deprecated flag handling

Several sidecars (for example, external-provisioner) deprecate command
line flags. When that happens, the flag must still be treated the same
way as before (i.e. optional values for boolean, required values for
non-boolean), but trying to use the flag needs to trigger a
deprecation warning.

An alternative to this code is to let the flag set a variable and then
check the variable. But that only catches cases where the variable is
set to a non-default value and leaves the variable in the code, which
can lead to mistakes (see
kubernetes-csi/external-provisioner#239).

deprecatedflags/deprecatedflags.go

@@ -0,0 +1,57 @@
+/*
+Copyright 2019 The Kubernetes 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 deprecatedflags can be used to declare a flag that is no
+// longer supported.
+package deprecatedflags
+
+import (
+	"flag"
+
+	"k8s.io/klog"
+)
+
+// Add defines a deprecated option which used to take some kind of
+// value. The type of the argument no longer matters, it gets ignored
+// and instead using the option triggers a deprecation warning. The
+// return code can be safely ignored and is only provided to support
+// replacing functions like flag.Int in a global variable section.
+func Add(name string) bool {
+	flag.Var(deprecated{name: name}, name, "This option is deprecated.")
+	return true
+}
+
+// AddBool defines a deprecated boolean option. Otherwise it behaves
+// like Add.
+func AddBool(name string) bool {
+	flag.Var(deprecated{name: name, isBool: true}, name, "This option is deprecated.")
+	return true
+}
+
+type deprecated struct {
+	name   string
+	isBool bool
+}
+
+var _ flag.Value = deprecated{}
+
+func (d deprecated) String() string { return "" }
+func (d deprecated) Set(value string) error {
+	klog.Warningf("Warning: option %s=%q is deprecated and has no effect", d.name, value)
+	return nil
+}
+func (d deprecated) Type() string     { return "" }
+func (d deprecated) IsBoolFlag() bool { return d.isBool }