cmd/tailscale/cli: use double hypens, make default usage func more clear

Mash up some code from ffcli and std's flag package to make a default
usage func that's super explicit for those not familiar with the Go
style flags. Only show double hyphens in usage text (but still accept both),
and show default values, and only show the proper usage of boolean flags.

Fixes #1353
Fixes #1529

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/cli.go

@@ -9,6 +9,7 @@ package cli
 import (
 	"context"
 	"flag"
+	"fmt"
 	"log"
 	"net"
 	"os"
@@ -16,6 +17,7 @@ import (
 	"runtime"
 	"strings"
 	"syscall"
+	"text/tabwriter"
 
 	"github.com/peterbourgon/ff/v2/ffcli"
 	"tailscale.com/ipn"
@@ -53,9 +55,7 @@ func Run(args []string) error {
 		ShortUsage: "tailscale [flags] <subcommand> [command flags]",
 		ShortHelp:  "The easiest, most secure way to use WireGuard.",
 		LongHelp: strings.TrimSpace(`
-For help on subcommands, add -help after: "tailscale status -help".
-
-All flags can use single or double hyphen prefixes (-help or --help).
+For help on subcommands, add --help after: "tailscale status --help".
 
 This CLI is still under active development. Commands and flags will
 change in the future.
@@ -68,8 +68,12 @@ change in the future.
 			pingCmd,
 			versionCmd,
 		},
-		FlagSet: rootfs,
-		Exec:    func(context.Context, []string) error { return flag.ErrHelp },
+		FlagSet:   rootfs,
+		Exec:      func(context.Context, []string) error { return flag.ErrHelp },
+		UsageFunc: usageFunc,
+	}
+	for _, c := range rootCmd.Subcommands {
+		c.UsageFunc = usageFunc
 	}
 
 	// Don't advertise the debug command, but it exists.
@@ -147,3 +151,72 @@ func strSliceContains(ss []string, s string) bool {
 	}
 	return false
 }
+
+func usageFunc(c *ffcli.Command) string {
+	var b strings.Builder
+
+	fmt.Fprintf(&b, "USAGE\n")
+	if c.ShortUsage != "" {
+		fmt.Fprintf(&b, "  %s\n", c.ShortUsage)
+	} else {
+		fmt.Fprintf(&b, "  %s\n", c.Name)
+	}
+	fmt.Fprintf(&b, "\n")
+
+	if c.LongHelp != "" {
+		fmt.Fprintf(&b, "%s\n\n", c.LongHelp)
+	}
+
+	if len(c.Subcommands) > 0 {
+		fmt.Fprintf(&b, "SUBCOMMANDS\n")
+		tw := tabwriter.NewWriter(&b, 0, 2, 2, ' ', 0)
+		for _, subcommand := range c.Subcommands {
+			fmt.Fprintf(tw, "  %s\t%s\n", subcommand.Name, subcommand.ShortHelp)
+		}
+		tw.Flush()
+		fmt.Fprintf(&b, "\n")
+	}
+
+	if countFlags(c.FlagSet) > 0 {
+		fmt.Fprintf(&b, "FLAGS\n")
+		tw := tabwriter.NewWriter(&b, 0, 2, 2, ' ', 0)
+		c.FlagSet.VisitAll(func(f *flag.Flag) {
+			var s string
+			name, usage := flag.UnquoteUsage(f)
+			if isBoolFlag(f) {
+				s = fmt.Sprintf("  --%s, --%s=false", f.Name, f.Name)
+			} else {
+				s = fmt.Sprintf("  --%s", f.Name) // Two spaces before --; see next two comments.
+				if len(name) > 0 {
+					s += " " + name
+				}
+			}
+			// Four spaces before the tab triggers good alignment
+			// for both 4- and 8-space tab stops.
+			s += "\n    \t"
+			s += strings.ReplaceAll(usage, "\n", "\n    \t")
+
+			if f.DefValue != "" {
+				s += fmt.Sprintf(" (default %s)", f.DefValue)
+			}
+
+			fmt.Fprintln(&b, s)
+		})
+		tw.Flush()
+		fmt.Fprintf(&b, "\n")
+	}
+
+	return strings.TrimSpace(b.String())
+}
+
+func isBoolFlag(f *flag.Flag) bool {
+	bf, ok := f.Value.(interface {
+		IsBoolFlag() bool
+	})
+	return ok && bf.IsBoolFlag()
+}
+
+func countFlags(fs *flag.FlagSet) (n int) {
+	fs.VisitAll(func(*flag.Flag) { n++ })
+	return n
+}