Restructure code to let linker perform deadcode elimination step

Cobra, in its default configuration, will execute a template to generate
help, usage and version outputs. Text/template execution calls MethodByName
and MethodByName disables dead code elimination in the Go linker, therefore
all programs that make use of cobra will be linked with dead code
elimination disabled, even if they end up replacing the default usage, help
and version formatters with a custom function and no actual text/template
evaluations are ever made at runtime.

Dead code elimination in the linker helps reduce disk space and memory
utilization of programs. For example, for the simple example program used by
TestDeadcodeElimination 40% of the final executable size is dead code. For a
more realistic example, 12% of the size of Delve's executable is deadcode.

This PR changes Cobra so that, in its default configuration, it does not
automatically inhibit deadcode elimination by:

1. changing Cobra's default behavior to emit output for usage and help using
   simple Go functions instead of template execution
2. quarantining all calls to template execution into SetUsageTemplate,
   SetHelpTemplate and SetVersionTemplate so that the linker can statically
   determine if they are reachable

cobra.go

@@ -176,12 +176,16 @@ func rpad(s string, padding int) string {
 	return fmt.Sprintf(formattedString, s)
 }
 
-// tmpl executes the given template text on data, writing the result to w.
+func tmpl(text string) *tmplFunc {
-func tmpl(w io.Writer, text string, data interface{}) error {
+	return &tmplFunc{
+		tmpl: text,
+		fn: func(w io.Writer, data interface{}) error {
 			t := template.New("top")
 			t.Funcs(templateFuncs)
 			template.Must(t.Parse(text))
 			return t.Execute(w, data)
+		},
+	}
 }
 
 // ld compares two strings and returns the levenshtein distance between them.

cobra_test.go

@@ -15,6 +15,10 @@
 package cobra
 
 import (
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
 	"testing"
 	"text/template"
 )
@@ -222,3 +226,57 @@ func TestRpad(t *testing.T) {
 		})
 	}
 }
+
+func TestDeadcodeElimination(t *testing.T) {
+	// check that a simple program using cobra in its default configuration is
+	// linked with deadcode elimination enabled.
+	const (
+		dirname  = "test_deadcode"
+		progname = "test_deadcode_elimination"
+	)
+	os.Mkdir(dirname, 0770)
+	filename := filepath.Join(dirname, progname+".go")
+	err := os.WriteFile(filename, []byte(`package main
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/spf13/cobra"
+)
+
+var rootCmd = &cobra.Command{
+	Version: "1.0",
+	Use:     "example_program",
+	Short:   "example_program - test fixture to check that deadcode elimination is allowed",
+	Run: func(cmd *cobra.Command, args []string) {
+		fmt.Println("hello world")
+	},
+	Aliases: []string{"alias1", "alias2"},
+	Example: "stringer --help",
+}
+
+func main() {
+	if err := rootCmd.Execute(); err != nil {
+		fmt.Fprintf(os.Stderr, "Whoops. There was an error while executing your CLI '%s'", err)
+		os.Exit(1)
+	}
+}
+`), 0660)
+	if err != nil {
+		t.Fatalf("could not write test program: %v", err)
+	}
+	defer os.RemoveAll(dirname)
+	buf, err := exec.Command("go", "build", filename).CombinedOutput()
+	if err != nil {
+		t.Fatalf("could not compile test program: %s", string(buf))
+	}
+	defer os.Remove(progname)
+	buf, err = exec.Command("go", "tool", "nm", progname).CombinedOutput()
+	if err != nil {
+		t.Fatalf("could not run go tool nm: %v", err)
+	}
+	if strings.Contains(string(buf), "MethodByName") {
+		t.Error("compiled programs contains MethodByName symbol")
+	}
+}

command.go

@@ -168,12 +168,12 @@ type Command struct {
 	// usageFunc is usage func defined by user.
 	usageFunc func(*Command) error
 	// usageTemplate is usage template defined by user.
-	usageTemplate string
+	usageTemplate *tmplFunc
 	// flagErrorFunc is func defined by user and it's called when the parsing of
 	// flags returns an error.
 	flagErrorFunc func(*Command, error) error
 	// helpTemplate is help template defined by user.
-	helpTemplate string
+	helpTemplate *tmplFunc
 	// helpFunc is help func defined by user.
 	helpFunc func(*Command, []string)
 	// helpCommand is command with usage 'help'. If it's not defined by user,
@@ -186,7 +186,7 @@ type Command struct {
 	completionCommandGroupID string
 
 	// versionTemplate is the version template defined by user.
-	versionTemplate string
+	versionTemplate *tmplFunc
 
 	// errPrefix is the error message prefix defined by user.
 	errPrefix string
@@ -313,7 +313,7 @@ func (c *Command) SetUsageFunc(f func(*Command) error) {
 
 // SetUsageTemplate sets usage template. Can be defined by Application.
 func (c *Command) SetUsageTemplate(s string) {
-	c.usageTemplate = s
+	c.usageTemplate = tmpl(s)
 }
 
 // SetFlagErrorFunc sets a function to generate an error when flag parsing
@@ -349,12 +349,12 @@ func (c *Command) SetCompletionCommandGroupID(groupID string) {
 
 // SetHelpTemplate sets help template to be used. Application can use it to set custom template.
 func (c *Command) SetHelpTemplate(s string) {
-	c.helpTemplate = s
+	c.helpTemplate = tmpl(s)
 }
 
 // SetVersionTemplate sets version template to be used. Application can use it to set custom template.
 func (c *Command) SetVersionTemplate(s string) {
-	c.versionTemplate = s
+	c.versionTemplate = tmpl(s)
 }
 
 // SetErrPrefix sets error message prefix to be used. Application can use it to set custom prefix.
@@ -435,7 +435,11 @@ func (c *Command) UsageFunc() (f func(*Command) error) {
 	}
 	return func(c *Command) error {
 		c.mergePersistentFlags()
-		err := tmpl(c.OutOrStderr(), c.UsageTemplate(), c)
+		fn := defaultUsageFunc
+		if c.usageTemplate != nil {
+			fn = c.usageTemplate.fn
+		}
+		err := fn(c.OutOrStderr(), c)
 		if err != nil {
 			c.PrintErrln(err)
 		}
@@ -461,9 +465,13 @@ func (c *Command) HelpFunc() func(*Command, []string) {
 	}
 	return func(c *Command, a []string) {
 		c.mergePersistentFlags()
+		fn := defaultHelpFunc
+		if c.helpTemplate != nil {
+			fn = c.helpTemplate.fn
+		}
 		// The help should be sent to stdout
 		// See https://github.com/spf13/cobra/issues/1002
-		err := tmpl(c.OutOrStdout(), c.HelpTemplate(), c)
+		err := fn(c.OutOrStdout(), c)
 		if err != nil {
 			c.PrintErrln(err)
 		}
@@ -545,70 +553,38 @@ func (c *Command) NamePadding() int {
 
 // UsageTemplate returns usage template for the command.
 func (c *Command) UsageTemplate() string {
-	if c.usageTemplate != "" {
+	if c.usageTemplate != nil {
-		return c.usageTemplate
+		return c.usageTemplate.tmpl
 	}
 
 	if c.HasParent() {
 		return c.parent.UsageTemplate()
 	}
-	return `Usage:{{if .Runnable}}
+	return defaultUsageTemplate
-  {{.UseLine}}{{end}}{{if .HasAvailableSubCommands}}
-  {{.CommandPath}} [command]{{end}}{{if gt (len .Aliases) 0}}
-
-Aliases:
-  {{.NameAndAliases}}{{end}}{{if .HasExample}}
-
-Examples:
-{{.Example}}{{end}}{{if .HasAvailableSubCommands}}{{$cmds := .Commands}}{{if eq (len .Groups) 0}}
-
-Available Commands:{{range $cmds}}{{if (or .IsAvailableCommand (eq .Name "help"))}}
-  {{rpad .Name .NamePadding }} {{.Short}}{{end}}{{end}}{{else}}{{range $group := .Groups}}
-
-{{.Title}}{{range $cmds}}{{if (and (eq .GroupID $group.ID) (or .IsAvailableCommand (eq .Name "help")))}}
-  {{rpad .Name .NamePadding }} {{.Short}}{{end}}{{end}}{{end}}{{if not .AllChildCommandsHaveGroup}}
-
-Additional Commands:{{range $cmds}}{{if (and (eq .GroupID "") (or .IsAvailableCommand (eq .Name "help")))}}
-  {{rpad .Name .NamePadding }} {{.Short}}{{end}}{{end}}{{end}}{{end}}{{end}}{{if .HasAvailableLocalFlags}}
-
-Flags:
-{{.LocalFlags.FlagUsages | trimTrailingWhitespaces}}{{end}}{{if .HasAvailableInheritedFlags}}
-
-Global Flags:
-{{.InheritedFlags.FlagUsages | trimTrailingWhitespaces}}{{end}}{{if .HasHelpSubCommands}}
-
-Additional help topics:{{range .Commands}}{{if .IsAdditionalHelpTopicCommand}}
-  {{rpad .CommandPath .CommandPathPadding}} {{.Short}}{{end}}{{end}}{{end}}{{if .HasAvailableSubCommands}}
-
-Use "{{.CommandPath}} [command] --help" for more information about a command.{{end}}
-`
 }
 
 // HelpTemplate return help template for the command.
 func (c *Command) HelpTemplate() string {
-	if c.helpTemplate != "" {
+	if c.helpTemplate != nil {
-		return c.helpTemplate
+		return c.helpTemplate.tmpl
 	}
 
 	if c.HasParent() {
 		return c.parent.HelpTemplate()
 	}
-	return `{{with (or .Long .Short)}}{{. | trimTrailingWhitespaces}}
+	return defaultHelpTemplate
-
-{{end}}{{if or .Runnable .HasSubCommands}}{{.UsageString}}{{end}}`
 }
 
 // VersionTemplate return version template for the command.
 func (c *Command) VersionTemplate() string {
-	if c.versionTemplate != "" {
+	if c.versionTemplate != nil {
-		return c.versionTemplate
+		return c.versionTemplate.tmpl
 	}
 
 	if c.HasParent() {
 		return c.parent.VersionTemplate()
 	}
-	return `{{with .DisplayName}}{{printf "%s " .}}{{end}}{{printf "version %s" .Version}}
+	return defaultVersionTemplate
-`
 }
 
 // ErrPrefix return error message prefix for the command
@@ -915,7 +891,11 @@ func (c *Command) execute(a []string) (err error) {
 			return err
 		}
 		if versionVal {
-			err := tmpl(c.OutOrStdout(), c.VersionTemplate(), c)
+			fn := defaultVersionFunc
+			if c.versionTemplate != nil {
+				fn = c.versionTemplate.fn
+			}
+			err := fn(c.OutOrStdout(), c)
 			if err != nil {
 				c.Println(err)
 			}
@@ -1897,3 +1877,141 @@ func commandNameMatches(s string, t string) bool {
 
 	return s == t
 }
+
+// tmplFunc holds a template and a function that will execute said template.
+type tmplFunc struct {
+	tmpl string
+	fn   func(io.Writer, interface{}) error
+}
+
+var defaultUsageTemplate = `Usage:{{if .Runnable}}
+  {{.UseLine}}{{end}}{{if .HasAvailableSubCommands}}
+  {{.CommandPath}} [command]{{end}}{{if gt (len .Aliases) 0}}
+
+Aliases:
+  {{.NameAndAliases}}{{end}}{{if .HasExample}}
+
+Examples:
+{{.Example}}{{end}}{{if .HasAvailableSubCommands}}{{$cmds := .Commands}}{{if eq (len .Groups) 0}}
+
+Available Commands:{{range $cmds}}{{if (or .IsAvailableCommand (eq .Name "help"))}}
+  {{rpad .Name .NamePadding }} {{.Short}}{{end}}{{end}}{{else}}{{range $group := .Groups}}
+
+{{.Title}}{{range $cmds}}{{if (and (eq .GroupID $group.ID) (or .IsAvailableCommand (eq .Name "help")))}}
+  {{rpad .Name .NamePadding }} {{.Short}}{{end}}{{end}}{{end}}{{if not .AllChildCommandsHaveGroup}}
+
+Additional Commands:{{range $cmds}}{{if (and (eq .GroupID "") (or .IsAvailableCommand (eq .Name "help")))}}
+  {{rpad .Name .NamePadding }} {{.Short}}{{end}}{{end}}{{end}}{{end}}{{end}}{{if .HasAvailableLocalFlags}}
+
+Flags:
+{{.LocalFlags.FlagUsages | trimTrailingWhitespaces}}{{end}}{{if .HasAvailableInheritedFlags}}
+
+Global Flags:
+{{.InheritedFlags.FlagUsages | trimTrailingWhitespaces}}{{end}}{{if .HasHelpSubCommands}}
+
+Additional help topics:{{range .Commands}}{{if .IsAdditionalHelpTopicCommand}}
+  {{rpad .CommandPath .CommandPathPadding}} {{.Short}}{{end}}{{end}}{{end}}{{if .HasAvailableSubCommands}}
+
+Use "{{.CommandPath}} [command] --help" for more information about a command.{{end}}
+`
+
+// defaultUsageFunc is equivalent to executing defaultUsageTemplate. The two should be changed in sync.
+func defaultUsageFunc(w io.Writer, in interface{}) error {
+	c := in.(*Command)
+	fmt.Fprint(w, "Usage:")
+	if c.Runnable() {
+		fmt.Fprintf(w, "\n  %s", c.UseLine())
+	}
+	if c.HasAvailableSubCommands() {
+		fmt.Fprintf(w, "\n  %s [command]", c.CommandPath())
+	}
+	if len(c.Aliases) > 0 {
+		fmt.Fprintf(w, "\n\nAliases:\n")
+		fmt.Fprintf(w, "  %s", c.NameAndAliases())
+	}
+	if c.HasExample() {
+		fmt.Fprintf(w, "\n\nExamples:\n")
+		fmt.Fprintf(w, "%s", c.Example)
+	}
+	if c.HasAvailableSubCommands() {
+		cmds := c.Commands()
+		if len(c.Groups()) == 0 {
+			fmt.Fprintf(w, "\n\nAvailable Commands:")
+			for _, subcmd := range cmds {
+				if subcmd.IsAvailableCommand() || subcmd.Name() == "help" {
+					fmt.Fprintf(w, "\n  %s %s", rpad(subcmd.Name(), subcmd.NamePadding()), subcmd.Short)
+				}
+			}
+		} else {
+			for _, group := range c.Groups() {
+				fmt.Fprintf(w, "\n\n%s", group.Title)
+				for _, subcmd := range cmds {
+					if subcmd.GroupID == group.ID && (subcmd.IsAvailableCommand() || subcmd.Name() == "help") {
+						fmt.Fprintf(w, "\n  %s %s", rpad(subcmd.Name(), subcmd.NamePadding()), subcmd.Short)
+					}
+				}
+			}
+			if !c.AllChildCommandsHaveGroup() {
+				fmt.Fprintf(w, "\n\nAdditional Commands:")
+				for _, subcmd := range cmds {
+					if subcmd.GroupID == "" && (subcmd.IsAvailableCommand() || subcmd.Name() == "help") {
+						fmt.Fprintf(w, "\n  %s %s", rpad(subcmd.Name(), subcmd.NamePadding()), subcmd.Short)
+					}
+				}
+			}
+		}
+	}
+	if c.HasAvailableLocalFlags() {
+		fmt.Fprintf(w, "\n\nFlags:\n")
+		fmt.Fprint(w, trimRightSpace(c.LocalFlags().FlagUsages()))
+	}
+	if c.HasAvailableInheritedFlags() {
+		fmt.Fprintf(w, "\n\nGlobal Flags:\n")
+		fmt.Fprint(w, trimRightSpace(c.InheritedFlags().FlagUsages()))
+	}
+	if c.HasHelpSubCommands() {
+		fmt.Fprintf(w, "\n\nAdditional help topcis:")
+		for _, subcmd := range c.Commands() {
+			if subcmd.IsAdditionalHelpTopicCommand() {
+				fmt.Fprintf(w, "\n %s %s", rpad(subcmd.CommandPath(), subcmd.CommandPathPadding()), subcmd.Short)
+			}
+		}
+	}
+	if c.HasAvailableSubCommands() {
+		fmt.Fprintf(w, "\n\nUse \"%s [command] --help\" for more information about a command.", c.CommandPath())
+	}
+	fmt.Fprintln(w)
+	return nil
+}
+
+var defaultHelpTemplate = `{{with (or .Long .Short)}}{{. | trimTrailingWhitespaces}}
+
+{{end}}{{if or .Runnable .HasSubCommands}}{{.UsageString}}{{end}}`
+
+// defaultHelpFunc is equivalent to executing defaultHelpTemplate. The two should be changed in sync.
+func defaultHelpFunc(w io.Writer, in interface{}) error {
+	c := in.(*Command)
+	usage := c.Long
+	if usage == "" {
+		usage = c.Short
+	}
+	usage = trimRightSpace(usage)
+	if usage != "" {
+		fmt.Fprintln(w, usage)
+		fmt.Fprintln(w)
+	}
+	if c.Runnable() || c.HasSubCommands() {
+		fmt.Fprint(w, c.UsageString())
+	}
+	return nil
+}
+
+var defaultVersionTemplate = `{{with .DisplayName}}{{printf "%s " .}}{{end}}{{printf "version %s" .Version}}
+`
+
+// defaultVersionFunc is equivalent to executing defaultVersionTemplate. The two should be changed in sync.
+func defaultVersionFunc(w io.Writer, in interface{}) error {
+	c := in.(*Command)
+	_, err := fmt.Fprintf(w, "%s version %s\n", c.DisplayName(), c.Version)
+	return err
+}