Ensure slice types can safely round-trip through flag.FlagSet

context.go

@@ -362,7 +362,8 @@ func lookupBoolT(name string, set *flag.FlagSet) bool {
 
 func copyFlag(name string, ff *flag.Flag, set *flag.FlagSet) {
 	switch ff.Value.(type) {
-	case *StringSlice:
+	case Serializeder:
+		set.Set(name, ff.Value.(Serializeder).Serialized())
 	default:
 		set.Set(name, ff.Value.String())
 	}

flag.go

@@ -1,6 +1,7 @@
 package cli
 
 import (
+	"encoding/json"
 	"flag"
 	"fmt"
 	"os"
@@ -10,6 +11,10 @@ import (
 	"time"
 )
 
+var (
+	slPfx = fmt.Sprintf("sl:::%d:::", time.Now().UTC().UnixNano())
+)
+
 // This flag enables bash-completion for all commands and subcommands
 var BashCompletionFlag = BoolFlag{
 	Name: "generate-bash-completion",
@@ -29,6 +34,11 @@ var HelpFlag = BoolFlag{
 	Usage: "show help",
 }
 
+// Serializeder is used to circumvent the limitations of flag.FlagSet.Set
+type Serializeder interface {
+	Serialized() string
+}
+
 // Flag is a common interface related to parsing flags in cli.
 // For more advanced flag parsing techniques, it is recommended that
 // this interface be implemented.
@@ -130,6 +140,13 @@ func (f *StringSlice) Set(value string) error {
 		f.hasBeenSet = true
 	}
 
+	if strings.HasPrefix(value, slPfx) {
+		v := []string{}
+		_ = json.Unmarshal([]byte(strings.Replace(value, slPfx, "", 1)), v)
+		f.slice = append(f.slice, v...)
+		return nil
+	}
+
 	f.slice = append(f.slice, value)
 	return nil
 }
@@ -139,6 +156,12 @@ func (f *StringSlice) String() string {
 	return fmt.Sprintf("%s", f.slice)
 }
 
+// Serialized allows StringSlice to fulfill Serializeder
+func (f *StringSlice) Serialized() string {
+	jsonBytes, _ := json.Marshal(f.slice)
+	return fmt.Sprintf("%s%s", slPfx, string(jsonBytes))
+}
+
 // Value returns the slice of strings set by this flag
 func (f *StringSlice) Value() []string {
 	return f.slice
@@ -219,6 +242,13 @@ func (i *IntSlice) Set(value string) error {
 		i.hasBeenSet = true
 	}
 
+	if strings.HasPrefix(slPfx, value) {
+		v := []int{}
+		_ = json.Unmarshal([]byte(strings.Replace(value, slPfx, "", 1)), v)
+		i.slice = append(i.slice, v...)
+		return nil
+	}
+
 	tmp, err := strconv.Atoi(value)
 	if err != nil {
 		return err