Add connection pool queuing strategies in HostClient. (#1238)

* At first, we implement LIFO(conventional) and FIFO which indicate in #1236
* Change default strategy FIFO

client.go

@@ -618,6 +618,14 @@ type RetryIfFunc func(request *Request) bool
 // TransportFunc wraps every request/response.
 type TransportFunc func(*Request, *Response) error
 
+// ConnPoolStrategyType define strategy of connection pool enqueue/dequeue
+type ConnPoolStrategyType int
+
+const (
+	FIFO ConnPoolStrategyType = iota
+	LIFO
+)
+
 // HostClient balances http requests among hosts listed in Addr.
 //
 // HostClient may be used for balancing load among multiple upstream hosts.
@@ -772,6 +780,9 @@ type HostClient struct {
 	// Transport defines a transport-like mechanism that wraps every request/response.
 	Transport TransportFunc
 
+	// Connection pool strategy. Can be either LIFO or FIFO (default).
+	ConnPoolStrategy ConnPoolStrategyType
+
 	clientName  atomic.Value
 	lastUseTime uint32
 
@@ -1497,6 +1508,10 @@ var (
 	// to broken server.
 	ErrConnectionClosed = errors.New("the server closed connection before returning the first response byte. " +
 		"Make sure the server returns 'Connection: close' response header before closing the connection")
+
+	// ErrConnPoolStrategyNotImpl is returned when HostClient.ConnPoolStrategy is not implement yet.
+	// If you see this error, then you need to check your HostClient configuration.
+	ErrConnPoolStrategyNotImpl = errors.New("connection pool strategy is not implement")
 )
 
 type timeoutError struct{}
@@ -1544,10 +1559,20 @@ func (c *HostClient) acquireConn(reqTimeout time.Duration, connectionClose bool)
 			}
 		}
 	} else {
-		n--
-		cc = c.conns[n]
-		c.conns[n] = nil
-		c.conns = c.conns[:n]
+		switch c.ConnPoolStrategy {
+		case LIFO:
+			n--
+			cc = c.conns[n]
+			c.conns[n] = nil
+			c.conns = c.conns[:n]
+		case FIFO:
+			cc = c.conns[0]
+			copy(c.conns, c.conns[1:])
+			c.conns[n-1] = nil
+			c.conns = c.conns[:n-1]
+		default:
+			return nil, ErrConnPoolStrategyNotImpl
+		}
 	}
 	c.connsLock.Unlock()