Print help that was asked for to stdout (closes #179)

--help, -h and the help subcommand now print to stdout and exit 0, so
the help can be paged and searched. The usage text of a usage error
stays on stderr with exit 2, as does help for an unknown subcommand
("sfdupes help scna"). A failed write of the help exits 1.

Cobra prints help to the command's output writer, which stays stderr,
drops a failed write, and returns no error after help. So run replaces
the help function and the help subcommand: the help goes into a buffer,
then to stdout, and the error sets the exit code.

README "Settings" and goal 4 say so.

Model: opus-5-5

internal/sfdupes/cli.go

@@ -3,6 +3,7 @@
 package sfdupes
 
 import (
+	"bytes"
 	"context"
 	"errors"
 	"fmt"
@@ -176,9 +177,32 @@ func run(args []string, stdout, stderr io.Writer) int {
 	}
 
 	root := newRootCommand(stdout, stderr)
+
+	// Help that was asked for, with --help, -h or the help subcommand,
+	// goes to stdout. Execute returns no error after help, so helpErr
+	// keeps the one printHelp returns. Cobra's own help subcommand is
+	// replaced because it answers "sfdupes help scna" with the help of
+	// sfdupes.
+	var helpErr error
+
+	cobraHelp := root.HelpFunc()
+	root.SetHelpFunc(func(cmd *cobra.Command, _ []string) {
+		helpErr = printHelp(cmd, cmd.Flags().Args(), cobraHelp, stdout)
+	})
+	root.SetHelpCommand(&cobra.Command{
+		Use:   "help [command]",
+		Short: "Help about any command",
+		Run: func(_ *cobra.Command, names []string) {
+			helpErr = printHelp(root, names, cobraHelp, stdout)
+		},
+	})
+
 	root.SetArgs(args)
 
 	err := root.Execute()
+	if err == nil {
+		err = helpErr
+	}
 
 	var fatal fatalError
 
@@ -195,15 +219,62 @@ func run(args []string, stdout, stderr io.Writer) int {
 
 		return exitFatal
 	default:
-		// A usage error, which cobra has already reported on stderr.
+		// A usage error, which cobra or printHelp has already reported
+		// on stderr.
 		return exitUsage
 	}
 }
 
+// printHelp prints help that was asked for to stdout. It is the help of
+// cmd, or, for a command with subcommands, of the subcommand that
+// operands name: scan in "sfdupes help scan". Operands that name none,
+// as in "sfdupes help scna" or "sfdupes scna --help", are a usage
+// error, reported on stderr as for "sfdupes scna".
+//
+// cobraHelp is cobra's own help function. It prints to the command's
+// output writer, which is stderr, and drops a failed write. So the help
+// goes into a buffer first, and a failed write of the buffer to stdout
+// is a fatal error.
+func printHelp(
+	cmd *cobra.Command, operands []string,
+	cobraHelp func(*cobra.Command, []string), stdout io.Writer,
+) error {
+	if cmd.HasSubCommands() {
+		cmd, operands, _ = cmd.Find(operands)
+	}
+
+	// When the help subcommand runs instead of cmd, cobra has not yet
+	// added the --help flag that cmd's help and usage text list.
+	cmd.InitDefaultHelpFlag()
+
+	// Operands left over here name no subcommand. They are reported as
+	// cobra reports "sfdupes scna", which cobra.NoArgs rejects.
+	if cmd.HasSubCommands() && len(operands) > 0 {
+		err := cobra.NoArgs(cmd, operands)
+		cmd.PrintErrln(cmd.ErrPrefix(), err.Error())
+		cmd.Println(cmd.UsageString())
+
+		return err
+	}
+
+	out := cmd.OutOrStderr()
+	help := new(bytes.Buffer)
+	cmd.SetOut(help)
+	cobraHelp(cmd, nil)
+	cmd.SetOut(out)
+
+	_, err := stdout.Write(help.Bytes())
+	if err != nil {
+		return fatalError{err: fmt.Errorf("write stdout: %w", err)}
+	}
+
+	return nil
+}
+
 // newRootCommand builds the command tree. Everything on stdout is
 // machine-readable data, the version line included, or help that was
-// asked for. The rest of the human-facing output (usage, errors) goes
-// to stderr.
+// asked for, which run prints. The rest of the human-facing output
+// (usage, errors) goes to stderr.
 func newRootCommand(stdout, stderr io.Writer) *cobra.Command {
 	var showVersion bool
 
@@ -242,15 +313,6 @@ func newRootCommand(stdout, stderr io.Writer) *cobra.Command {
 	root.SetErr(stderr)
 	root.CompletionOptions.DisableDefaultCmd = true
 
-	// Cobra's default help also prints to the output writer. Help that
-	// was asked for, with --help, -h or the help subcommand, goes to
-	// stdout instead.
-	printHelp := root.HelpFunc()
-	root.SetHelpFunc(func(cmd *cobra.Command, args []string) {
-		cmd.SetOut(stdout)
-		printHelp(cmd, args)
-	})
-
 	// Cobra's built-in version flag prints to the output writer,
 	// stderr; this one prints to stdout.
 	root.Flags().BoolVarP(&showVersion, "version", "v", false,

internal/sfdupes/cli_test.go

@@ -628,6 +628,14 @@ func TestRunNamesDefaultDatabase(t *testing.T) {
 
 func TestRunUsageErrors(t *testing.T) {
 	// Usage errors keep exiting 2 with cobra's own report on stderr.
+	// An unknown subcommand is reported the same way with or without a
+	// request for help: the line naming it, then the usage text.
+	const (
+		unknown       = "nope"
+		unknownReport = "unknown command \"" + unknown +
+			"\" for \"sfdupes\"\n" + usageMarker
+	)
+
 	cases := map[string]struct {
 		args []string
 		want string
@@ -637,7 +645,13 @@ func TestRunUsageErrors(t *testing.T) {
 		"info with args":     {[]string{cmdInfo, "x"}, usageMarker},
 		"dwim with args":     {[]string{cmdDwim, "x"}, usageMarker},
 		"unknown flag":       {[]string{cmdScan, "--nope", "/"}, usageMarker},
-		"unknown subcommand": {[]string{"nope"}, "unknown command"},
+		"unknown subcommand": {[]string{unknown}, unknownReport},
+		"unknown subcommand with --help": {
+			[]string{unknown, helpFlag}, unknownReport,
+		},
+		"help for an unknown subcommand": {
+			[]string{helpCommand, unknown}, unknownReport,
+		},
 	}
 
 	for name, tc := range cases {
@@ -723,6 +737,13 @@ func TestRunHelp(t *testing.T) {
 				args, stdout.String())
 		}
 
+		// The help subcommand prints the same help as --help, which
+		// lists --help itself.
+		if !strings.Contains(stdout.String(), "-h, "+helpFlag) {
+			t.Errorf("run(%v) stdout = %q, want it to list %s",
+				args, stdout.String(), helpFlag)
+		}
+
 		if got := stderr.String(); got != "" {
 			t.Errorf("run(%v) stderr = %q, want nothing", args, got)
 		}
@@ -928,6 +949,32 @@ func TestRunVersionWriteFailureIsFatal(t *testing.T) {
 	}
 }
 
+func TestRunHelpWriteFailureIsFatal(t *testing.T) {
+	t.Parallel()
+
+	// README §Error handling: a failed write to stdout is a fatal error,
+	// exit 1, reported on stderr starting with "sfdupes:". Help goes to
+	// stdout, so that holds for help too.
+	cases := [][]string{
+		{helpFlag}, {"-h"}, {helpCommand},
+		{cmdScan, helpFlag}, {helpCommand, cmdScan},
+	}
+
+	for _, args := range cases {
+		var stderr bytes.Buffer
+
+		code := run(args, failingWriter{}, &stderr)
+		if code != exitFatal {
+			t.Errorf("run(%v) = %d, want %d", args, code, exitFatal)
+		}
+
+		want := "sfdupes: write stdout: " + errWriteFailed.Error() + "\n"
+		if got := stderr.String(); got != want {
+			t.Errorf("run(%v) stderr = %q, want %q", args, got, want)
+		}
+	}
+}
+
 // scanFixture builds a small tree holding one duplicate pair and one
 // unreadable file and scans it into the database the caller has
 // pointed SFDUPES_DATABASE at. The unreadable file makes the scan warn