GODRIVER-2276 Use OP_MSG and hello with loadBalanced

mongo/integration/client_test.go

@@ -651,6 +651,27 @@ func TestClient(t *testing.T) {
 				"expected 'OP_MSG' OpCode in wire message, got %q", pair.Sent.OpCode.String())
 		}
 	})
+
+	// Test that OP_MSG is used for handshakes when loadBalanced is true.
+	opMsgLBOpts := mtest.NewOptions().ClientType(mtest.Proxy).MinServerVersion("5.0").Topologies(mtest.LoadBalanced)
+	mt.RunOpts("OP_MSG used for handshakes when loadBalanced is true", opMsgLBOpts, func(mt *mtest.T) {
+		err := mt.Client.Ping(context.Background(), mtest.PrimaryRp)
+		assert.Nil(mt, err, "Ping error: %v", err)
+
+		msgPairs := mt.GetProxiedMessages()
+		assert.True(mt, len(msgPairs) >= 3, "expected at least 3 events, got %v", len(msgPairs))
+
+		// First three messages should be connection handshakes: one for the heartbeat connection, another for the
+		// application connection, and a final one for the RTT monitor connection.
+		for idx, pair := range msgPairs[:3] {
+			assert.Equal(mt, "hello", pair.CommandName, "expected command name 'hello' at index %d, got %s", idx,
+				pair.CommandName)
+
+			// Assert that appended OpCode is OP_MSG when loadBalanced is true.
+			assert.Equal(mt, wiremessage.OpMsg, pair.Sent.OpCode,
+				"expected 'OP_MSG' OpCode in wire message, got %q", pair.Sent.OpCode.String())
+		}
+	})
 }
 
 func TestClientStress(t *testing.T) {

x/mongo/driver/operation.go

@@ -813,10 +813,10 @@ func (Operation) decompressWireMessage(wm []byte) ([]byte, error) {
 
 func (op Operation) createWireMessage(ctx context.Context, dst []byte,
 	desc description.SelectedServer, conn Connection) ([]byte, startedInformation, error) {
-
-	// If API version is not declared and wire version is unknown or less than 6, use OP_QUERY.
-	// Otherwise, use OP_MSG.
-	if op.ServerAPI == nil && (desc.WireVersion == nil || desc.WireVersion.Max < wiremessage.OpmsgWireVersion) {
+	// If topology is not LoadBalanced, API version is not declared, and wire version is unknown
+	// or less than 6, use OP_QUERY. Otherwise, use OP_MSG.
+	if desc.Kind != description.LoadBalanced && op.ServerAPI == nil &&
+		(desc.WireVersion == nil || desc.WireVersion.Max < wiremessage.OpmsgWireVersion) {
 		return op.createQueryWireMessage(dst, desc)
 	}
 	return op.createMsgWireMessage(ctx, dst, desc, conn)

x/mongo/driver/operation/hello.go

@@ -159,7 +159,9 @@ func (h *Hello) handshakeCommand(dst []byte, desc description.SelectedServer) ([
 
 // command appends all necessary command fields.
 func (h *Hello) command(dst []byte, desc description.SelectedServer) ([]byte, error) {
-	if h.serverAPI != nil || desc.Server.HelloOK {
+	// Use "hello" if topology is LoadBalanced, API version is declared or server
+	// has responded with "helloOk". Otherwise, use legacy hello.
+	if desc.Kind == description.LoadBalanced || h.serverAPI != nil || desc.Server.HelloOK {
 		dst = bsoncore.AppendInt32Element(dst, "hello", 1)
 	} else {
 		dst = bsoncore.AppendInt32Element(dst, internal.LegacyHello, 1)