Add canceled acquire counting

pool.go

@@ -64,8 +64,9 @@ type Pool struct {
 	destructor  Destructor
 	maxSize     int
 
-	acquireCount     int64
-	slowAcquireCount int64
+	acquireCount         int64
+	slowAcquireCount     int64
+	canceledAcquireCount int64
 
 	closed bool
 }
@@ -106,6 +107,7 @@ type Stat struct {
 	maxResources          int
 	acquireCount          int64
 	slowAcquireCount      int64
+	canceledAcquireCount  int64
 }
 
 // TotalResource returns the total number of resources in the pool.
@@ -145,13 +147,20 @@ func (s *Stat) SlowAcquireCount() int64 {
 	return s.slowAcquireCount
 }
 
+// CanceledAcquireCount returns the number of acquires from the pool
+// that were canceled by a context.
+func (s *Stat) CanceledAcquireCount() int64 {
+	return s.canceledAcquireCount
+}
+
 // Stat returns the current pool statistics.
 func (p *Pool) Stat() *Stat {
 	p.cond.L.Lock()
 	s := &Stat{
-		maxResources:     p.maxSize,
-		acquireCount:     p.acquireCount,
-		slowAcquireCount: p.slowAcquireCount,
+		maxResources:         p.maxSize,
+		acquireCount:         p.acquireCount,
+		slowAcquireCount:     p.slowAcquireCount,
+		canceledAcquireCount: p.canceledAcquireCount,
 	}
 
 	for _, res := range p.allResources {
@@ -174,16 +183,18 @@ func (p *Pool) Stat() *Stat {
 // maximum capacity it will block until a resource is available. ctx can be used
 // to cancel the Acquire.
 func (p *Pool) Acquire(ctx context.Context) (*Resource, error) {
+	p.cond.L.Lock()
 	if doneChan := ctx.Done(); doneChan != nil {
 		select {
 		case <-ctx.Done():
+			p.canceledAcquireCount += 1
+			p.cond.L.Unlock()
 			return nil, ctx.Err()
 		default:
 		}
 	}
 
 	slowAcquire := false
-	p.cond.L.Lock()
 
 	for {
 		if p.closed {
@@ -218,6 +229,15 @@ func (p *Pool) Acquire(ctx context.Context) (*Resource, error) {
 			if err != nil {
 				p.allResources = removeResource(p.allResources, res)
 				p.destructWG.Done()
+
+				select {
+				case <-ctx.Done():
+					if err == ctx.Err() {
+						p.canceledAcquireCount += 1
+					}
+				default:
+				}
+
 				p.cond.L.Unlock()
 				return nil, err
 			}
@@ -242,6 +262,8 @@ func (p *Pool) Acquire(ctx context.Context) (*Resource, error) {
 
 			select {
 			case <-ctx.Done():
+				p.canceledAcquireCount += 1
+
 				// Allow goroutine waiting for signal to exit. Re-signal since we couldn't
 				// do anything with it. Another goroutine might be waiting.
 				go func() {