TryAcquire created resource in background instead of blocking

refs #14

pool.go

@@ -265,10 +265,30 @@ func (p *Pool) Acquire(ctx context.Context) (*Resource, error) {
 	return p.doAcquire(ctx, true)
 }
 
-// TryAcquire gets a resource from the pool. TryAcquire is the same as Acquire except
-// it returns ErrNotAvailable if the pool is at maximum capacity and no resources are available.
+// TryAcquire gets a resource from the pool if one is immediately available. If not, it returns ErrNotAvailable. If no
+// resources are available but the pool has room to grow, a resource will be created in the background. ctx is only
+// used to cancel the background creation.
 func (p *Pool) TryAcquire(ctx context.Context) (*Resource, error) {
-	return p.doAcquire(ctx, false)
+	p.cond.L.Lock()
+	defer p.cond.L.Unlock()
+
+	if p.closed {
+		return nil, ErrClosedPool
+	}
+
+	// If a resource is available now
+	if len(p.idleResources) > 0 {
+		res := p.idleResources[len(p.idleResources)-1]
+		p.idleResources[len(p.idleResources)-1] = nil // Avoid memory leak
+		p.idleResources = p.idleResources[:len(p.idleResources)-1]
+		p.acquireCount += 1
+		res.status = resourceStatusAcquired
+		return res, nil
+	}
+
+	go p.CreateResource(ctx)
+
+	return nil, ErrNotAvailable
 }
 
 // doAcquire implements shared logic behind Acquire and TryAcquire. If block is true

pool_test.go

@@ -175,14 +175,21 @@ func TestPoolAcquireReusesResources(t *testing.T) {
 	assert.Equal(t, 1, createCounter.Value())
 }
 
-func TestPoolTryAcquireDoesNotBlock(t *testing.T) {
+func TestPoolTryAcquire(t *testing.T) {
 	constructor, createCounter := createConstructor()
 	pool := puddle.NewPool(constructor, stubDestructor, 1)
 
+	// Pool is initially empty so TryAcquire fails but starts construction of resource in the background.
 	res, err := pool.TryAcquire(context.Background())
+	require.EqualError(t, err, puddle.ErrNotAvailable.Error())
+	assert.Nil(t, res)
+
+	// Wait for background creation to complete.
+	time.Sleep(100 * time.Millisecond)
+
+	res, err = pool.TryAcquire(context.Background())
 	require.NoError(t, err)
 	assert.Equal(t, 1, res.Value())
-
 	defer res.Release()
 
 	res, err = pool.TryAcquire(context.Background())