Split plugin framework into its own package

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

pkg/plugin/framework/client_dispenser.go

@@ -0,0 +1,75 @@
+/*
+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/sirupsen/logrus"
+	"google.golang.org/grpc"
+)
+
+// clientBase implements client and contains shared fields common to all clients.
+type clientBase struct {
+	plugin string
+	logger logrus.FieldLogger
+}
+
+type ClientDispenser interface {
+	ClientFor(name string) interface{}
+}
+
+// clientDispenser supports the initialization and retrieval of multiple implementations for a single plugin kind, such as
+// "aws" and "azure" implementations of the object store plugin.
+type clientDispenser struct {
+	// logger is the log the plugin should use.
+	logger logrus.FieldLogger
+	// clienConn is shared among all implementations for this client.
+	clientConn *grpc.ClientConn
+	// initFunc returns a client that implements a plugin interface, such as ObjectStore.
+	initFunc clientInitFunc
+	// clients keeps track of all the initialized implementations.
+	clients map[string]interface{}
+}
+
+type clientInitFunc func(base *clientBase, clientConn *grpc.ClientConn) interface{}
+
+// newClientDispenser creates a new clientDispenser.
+func newClientDispenser(logger logrus.FieldLogger, clientConn *grpc.ClientConn, initFunc clientInitFunc) *clientDispenser {
+	return &clientDispenser{
+		clientConn: clientConn,
+		logger:     logger,
+		initFunc:   initFunc,
+		clients:    make(map[string]interface{}),
+	}
+}
+
+// ClientFor returns a gRPC client stub for the implementation of a plugin named name. If the client stub does not
+// currently exist, clientFor creates it.
+func (cd *clientDispenser) ClientFor(name string) interface{} {
+	if client, found := cd.clients[name]; found {
+		return client
+	}
+
+	base := &clientBase{
+		plugin: name,
+		logger: cd.logger,
+	}
+	// Initialize the plugin (e.g. newBackupItemActionGRPCClient())
+	client := cd.initFunc(base, cd.clientConn)
+	cd.clients[name] = client
+
+	return client
+}