Fix connect leak from clientpool

It's also fixed upstream in thrift and will be part of thrift 0.16.0,
but the double fix is harmless and will help us when it's used on older
thrift versions.

clientpool/channel.go

@@ -60,6 +60,14 @@ func (cp *channelPool) Get() (client Client, err error) {
 		if c.IsOpen() {
 			return c, nil
 		}
+		// For thrift connections, IsOpen could return false in both explicit and
+		// implicit closed situations.
+		// In implicit closed situation, IsOpen does a connectivity check and
+		// returns false if that check fails. In such case we should still close the
+		// connection explicitly to avoid resource leak.
+		// In explicit situation, calling Close again will just return an already
+		// closed error, which is harmless here.
+		c.Close()
 	default:
 	}
 
@@ -85,6 +93,11 @@ func (cp *channelPool) Release(c Client) error {
 	defer atomic.AddInt32(&cp.numActive, -1)
 
 	if !c.IsOpen() {
+		// Even when c.IsOpen reported false, still call Close explicitly to avoid
+		// connection leaks. At worst case scenario it just returns an already
+		// closed error, which is still harmless.
+		c.Close()
+
 		newC, err := cp.opener()
 		if err != nil {
 			return err