pool: gracefully close pool by giving users time to free connection.

pool.go

@@ -66,6 +66,7 @@ func (l *connList) Add(cn *conn) {
 	l.mx.Unlock()
 }
 
+// Remove closes connection and removes it from the list.
 func (l *connList) Remove(cn *conn) error {
 	defer l.mx.Unlock()
 	l.mx.Lock()
@@ -172,8 +173,8 @@ func (p *connPool) First() *conn {
 }
 
 // wait waits for free non-idle connection. It returns nil on timeout.
-func (p *connPool) wait(timeout time.Duration) *conn {
-	deadline := time.After(timeout)
+func (p *connPool) wait() *conn {
+	deadline := time.After(p.opt.PoolTimeout)
 	for {
 		select {
 		case cn := <-p.freeConns:
@@ -208,14 +209,13 @@ func (p *connPool) new() (*conn, error) {
 	return cn, nil
 }
 
-// Get returns existed connection from the pool or creates a new one
-// if needed.
+// Get returns existed connection from the pool or creates a new one.
 func (p *connPool) Get() (*conn, error) {
 	if p.closed() {
 		return nil, errClosed
 	}
 
-	// Fetch first non-idle connection, if available
+	// Fetch first non-idle connection, if available.
 	if cn := p.First(); cn != nil {
 		return cn, nil
 	}
@@ -231,8 +231,8 @@ func (p *connPool) Get() (*conn, error) {
 		return cn, nil
 	}
 
-	// Otherwise, wait for the available connection
-	if cn := p.wait(p.opt.PoolTimeout); cn != nil {
+	// Otherwise, wait for the available connection.
+	if cn := p.wait(); cn != nil {
 		return cn, nil
 	}
 
@@ -277,11 +277,25 @@ func (p *connPool) FreeLen() int {
 	return len(p.freeConns)
 }
 
-func (p *connPool) Close() error {
+func (p *connPool) Close() (retErr error) {
 	if !atomic.CompareAndSwapInt32(&p._closed, 0, 1) {
 		return errClosed
 	}
-	return p.conns.Close()
+	// First close free connections.
+	for p.Len() > 0 {
+		cn := p.wait()
+		if cn == nil {
+			break
+		}
+		if err := p.conns.Remove(cn); err != nil {
+			retErr = err
+		}
+	}
+	// Then close the rest.
+	if err := p.conns.Close(); err != nil {
+		retErr = err
+	}
+	return retErr
 }
 
 func (p *connPool) reaper() {