TUN-3995: Optional --features flag for tunnel run.

These features will be included in the ClientInfo.Features field when
running a named tunnel.

cmd/cloudflared/tunnel/config_test.go

@@ -0,0 +1,13 @@
+package tunnel
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestDedup(t *testing.T) {
+	expected := []string{"a", "b"}
+	actual := dedup([]string{"a", "b", "a"})
+	require.Equal(t, expected, actual)
+}

cmd/cloudflared/tunnel/configuration.go

@@ -201,9 +201,10 @@ func prepareTunnelConfig(
 		if err != nil {
 			return nil, ingress.Ingress{}, errors.Wrap(err, "can't generate clientUUID")
 		}
+		features := append(c.StringSlice("features"), origin.FeatureSerializedHeaders)
 		namedTunnel.Client = tunnelpogs.ClientInfo{
 			ClientID: clientUUID[:],
-			Features: []string{origin.FeatureSerializedHeaders},
+			Features: dedup(features),
 			Version:  version,
 			Arch:     fmt.Sprintf("%s_%s", buildInfo.GoOS, buildInfo.GoArch),
 		}
@@ -301,3 +302,22 @@ func isWarpRoutingEnabled(warpConfig config.WarpRoutingConfig, isNamedTunnel boo
 func isRunningFromTerminal() bool {
 	return terminal.IsTerminal(int(os.Stdout.Fd()))
 }
+
+// Remove any duplicates from the slice
+func dedup(slice []string) []string {
+
+	// Convert the slice into a set
+	set := make(map[string]bool, 0)
+	for _, str := range slice {
+		set[str] = true
+	}
+
+	// Convert the set back into a slice
+	keys := make([]string, len(set))
+	i := 0
+	for str := range set {
+		keys[i] = str
+		i++
+	}
+	return keys
+}

cmd/cloudflared/tunnel/subcommands.go

@@ -87,6 +87,11 @@ var (
 			"overwrite the previous tunnel. If you want to use a single hostname with multiple " +
 			"tunnels, you can do so with Cloudflare's Load Balancer product.",
 	})
+	featuresFlag = altsrc.NewStringSliceFlag(&cli.StringSliceFlag{
+		Name:    "features",
+		Aliases: []string{"F"},
+		Usage:   "Opt into various features that are still being developed or tested.",
+	})
 	credentialsFileFlag = altsrc.NewStringFlag(&cli.StringFlag{
 		Name:    CredFileFlag,
 		Aliases: []string{CredFileFlagAlias},
@@ -370,6 +375,7 @@ func buildRunCommand() *cli.Command {
 		forceFlag,
 		credentialsFileFlag,
 		selectProtocolFlag,
+		featuresFlag,
 	}
 	flags = append(flags, configureProxyFlags(false)...)
 	return &cli.Command{