitGroup{}
		wg.Add(N)
		expected := []int{}

		//schedule a lot of work
		for i := 0; i < N; i++ {
			i := i
			go func() {
				semaphore <- struct{}{}
				lock.Lock()
				results = append(results, i)
				lock.Unlock()
				<-semaphore
				wg.Done()
			}()
			time.Sleep(time.Millisecond)
			expected = append(expected, i)
		}

		//start running the work
		for i := 0; i < maxConcurrent; i++ {
			<-semaphore
		}

		wg.Wait()
		for i := 0; i < len(expected); i++ {
			if results[i] != expected[i] {
				fmt.Println("MISMATCH:", results[i], expected[i])
			}
		}
		Ω(results).Should(Equal(expected))
	})
})