Remove requirement for custom RPCClient (#5405)

This change is a simplification over existing
code since it is not required to have a separate
RPCClient structure instead keep authRPCClient can
do the same job.

There is no code which directly uses netRPCClient(),
keeping authRPCClient is better and simpler. This
simplication also allows for removal of multiple
levels of locking code per object.

Observed in #5160

cmd/auth-rpc-client_test.go

@@ -16,7 +16,12 @@
 
 package cmd
 
-import "testing"
+import (
+	"crypto/x509"
+	"os"
+	"path"
+	"testing"
+)
 
 // Tests authorized RPC client.
 func TestAuthRPCClient(t *testing.T) {
@@ -53,3 +58,81 @@ func TestAuthRPCClient(t *testing.T) {
 		t.Fatalf("Unexpected node value %s, but expected %s", authRPC.ServiceEndpoint(), authCfg.serviceEndpoint)
 	}
 }
+
+// Test rpc dial test.
+func TestRPCDial(t *testing.T) {
+	prevRootCAs := globalRootCAs
+	defer func() {
+		globalRootCAs = prevRootCAs
+	}()
+
+	rootPath, err := newTestConfig(globalMinioDefaultRegion)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(rootPath)
+
+	testServer := StartTestServer(t, "")
+	defer testServer.Stop()
+
+	cert, key, err := generateTLSCertKey("127.0.0.1")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Set global root CAs.
+	globalRootCAs = x509.NewCertPool()
+	globalRootCAs.AppendCertsFromPEM(cert)
+
+	testServerTLS := StartTestTLSServer(t, "", cert, key)
+	defer testServerTLS.Stop()
+
+	adminEndpoint := path.Join(minioReservedBucketPath, adminPath)
+	testCases := []struct {
+		serverAddr     string
+		serverEndpoint string
+		success        bool
+		secure         bool
+	}{
+		// Empty server addr should fail.
+		{
+			serverAddr:     "",
+			serverEndpoint: adminEndpoint,
+			success:        false,
+		},
+		// Unexpected server addr should fail.
+		{
+			serverAddr:     "example.com",
+			serverEndpoint: adminEndpoint,
+			success:        false,
+		},
+		// Server addr connects but fails for CONNECT call.
+		{
+			serverAddr:     "example.com:80",
+			serverEndpoint: "/",
+			success:        false,
+		},
+		// Successful connecting to insecure RPC server.
+		{
+			serverAddr:     testServer.Server.Listener.Addr().String(),
+			serverEndpoint: adminEndpoint,
+			success:        true,
+		},
+		// Successful connecting to secure RPC server.
+		{
+			serverAddr:     testServerTLS.Server.Listener.Addr().String(),
+			serverEndpoint: adminEndpoint,
+			success:        true,
+			secure:         true,
+		},
+	}
+	for i, testCase := range testCases {
+		_, err = rpcDial(testCase.serverAddr, testCase.serverEndpoint, testCase.secure)
+		if err != nil && testCase.success {
+			t.Errorf("Test %d: Expected success but found failure instead %s", i+1, err)
+		}
+		if err == nil && !testCase.success {
+			t.Errorf("Test %d: Expected failure but found success instead", i+1)
+		}
+	}
+}