Split plugin framework into its own package

Signed-off-by: Carlisia <carlisiac@vmware.com>

pkg/plugin/framework/server_mux.go

@@ -0,0 +1,77 @@
+/*
+Copyright 2018, 2019 the Velero contributors.
+
+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 framework
+
+import (
+	"github.com/pkg/errors"
+	"github.com/sirupsen/logrus"
+	"k8s.io/apimachinery/pkg/util/sets"
+)
+
+// HandlerInitializer is a function that initializes and returns a new instance of one of Velero's plugin interfaces
+// (ObjectStore, BlockStore, BackupItemAction, RestoreItemAction).
+type HandlerInitializer func(logger logrus.FieldLogger) (interface{}, error)
+
+// serverMux manages multiple implementations of a single plugin kind, such as pod and pvc BackupItemActions.
+type serverMux struct {
+	kind         PluginKind
+	initializers map[string]HandlerInitializer
+	handlers     map[string]interface{}
+	serverLog    logrus.FieldLogger
+}
+
+// newServerMux returns a new serverMux.
+func newServerMux(logger logrus.FieldLogger) *serverMux {
+	return &serverMux{
+		initializers: make(map[string]HandlerInitializer),
+		handlers:     make(map[string]interface{}),
+		serverLog:    logger,
+	}
+}
+
+// register registers the initializer for name.
+func (m *serverMux) register(name string, f HandlerInitializer) {
+	// TODO(ncdc): return an error on duplicate registrations for the same name.
+	m.initializers[name] = f
+}
+
+// names returns a list of all registered implementations.
+func (m *serverMux) names() []string {
+	return sets.StringKeySet(m.initializers).List()
+}
+
+// getHandler returns the instance for a plugin with the given name. If an instance has already been initialized,
+// that is returned. Otherwise, the instance is initialized by calling its initialization function.
+func (m *serverMux) getHandler(name string) (interface{}, error) {
+	if instance, found := m.handlers[name]; found {
+		return instance, nil
+	}
+
+	initializer, found := m.initializers[name]
+	if !found {
+		return nil, errors.Errorf("unknown %v plugin: %s", m.kind, name)
+	}
+
+	instance, err := initializer(m.serverLog)
+	if err != nil {
+		return nil, err
+	}
+
+	m.handlers[name] = instance
+
+	return m.handlers[name], nil
+}