utils/cobrautil/templates: improve flags printing

This patch slightly changes the kubectl based flag output to be more similar to curl help.

Changes:
* Use quoted name for value description - like in curl
* Use (default XXX) - like in the stock cobra output
* Add (env FORWARDER_XXX) - this is Forwarder specific, it simplifies working with containers and Systemd where flags are specified as environment variables.

Sample output:

    -n, --dns-server (env FORWARDER_DNS_SERVER)
        DNS server(s) to use instead of system default, format: <ip>[:<port>] (default port is 53). If specified
        multiple times, the first one is used as primary server, the rest are used as a fallback.

    --dns-timeout duration (default 5s) (env FORWARDER_DNS_TIMEOUT)
        Timeout for dialing DNS servers.

cmd/forwarder/root.go

@@ -45,15 +45,5 @@ func rootCommand() *cobra.Command {
 		version.Command(),
 	)
 
-	decorate(cmd)
-
 	return cmd
 }
-
-func decorate(cmd *cobra.Command) {
-	cobrautil.AppendEnvToUsage(cmd, envPrefix)
-
-	for _, cmd := range cmd.Commands() {
-		decorate(cmd)
-	}
-}

utils/cobrautil/templates/help_flags_printer.go

@@ -32,14 +32,16 @@ const offset = 10
 // processes the help flag and print
 // it to i/o writer
 type HelpFlagPrinter struct {
+	envPrefix string
 	wrapLimit uint
 	out       io.Writer
 }
 
 // NewHelpFlagPrinter will initialize a HelpFlagPrinter given the
 // i/o writer
-func NewHelpFlagPrinter(out io.Writer, wrapLimit uint) *HelpFlagPrinter {
+func NewHelpFlagPrinter(out io.Writer, envPrefix string, wrapLimit uint) *HelpFlagPrinter {
 	return &HelpFlagPrinter{
+		envPrefix: envPrefix,
 		wrapLimit: wrapLimit,
 		out:       out,
 	}
@@ -48,7 +50,7 @@ func NewHelpFlagPrinter(out io.Writer, wrapLimit uint) *HelpFlagPrinter {
 // PrintHelpFlag will beautify the help flags and print it out to p.out
 func (p *HelpFlagPrinter) PrintHelpFlag(flag *flag.Flag) {
 	formatBuf := new(bytes.Buffer)
-	writeFlag(formatBuf, flag)
+	writeFlag(formatBuf, flag, p.envPrefix)
 
 	wrappedStr := formatBuf.String()
 	flagAndUsage := strings.Split(formatBuf.String(), "\n")
@@ -67,10 +69,40 @@ func (p *HelpFlagPrinter) PrintHelpFlag(flag *flag.Flag) {
 
 // writeFlag will output the help flag based
 // on the format provided by getFlagFormat to i/o writer
-func writeFlag(out io.Writer, f *flag.Flag) {
+func writeFlag(out io.Writer, f *flag.Flag, envPrefix string) {
+	val, usage := flag.UnquoteUsage(f)
+	if val == "string" {
+		val = ""
+	}
+	if val != "" {
+		val = " " + val
+	}
+
+	def := f.DefValue
+	if def == "[]" {
+		def = ""
+	}
+	if def != "" {
+		if f.Value.Type() == "string" {
+			def = fmt.Sprintf(" (default '%s')", f.DefValue)
+		} else {
+			def = fmt.Sprintf(" (default %s)", f.DefValue)
+		}
+	}
+
 	deprecated := ""
 	if f.Deprecated != "" {
 		deprecated = fmt.Sprintf(" (DEPRECATED: %s)", f.Deprecated)
 	}
-	fmt.Fprintf(out, getFlagFormat(f), f.Shorthand, f.Name, f.DefValue, f.Usage, deprecated)
+
+	env := fmt.Sprintf(" (env %s)", envName(envPrefix, f.Name))
+
+	fmt.Fprintf(out, getFlagFormat(f), f.Shorthand, f.Name, val, def, env, usage, deprecated)
+}
+
+func envName(envPrefix, flagName string) string {
+	name := flagName
+	name = strings.ReplaceAll(name, "-", "_")
+	name = fmt.Sprintf("%s_%s", envPrefix, name)
+	return strings.ToUpper(name)
 }

utils/cobrautil/templates/templater.go

@@ -125,7 +125,7 @@ func (templater *templater) templateFuncs(exposedFlags ...string) template.FuncM
 		"appendIfNotPresent":  appendIfNotPresent,
 		"flagsNotIntersected": flagsNotIntersected,
 		"visibleFlags":        visibleFlags,
-		"flagsUsages":         flagsUsages,
+		"flagsUsages":         templater.flagsUsages,
 		"cmdGroups":           templater.cmdGroups,
 		"cmdGroupsString":     templater.cmdGroupsString,
 		"rootCmd":             templater.rootCmdName,
@@ -225,9 +225,9 @@ func (t *templater) usageLine(c *cobra.Command) string {
 }
 
 // flagsUsages will print out the kubectl help flags
-func flagsUsages(f *flag.FlagSet) (string, error) {
+func (t *templater) flagsUsages(f *flag.FlagSet) (string, error) {
 	flagBuf := new(bytes.Buffer)
-	printer := NewHelpFlagPrinter(flagBuf, DefaultWrapLimit)
+	printer := NewHelpFlagPrinter(flagBuf, t.RootCmd.Name(), DefaultWrapLimit)
 
 	f.VisitAll(func(flag *flag.Flag) {
 		if flag.Hidden {
@@ -241,12 +241,7 @@ func flagsUsages(f *flag.FlagSet) (string, error) {
 
 // getFlagFormat will output the flag format
 func getFlagFormat(f *flag.Flag) string {
-	var format string
-	format = "--%s=%s:\n%s%s"
-	if f.Value.Type() == "string" {
-		format = "--%s='%s':\n%s%s"
-	}
-
+	format := "--%s%s%s%s\n%s%s"
 	if len(f.Shorthand) > 0 {
 		format = "    -%s, " + format
 	} else {