Added fifo mutex

sync.go

@@ -554,3 +554,51 @@ func (d *AtomicDuration) Duration() time.Duration {
 	defer d.m.Unlock()
 	return d.d
 }
+
+// FIFOMutex is a mutex guaranteeing FIFO order
+type FIFOMutex struct {
+	busy    bool
+	m       sync.Mutex // Locks busy and waiting
+	waiting []*sync.Cond
+}
+
+func (m *FIFOMutex) Lock() {
+	// No need to wait
+	m.m.Lock()
+	if !m.busy {
+		m.busy = true
+		m.m.Unlock()
+		return
+	}
+
+	// Create cond
+	c := sync.NewCond(&sync.Mutex{})
+
+	// Make sure to lock cond when waiting mutex is still held
+	c.L.Lock()
+
+	// Add to waiting queue
+	m.waiting = append(m.waiting, c)
+	m.m.Unlock()
+
+	// Wait
+	c.Wait()
+}
+
+func (m *FIFOMutex) Unlock() {
+	// Lock
+	m.m.Lock()
+	defer m.m.Unlock()
+
+	// Waiting queue is empty
+	if len(m.waiting) == 0 {
+		m.busy = false
+		return
+	}
+
+	// Signal and remove first item in waiting queue
+	m.waiting[0].L.Lock()
+	m.waiting[0].Signal()
+	m.waiting[0].L.Unlock()
+	m.waiting = m.waiting[1:]
+}

sync_test.go

@@ -192,3 +192,29 @@ func TestDebugMutex(t *testing.T) {
 		t.Fatalf("%s doesn't contain %s", g, s)
 	}
 }
+
+func TestFIFOMutex(t *testing.T) {
+	m := FIFOMutex{}
+	var r []int
+	m.Lock()
+	wg := sync.WaitGroup{}
+	testFIFOMutex(1, &m, &r, &wg)
+	m.Unlock()
+	wg.Wait()
+	if e, g := []int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}, r; !reflect.DeepEqual(e, g) {
+		t.Fatalf("expected %v, got %v", e, g)
+	}
+}
+
+func testFIFOMutex(i int, m *FIFOMutex, r *[]int, wg *sync.WaitGroup) {
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		if i < 10 {
+			testFIFOMutex(i+1, m, r, wg)
+		}
+		m.Lock()
+		*r = append(*r, i)
+		m.Unlock()
+	}()
+}