Update the plugin watcher interface

pkg/kubelet/util/pluginwatcher/types.go

+/*
+Copyright 2018 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 pluginwatcher
+
+// PluginHandler is an interface a client of the pluginwatcher API needs to implement in
+// order to consume plugins
+// The PluginHandler follows the simple following state machine:
+//
+//                         +--------------------------------------+
+//                         |            ReRegistration            |
+//                         | Socket created with same plugin name |
+//                         |                                      |
+//                         |                                      |
+//    Socket Created       v                                      +        Socket Deleted
+// +------------------> Validate +---------------------------> Register +------------------> DeRegister
+//                         +                                      +                              +
+//                         |                                      |                              |
+//                         | Error                                | Error                        |
+//                         |                                      |                              |
+//                         v                                      v                              v
+//                        Out                                    Out                            Out
+//
+// The pluginwatcher module follows strictly and sequentially this state machine for each *plugin name*.
+// e.g: If you are Registering a plugin foo, you cannot get a DeRegister call for plugin foo
+//      until the Register("foo") call returns. Nor will you get a Validate("foo", "Different endpoint", ...)
+//      call until the Register("foo") call returns.
+//
+// ReRegistration: Socket created with same plugin name, usually for a plugin update
+// e.g: plugin with name foo registers at foo.com/foo-1.9.7 later a plugin with name foo
+//      registers at foo.com/foo-1.9.9
+//
+// DeRegistration: When ReRegistration happens only the deletion of the new socket will trigger a DeRegister call
+
+type PluginHandler interface {
+	// Validate returns an error if the information provided by
+	// the potential plugin is erroneous (unsupported version, ...)
+	ValidatePlugin(pluginName string, endpoint string, versions []string) error
+	// RegisterPlugin is called so that the plugin can be register by any
+	// plugin consumer
+	// Error encountered here can still be Notified to the plugin.
+	RegisterPlugin(pluginName, endpoint string) error
+	// DeRegister is called once the pluginwatcher observes that the socket has
+	// been deleted.
+	DeRegisterPlugin(pluginName string)
+}