[flagutil] Add flag type for string sets

Signed-off-by: Andrew Mason <amason@slack-corp.com>

go/flagutil/sets.go

@@ -0,0 +1,80 @@
+/*
+Copyright 2021 The Vitess Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+	http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package flagutil
+
+import (
+	"flag"
+	"strings"
+
+	"github.com/spf13/pflag"
+	"k8s.io/apimachinery/pkg/util/sets"
+)
+
+var (
+	_ flag.Value  = (*StringSetFlag)(nil)
+	_ pflag.Value = (*StringSetFlag)(nil)
+)
+
+// StringSetFlag can be used to collect multiple instances of a flag into a set
+// of values.
+//
+// For example, defining the following:
+//
+//		var x flagutil.StringSetFlag
+//		flag.Var(&x, "foo", "")
+//
+// And then specifying "-foo x -foo y -foo x", will result in a set of {x, y}.
+//
+// In addition to implemnting the standard flag.Value interface, it also
+// provides an implementation of pflag.Value, so it is usable in libraries like
+// cobra.
+type StringSetFlag struct {
+	set sets.String
+}
+
+// ToSet returns the underlying string set, or an empty set if the underlying
+// set is nil.
+func (set *StringSetFlag) ToSet() sets.String {
+	if set.set == nil {
+		set.set = sets.NewString()
+	}
+
+	return set.set
+}
+
+// Set is part of the pflag.Value and flag.Value interfaces.
+func (set *StringSetFlag) Set(s string) error {
+	if set.set == nil {
+		set.set = sets.NewString(s)
+		return nil
+	}
+
+	set.set.Insert(s)
+	return nil
+}
+
+// String is part of the pflag.Value and flag.Value interfaces.
+func (set *StringSetFlag) String() string {
+	if set.set == nil {
+		return ""
+	}
+
+	return strings.Join(set.set.List(), ", ")
+}
+
+// Type is part of the pflag.Value interface.
+func (set *StringSetFlag) Type() string { return "StringSetFlag" }