Drain response body properly for http connection pool (#6415)

Currently Go http connection pool was not being properly
utilized leading to degrading performance as the number
of concurrent requests increased.

As recommended by Go implementation, we have to drain the
response body and close it.

cmd/utils.go

@@ -25,12 +25,14 @@ import (
 	"encoding/xml"
 	"fmt"
 	"io"
+	"io/ioutil"
 	"net"
 	"net/http"
 	"net/url"
 	"os"
 	"reflect"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/minio/minio/cmd/logger"
@@ -381,3 +383,32 @@ func isNetworkOrHostDown(err error) bool {
 	}
 	return false
 }
+
+var b512pool = sync.Pool{
+	New: func() interface{} {
+		buf := make([]byte, 512)
+		return &buf
+	},
+}
+
+// CloseResponse close non nil response with any response Body.
+// convenient wrapper to drain any remaining data on response body.
+//
+// Subsequently this allows golang http RoundTripper
+// to re-use the same connection for future requests.
+func CloseResponse(respBody io.ReadCloser) {
+	// Callers should close resp.Body when done reading from it.
+	// If resp.Body is not closed, the Client's underlying RoundTripper
+	// (typically Transport) may not be able to re-use a persistent TCP
+	// connection to the server for a subsequent "keep-alive" request.
+	if respBody != nil {
+		// Drain any remaining Body and then close the connection.
+		// Without this closing connection would disallow re-using
+		// the same connection for future uses.
+		//  - http://stackoverflow.com/a/17961593/4465767
+		bufp := b512pool.Get().(*[]byte)
+		defer b512pool.Put(bufp)
+		io.CopyBuffer(ioutil.Discard, respBody, *bufp)
+		respBody.Close()
+	}
+}