Pass ProtocolInfo to Init

client_test.go

@@ -64,11 +64,13 @@ func TestStartFailed(t *testing.T) {
 	c.Assert(client.Close(), qt.IsNil)
 }
 
+const clientVersion = 3
+
 func newTestClientForServer(t testing.TB, server string, codec codecs.Codec, cfg model.ExampleConfig, env ...string) *execrpc.Client[model.ExampleConfig, model.ExampleRequest, model.ExampleMessage, model.ExampleReceipt] {
 	client, err := execrpc.StartClient(
 		execrpc.ClientOptions[model.ExampleConfig, model.ExampleRequest, model.ExampleMessage, model.ExampleReceipt]{
 			ClientRawOptions: execrpc.ClientRawOptions{
-				Version: 1,
+				Version: clientVersion,
 				Cmd:     "go",
 				Dir:     "./examples/servers/" + server,
 				Args:    []string{"run", "."},
@@ -224,7 +226,7 @@ func TestTyped(t *testing.T) {
 		client, err := execrpc.StartClient(
 			execrpc.ClientOptions[model.ExampleConfig, model.ExampleRequest, model.ExampleMessage, model.ExampleReceipt]{
 				ClientRawOptions: execrpc.ClientRawOptions{
-					Version: 1,
+					Version: clientVersion,
 					Cmd:     "go",
 					Dir:     "./examples/servers/typed",
 					Args:    []string{"run", "."},

examples/servers/readmeexample/main.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"fmt"
 	"hash"
 	"hash/fnv"
 	"log"
@@ -29,7 +30,10 @@ func main() {
 			// Optional function to initialize the server
 			// with the client configuration.
 			// This will be called once on server start.
-			Init: func(cfg model.ExampleConfig) error {
+			Init: func(cfg model.ExampleConfig, procol execrpc.ProtocolInfo) error {
+				if procol.Version != 3 {
+					return fmt.Errorf("unsupported protocol version: %d", procol.Version)
+				}
 				clientConfig = cfg
 				return clientConfig.Init()
 			},

examples/servers/typed/main.go

@@ -43,7 +43,10 @@ func main() {
 		execrpc.ServerOptions[model.ExampleConfig, model.ExampleRequest, model.ExampleMessage, model.ExampleReceipt]{
 			GetHasher:     getHasher,
 			DelayDelivery: delayDelivery,
-			Init: func(cfg model.ExampleConfig) error {
+			Init: func(cfg model.ExampleConfig, protocol execrpc.ProtocolInfo) error {
+				if protocol.Version != 3 {
+					return fmt.Errorf("unsupported protocol version: %d", protocol.Version)
+				}
 				clientConfig = cfg
 				return clientConfig.Init()
 			},

server.go

@@ -76,15 +76,18 @@ func NewServer[C, Q, M, R any](opts ServerOptions[C, Q, M, R]) (*Server[C, Q, M,
 				return nil
 			}
 
-			var cfg C
+			var (
+				cfg          C
+				protocolInfo = ProtocolInfo{Version: message.Header.Version}
+			)
 			err := opts.Codec.Decode(message.Body, &cfg)
 			if err != nil {
 				m := createErrorMessage(err, message.Header, MessageStatusErrDecodeFailed)
 				d.SendMessage(m)
 				return nil
 			}
 
-			if err := opts.Init(cfg); err != nil {
+			if err := opts.Init(cfg, protocolInfo); err != nil {
 				m := createErrorMessage(err, message.Header, MessageStatusErrInitServerFailed)
 				d.SendMessage(m)
 				return nil
@@ -259,12 +262,20 @@ func createErrorMessage(err error, h Header, failureStatus uint16) Message {
 	return m
 }
 
+// ProtocolInfo is the protocol information passed to the server's Init function.
+type ProtocolInfo struct {
+	// The version passed down from the client.
+	// This usually represents a major version,
+	// so any increment should be considered a breaking change.
+	Version uint16 `json:"version"`
+}
+
 // ServerOptions is the options for a server.
 type ServerOptions[C, Q, M, R any] struct {
 	// Init is the function that will be called when the server is started.
 	// It can be used to initialize the server with the given configuration.
 	// If an error is returned, the server will stop.
-	Init func(C) error
+	Init func(C, ProtocolInfo) error
 
 	// Handle is the function that will be called when a request is received.
 	Handle func(*Call[Q, M, R])