feat: default to list when dutctl is run without arguments

Extract the dispatch logic from start() into a small testable helper,
cover it with baseline dispatch tests, and make a bare `dutctl` (no
positional arguments) default to the list command. The usage text
documents the new no-args form.

Signed-off-by: Fabian Wienand <fabian.wienand@blindspot.software>

Author: RiSKeD

cmds/dutctl/dutctl.go

@@ -28,6 +28,7 @@ const usageAbstract = `dutctl - The client application of the DUT Control system
 `
 const usageSynopsis = `
 SYNOPSIS:
+	dutctl [options]
 	dutctl [options] list
 	dutctl [options] <device>
 	dutctl [options] <device> <command> [args...]
@@ -42,9 +43,11 @@ The optional args are passed to the command.
 To list all available devices, use the list command. If only a device is provided,
 dutctl list all available commands for the device.
 
-If a device, a command and the keyword help are provided, dutctl will show usage 
+If a device, a command and the keyword help are provided, dutctl will show usage
 information for the command.
 
+When dutctl is run without any positional arguments, it defaults to the list command.
+
 `
 
 const (
@@ -147,43 +150,43 @@ var errInvalidCmdline = fmt.Errorf("invalid command line")
 func (app *application) start() {
 	log.SetOutput(app.stdout)
 
-	if len(app.args) == 0 {
-		app.exit(errInvalidCmdline)
-	}
-
-	if app.args[0] == "version" {
+	if len(app.args) > 0 && app.args[0] == "version" {
 		app.printVersion()
 		app.exit(nil)
 	}
 
 	app.setupRPCClient()
+	app.exit(app.dispatch())
+}
+
+// dispatch decides which RPC to call based on app.args.
+// It is split out from start so it can be unit tested without os.Exit.
+func (app *application) dispatch() error {
+	if len(app.args) == 0 {
+		return app.listRPC()
+	}
 
 	if app.args[0] == "list" {
 		if len(app.args) > 1 {
-			app.exit(errInvalidCmdline)
+			return errInvalidCmdline
 		}
 
-		err := app.listRPC()
-		app.exit(err)
+		return app.listRPC()
 	}
 
 	if len(app.args) == 1 {
-		device := app.args[0]
-		err := app.commandsRPC(device)
-		app.exit(err)
+		return app.commandsRPC(app.args[0])
 	}
 
 	device := app.args[0]
 	command := app.args[1]
 	cmdArgs := app.args[2:]
 
 	if len(cmdArgs) > 0 && cmdArgs[0] == "help" {
-		err := app.detailsRPC(device, command, "help")
-		app.exit(err)
+		return app.detailsRPC(device, command, "help")
 	}
 
-	err := app.runRPC(device, command, cmdArgs)
-	app.exit(err)
+	return app.runRPC(device, command, cmdArgs)
 }
 
 // exit terminates the application. If the provided error is not nil, it is printed to

cmds/dutctl/dutctl_test.go

@@ -0,0 +1,189 @@
+// Copyright 2025 Blindspot Software
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+package main
+
+import (
+	"context"
+	"errors"
+	"io"
+	"testing"
+
+	"connectrpc.com/connect"
+
+	"github.com/BlindspotSoftware/dutctl/internal/output"
+	pb "github.com/BlindspotSoftware/dutctl/protobuf/gen/dutctl/v1"
+	"github.com/BlindspotSoftware/dutctl/protobuf/gen/dutctl/v1/dutctlv1connect"
+)
+
+// fakeDeviceServiceClient is a hand-written test double for
+// dutctlv1connect.DeviceServiceClient. Only the unary RPCs are
+// implemented; Run returns nil because the streaming path is not
+// exercised in these tests.
+type fakeDeviceServiceClient struct {
+	listDevices []string
+	listErr     error
+	listCalls   int
+
+	commandsCalls []string
+
+	detailsCalls []detailsCall
+}
+
+type detailsCall struct {
+	device, cmd, keyword string
+}
+
+func (f *fakeDeviceServiceClient) List(
+	_ context.Context, _ *connect.Request[pb.ListRequest],
+) (*connect.Response[pb.ListResponse], error) {
+	f.listCalls++
+
+	if f.listErr != nil {
+		return nil, f.listErr
+	}
+
+	return connect.NewResponse(&pb.ListResponse{Devices: f.listDevices}), nil
+}
+
+func (f *fakeDeviceServiceClient) Commands(
+	_ context.Context, req *connect.Request[pb.CommandsRequest],
+) (*connect.Response[pb.CommandsResponse], error) {
+	f.commandsCalls = append(f.commandsCalls, req.Msg.GetDevice())
+
+	return connect.NewResponse(&pb.CommandsResponse{}), nil
+}
+
+func (f *fakeDeviceServiceClient) Details(
+	_ context.Context, req *connect.Request[pb.DetailsRequest],
+) (*connect.Response[pb.DetailsResponse], error) {
+	f.detailsCalls = append(f.detailsCalls, detailsCall{
+		device:  req.Msg.GetDevice(),
+		cmd:     req.Msg.GetCmd(),
+		keyword: req.Msg.GetKeyword(),
+	})
+
+	return connect.NewResponse(&pb.DetailsResponse{}), nil
+}
+
+func (f *fakeDeviceServiceClient) Run(
+	_ context.Context,
+) *connect.BidiStreamForClient[pb.RunRequest, pb.RunResponse] {
+	return nil
+}
+
+// Compile-time assertion that the fake satisfies the interface.
+var _ dutctlv1connect.DeviceServiceClient = (*fakeDeviceServiceClient)(nil)
+
+// newTestApp builds an application with a fake RPC client and a
+// discarding formatter. Use it to drive dispatch in unit tests.
+func newTestApp(t *testing.T, fake *fakeDeviceServiceClient, args ...string) *application {
+	t.Helper()
+
+	return &application{
+		stdin:     io.NopCloser(nil),
+		stdout:    io.Discard,
+		stderr:    io.Discard,
+		exitFunc:  func(int) {},
+		args:      args,
+		rpcClient: fake,
+		formatter: output.New(output.Config{Stdout: io.Discard, Stderr: io.Discard}),
+	}
+}
+
+func TestDispatch(t *testing.T) {
+	tests := []struct {
+		name         string
+		args         []string
+		listDevices  []string
+		wantErrIs    error
+		wantListHit  int
+		wantCmdHits  []string
+		wantDetailHi []detailsCall
+	}{
+		{
+			name:        "no args defaults to list",
+			args:        nil,
+			wantListHit: 1,
+		},
+		{
+			name:        "explicit list",
+			args:        []string{"list"},
+			wantListHit: 1,
+		},
+		{
+			name:      "explicit list with extra args is invalid",
+			args:      []string{"list", "extra"},
+			wantErrIs: errInvalidCmdline,
+		},
+		{
+			name:        "single arg lists commands for that device",
+			args:        []string{"mydevice"},
+			wantCmdHits: []string{"mydevice"},
+		},
+		{
+			name: "device command help calls details",
+			args: []string{"mydevice", "power", "help"},
+			wantDetailHi: []detailsCall{
+				{device: "mydevice", cmd: "power", keyword: "help"},
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			fake := &fakeDeviceServiceClient{listDevices: tt.listDevices}
+			app := newTestApp(t, fake, tt.args...)
+
+			err := app.dispatch()
+
+			if tt.wantErrIs != nil {
+				if !errors.Is(err, tt.wantErrIs) {
+					t.Fatalf("dispatch error: want %v, got %v", tt.wantErrIs, err)
+				}
+			} else if err != nil {
+				t.Fatalf("dispatch: unexpected error: %v", err)
+			}
+
+			if fake.listCalls != tt.wantListHit {
+				t.Errorf("List calls: want %d, got %d", tt.wantListHit, fake.listCalls)
+			}
+
+			if !equalStrings(fake.commandsCalls, tt.wantCmdHits) {
+				t.Errorf("Commands calls: want %v, got %v", tt.wantCmdHits, fake.commandsCalls)
+			}
+
+			if !equalDetails(fake.detailsCalls, tt.wantDetailHi) {
+				t.Errorf("Details calls: want %v, got %v", tt.wantDetailHi, fake.detailsCalls)
+			}
+		})
+	}
+}
+
+func equalStrings(a, b []string) bool {
+	if len(a) != len(b) {
+		return false
+	}
+
+	for i := range a {
+		if a[i] != b[i] {
+			return false
+		}
+	}
+
+	return true
+}
+
+func equalDetails(a, b []detailsCall) bool {
+	if len(a) != len(b) {
+		return false
+	}
+
+	for i := range a {
+		if a[i] != b[i] {
+			return false
+		}
+	}
+
+	return true
+}