Customize help text and add ServerWithDoc (#7)

flags.go

@@ -17,6 +17,7 @@ package pluginrpc
 import (
 	"fmt"
 	"io"
+	"sort"
 	"strconv"
 	"strings"
 
@@ -32,6 +33,7 @@ const (
 	FormatFlagName = "format"
 
 	protocolVersion = 1
+	flagWrapping    = 140
 )
 
 type flags struct {
@@ -40,10 +42,13 @@ type flags struct {
 	format        Format
 }
 
-func parseFlags(output io.Writer, args []string) (*flags, []string, error) {
+func parseFlags(output io.Writer, args []string, spec Spec, doc string) (*flags, []string, error) {
 	flags := &flags{}
 	var formatString string
 	flagSet := pflag.NewFlagSet("plugin", pflag.ContinueOnError)
+	flagSet.Usage = func() {
+		_, _ = fmt.Fprint(output, getFlagUsage(flagSet, spec, doc))
+	}
 	flagSet.SetOutput(output)
 	flagSet.BoolVar(&flags.printProtocol, ProtocolFlagName, false, "Print the protocol to stdout and exit.")
 	flagSet.BoolVar(&flags.printSpec, SpecFlagName, false, "Print the spec to stdout in the specified format and exit.")
@@ -68,6 +73,35 @@ func parseFlags(output io.Writer, args []string) (*flags, []string, error) {
 	return flags, flagSet.Args(), nil
 }
 
+func getFlagUsage(flagSet *pflag.FlagSet, spec Spec, doc string) string {
+	var sb strings.Builder
+	if doc != "" {
+		_, _ = sb.WriteString(doc)
+		_, _ = sb.WriteString("\n\n")
+	}
+	_, _ = sb.WriteString("Commands:\n\n")
+	var argBasedProcedureStrings []string
+	var pathBasedProcedureStrings []string
+	for _, procedure := range spec.Procedures() {
+		if args := procedure.Args(); len(args) > 0 {
+			argBasedProcedureStrings = append(argBasedProcedureStrings, strings.Join(args, " "))
+		} else {
+			pathBasedProcedureStrings = append(pathBasedProcedureStrings, procedure.Path())
+		}
+	}
+	sort.Strings(argBasedProcedureStrings)
+	sort.Strings(pathBasedProcedureStrings)
+	for _, procedureString := range append(argBasedProcedureStrings, pathBasedProcedureStrings...) {
+		_, _ = sb.WriteString("  ")
+		_, _ = sb.WriteString(procedureString)
+		_, _ = sb.WriteString("\n")
+	}
+	_, _ = sb.WriteString("\nFlags:\n\n")
+	_, _ = sb.WriteString(flagSet.FlagUsagesWrapped(flagWrapping))
+	_, _ = sb.WriteString("  -h, --help            Show this help.\n")
+	return sb.String()
+}
+
 func marshalProtocol(value int) []byte {
 	return []byte(strconv.Itoa(value) + "\n")
 }

internal/example/cmd/echo-plugin/main.go

@@ -49,7 +49,11 @@ func newServer() (pluginrpc.Server, error) {
 	serverRegistrar := pluginrpc.NewServerRegistrar()
 	echoServiceServer := examplev1pluginrpc.NewEchoServiceServer(pluginrpc.NewHandler(spec), echoServiceHandler{})
 	examplev1pluginrpc.RegisterEchoServiceServer(serverRegistrar, echoServiceServer)
-	return pluginrpc.NewServer(spec, serverRegistrar)
+	return pluginrpc.NewServer(
+		spec,
+		serverRegistrar,
+		pluginrpc.ServerWithDoc("An example plugin that implements the EchoService."),
+	)
 }
 
 type echoServiceHandler struct{}

server.go

@@ -40,21 +40,35 @@ type Server interface {
 //
 // Once passed to this constructor, the ServerRegistrar can no longer have new
 // paths registered to it.
-func NewServer(spec Spec, serverRegistrar ServerRegistrar, _ ...ServerOption) (Server, error) {
-	return newServer(spec, serverRegistrar)
+func NewServer(spec Spec, serverRegistrar ServerRegistrar, options ...ServerOption) (Server, error) {
+	return newServer(spec, serverRegistrar, options...)
 }
 
 // ServerOption is an option for a new Server.
 type ServerOption func(*serverOptions)
 
+// ServerWithDoc will attach the given documentation to the server.
+//
+// This will add ths given docs as a prefix when the flag -h/--help is used.
+func ServerWithDoc(doc string) ServerOption {
+	return func(serverOptions *serverOptions) {
+		serverOptions.doc = doc
+	}
+}
+
 // *** PRIVATE ***
 
 type server struct {
 	spec             Spec
 	pathToHandleFunc map[string]func(context.Context, HandleEnv, ...HandleOption) error
+	doc              string
 }
 
-func newServer(spec Spec, serverRegistrar ServerRegistrar) (*server, error) {
+func newServer(spec Spec, serverRegistrar ServerRegistrar, options ...ServerOption) (*server, error) {
+	serverOptions := newServerOptions()
+	for _, option := range options {
+		option(serverOptions)
+	}
 	pathToHandleFunc, err := serverRegistrar.pathToHandleFunc()
 	if err != nil {
 		return nil, err
@@ -72,11 +86,12 @@ func newServer(spec Spec, serverRegistrar ServerRegistrar) (*server, error) {
 	return &server{
 		spec:             spec,
 		pathToHandleFunc: pathToHandleFunc,
+		doc:              serverOptions.doc,
 	}, nil
 }
 
 func (s *server) Serve(ctx context.Context, env Env) error {
-	flags, args, err := parseFlags(env.Stderr, env.Args)
+	flags, args, err := parseFlags(env.Stderr, env.Args, s.spec, s.doc)
 	if err != nil {
 		if errors.Is(err, pflag.ErrHelp) {
 			return nil
@@ -111,4 +126,10 @@ func (s *server) Serve(ctx context.Context, env Env) error {
 
 func (*server) isServer() {}
 
-type serverOptions struct{}
+type serverOptions struct {
+	doc string
+}
+
+func newServerOptions() *serverOptions {
+	return &serverOptions{}
+}