Implement HTTP POST based RPC (#5840)

Added support for new RPC support using HTTP POST.  RPC's 
arguments and reply are Gob encoded and sent as HTTP 
request/response body.

This patch also removes Go RPC based implementation.

cmd/rpc/client.go

@@ -0,0 +1,128 @@
+/*
+ * Minio Cloud Storage, (C) 2018 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 rpc
+
+import (
+	"bytes"
+	"context"
+	"crypto/tls"
+	"encoding/gob"
+	"errors"
+	"fmt"
+	"net"
+	"net/http"
+	"reflect"
+	"time"
+
+	xhttp "github.com/minio/minio/cmd/http"
+	xnet "github.com/minio/minio/pkg/net"
+)
+
+// DefaultRPCTimeout - default RPC timeout is one minute.
+const DefaultRPCTimeout = 1 * time.Minute
+
+// Client - http based RPC client.
+type Client struct {
+	httpClient *http.Client
+	serviceURL *xnet.URL
+}
+
+// Call - calls service method on RPC server.
+func (client *Client) Call(serviceMethod string, args, reply interface{}) error {
+	replyKind := reflect.TypeOf(reply).Kind()
+	if replyKind != reflect.Ptr {
+		return fmt.Errorf("rpc reply must be a pointer type, but found %v", replyKind)
+	}
+
+	data, err := gobEncode(args)
+	if err != nil {
+		return err
+	}
+
+	callRequest := CallRequest{
+		Method:   serviceMethod,
+		ArgBytes: data,
+	}
+
+	var buf bytes.Buffer
+	if err = gob.NewEncoder(&buf).Encode(callRequest); err != nil {
+		return err
+	}
+
+	response, err := client.httpClient.Post(client.serviceURL.String(), "", &buf)
+	if err != nil {
+		return err
+	}
+	defer response.Body.Close()
+
+	if response.StatusCode != http.StatusOK {
+		return fmt.Errorf("%v rpc call failed with error code %v", serviceMethod, response.StatusCode)
+	}
+
+	var callResponse CallResponse
+	if err := gob.NewDecoder(response.Body).Decode(&callResponse); err != nil {
+		return err
+	}
+
+	if callResponse.Error != "" {
+		return errors.New(callResponse.Error)
+	}
+
+	return gobDecode(callResponse.ReplyBytes, reply)
+}
+
+// Close - does nothing and presents for interface compatibility.
+func (client *Client) Close() error {
+	return nil
+}
+
+func newCustomDialContext(timeout time.Duration) func(ctx context.Context, network, addr string) (net.Conn, error) {
+	return func(ctx context.Context, network, addr string) (net.Conn, error) {
+		dialer := &net.Dialer{
+			Timeout:   timeout,
+			KeepAlive: timeout,
+			DualStack: true,
+		}
+
+		conn, err := dialer.DialContext(ctx, network, addr)
+		if err != nil {
+			return nil, err
+		}
+
+		return xhttp.NewTimeoutConn(conn, timeout, timeout), nil
+	}
+}
+
+// NewClient - returns new RPC client.
+func NewClient(serviceURL *xnet.URL, tlsConfig *tls.Config, timeout time.Duration) *Client {
+	return &Client{
+		httpClient: &http.Client{
+			// Transport is exactly same as Go default in https://golang.org/pkg/net/http/#RoundTripper
+			// except custom DialContext and TLSClientConfig.
+			Transport: &http.Transport{
+				Proxy:                 http.ProxyFromEnvironment,
+				DialContext:           newCustomDialContext(timeout),
+				MaxIdleConns:          100,
+				IdleConnTimeout:       90 * time.Second,
+				TLSHandshakeTimeout:   10 * time.Second,
+				ExpectContinueTimeout: 1 * time.Second,
+				TLSClientConfig:       tlsConfig,
+			},
+		},
+		serviceURL: serviceURL,
+	}
+}