Add flagx.Advanced variable and first advanced flag -httpx.tcp-network (

#123)

* Add flagx.Advanced flag set
* Add httpx advanced flag to select tcp network

flagx/advanced.go

@@ -0,0 +1,20 @@
+package flagx
+
+import "flag"
+
+var (
+	// Advanced is a *flag.FlagSet for advanced flags. Packages should add flags
+	// to Advanced when those flags should NOT be included in the default
+	// flag.CommandLine flag set. Advanced flags may be enabled by calling
+	// EnableAdvancedFlags _before_ calling flag.Parse().
+	Advanced = flag.NewFlagSet("advanced", flag.ExitOnError)
+)
+
+// EnableAdvancedFlags adds all flags registered with the Advanced flag set to
+// the default flag.CommandLine flag set. EnableAdvancedFlags should be called
+// before flag.Parse().
+func EnableAdvancedFlags() {
+	Advanced.VisitAll(func(f *flag.Flag) {
+		flag.Var(f.Value, f.Name, f.Usage)
+	})
+}

flagx/advanced_test.go

@@ -0,0 +1,47 @@
+package flagx
+
+import (
+	"flag"
+	"testing"
+
+	"github.com/go-test/deep"
+)
+
+func TestEnableAdvancedFlags(t *testing.T) {
+	t.Run("example", func(t *testing.T) {
+		// Add advanced flags.
+		e := Enum{
+			Options: []string{"a", "b"},
+			Value:   "b",
+		}
+		s := "advanced"
+		Advanced.Var(&e, "advanced-enum-flag", "advanced-usage")
+		Advanced.StringVar(&s, "advanced-string-flag", "", "advanced-usage")
+
+		// Add default flags.
+		// Reset the default command line flag to simplilfy checking tests.
+		flag.CommandLine = flag.NewFlagSet("default", flag.ContinueOnError)
+		s2 := "default"
+		flag.StringVar(&s2, "default-flag", "", "default-usage")
+
+		// Add advanced flags to default set.
+		EnableAdvancedFlags()
+
+		found := map[string]bool{}
+		expected := map[string]bool{
+			"advanced-enum-flag":   true,
+			"advanced-string-flag": true,
+			"default-flag":         true,
+		}
+		// Verify that they are present in the default set now.
+		flag.CommandLine.VisitAll(
+			func(f *flag.Flag) {
+				found[f.Name] = true
+				t.Logf("Found: %q", f.Name)
+			},
+		)
+		if diff := deep.Equal(found, expected); diff != nil {
+			t.Errorf("EnableAdvancedFlags() found the wrong advanced flags: %#v", diff)
+		}
+	})
+}

httpx/httpx.go

@@ -13,10 +13,24 @@ import (
 	"net/http"
 	"strings"
 	"time"
+
+	"github.com/m-lab/go/flagx"
 )
 
 var logFatalf = log.Fatalf
 
+// tcpNetwork defines the TCP network used by ListenAndServeAsync and
+// ListenAndServeTLSAsync HTTP(S) servers. See https://golang.org/pkg/net/#Dial
+var tcpNetwork = flagx.Enum{
+	Options: []string{"tcp", "tcp4", "tcp6"},
+	Value:   "tcp",
+}
+
+func init() {
+	// Add as an advanced flag.
+	flagx.Advanced.Var(&tcpNetwork, "httpx.tcp-network", "Controls the TCP stack used by httpx net.Listeners")
+}
+
 // The code here is adapted from https://golang.org/src/net/http/server.go?s=85391:85432#L2742
 
 // tcpKeepAliveListener sets TCP keep-alive timeouts on accepted
@@ -56,7 +70,7 @@ func serve(server *http.Server, listener net.Listener) {
 // contain the address and port which this server is listening on.
 func ListenAndServeAsync(server *http.Server) error {
 	// Start listening synchronously.
-	listener, err := net.Listen("tcp", server.Addr)
+	listener, err := net.Listen(tcpNetwork.Value, server.Addr)
 	if err != nil {
 		return err
 	}
@@ -87,7 +101,7 @@ func serveTLS(server *http.Server, listener net.Listener, certFile, keyFile stri
 // fatal error if the server dies for a reason besides ErrServerClosed.
 func ListenAndServeTLSAsync(server *http.Server, certFile, keyFile string) error {
 	// Start listening synchronously.
-	listener, err := net.Listen("tcp", server.Addr)
+	listener, err := net.Listen(tcpNetwork.Value, server.Addr)
 	if err != nil {
 		return err
 	}