Implement condition variable in worker pool

- Added a condition variable to the Pool struct to manage the pool.
- Created a NewPool function to initialize the pool with the condition variable.
- Modified the Submit method to wait if the pool is at capacity before adding a new worker.
- Added a signal to the worker's run method to notify the condition variable when a worker is done.
- Added a broadcast in the Pool's Release method to notify all waiting goroutines when the pool is released.

Signed-off-by: Daniel Hu <tao.hu@merico.dev>

pool.go

@@ -13,26 +13,37 @@ type Pool struct {
 	workers WorkerStack
 	// lock represents a mutex to protect the workers slice
 	lock sync.Mutex
+	// cond represents a condition variable to manage the pool
+	cond     *sync.Cond
+}
+
+func NewPool(capacity int) *Pool {
+	p := &Pool{
+		capacity: capacity,
+		workers:  new(workerStack),
+	}
+	p.cond = sync.NewCond(&p.lock)
+	return p
 }
 
 func (p *Pool) Submit(task Task) {
 	p.lock.Lock()
 	defer p.lock.Unlock()
 
-	if p.running < p.capacity {
-		worker := p.workers.Pop()
-		if worker == nil {
-			worker = &Worker{
-				pool: p,
-				task: make(chan Task),
-			}
+	for p.running == p.capacity {
+		p.cond.Wait()
+	}
+
+	worker := p.workers.Pop()
+	if worker == nil {
+		worker = &Worker{
+			pool: p,
+			task: make(chan Task),
 		}
-		worker.task <- task
-		worker.run()
-		p.running++
-	} else {
-		// TODO: handle the case when the pool is full
 	}
+	worker.task <- task
+	worker.run()
+	p.running++
 }
 
 func (p *Pool) Running() int {
@@ -52,4 +63,5 @@ func (p *Pool) Release() {
 		close(worker.task)
 	}
 	p.running = 0
+	p.cond.Broadcast()
 }

worker.go

@@ -25,6 +25,7 @@ func (w *Worker) run() {
 			w.pool.lock.Lock()
 			w.pool.workers.Push(w)
 			w.pool.running--
+			w.pool.cond.Signal()
 			w.pool.lock.Unlock()
 		}
 	}()