Make sure to drain body upon an error (#7197)

Also cleanup redundant code and use it at a common place

cmd/http/close.go

@@ -0,0 +1,52 @@
+/*
+ * Minio Cloud Storage, (C) 2019 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package http
+
+import (
+	"io"
+	"io/ioutil"
+	"sync"
+)
+
+var b512pool = sync.Pool{
+	New: func() interface{} {
+		buf := make([]byte, 512)
+		return &buf
+	},
+}
+
+// DrainBody 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 DrainBody(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()
+	}
+}