PLT-7407: Back-end plugin mechanism (#7177)

* begin backend plugin wip

* flesh out rpcplugin. everything done except for minor supervisor stubs

* done with basic plugin infrastructure

* simplify tests

* remove unused test lines

plugin/rpcplugin/hooks.go

@@ -0,0 +1,77 @@
+package rpcplugin
+
+import (
+	"io"
+	"net/rpc"
+
+	"github.com/mattermost/platform/plugin"
+)
+
+type LocalHooks struct {
+	hooks     plugin.Hooks
+	muxer     *Muxer
+	remoteAPI *RemoteAPI
+}
+
+func (h *LocalHooks) OnActivate(args int64, reply *struct{}) error {
+	stream := h.muxer.Connect(args)
+	if h.remoteAPI != nil {
+		h.remoteAPI.Close()
+	}
+	h.remoteAPI = ConnectAPI(stream, h.muxer)
+	return h.hooks.OnActivate(h.remoteAPI)
+}
+
+func (h *LocalHooks) OnDeactivate(args, reply *struct{}) error {
+	err := h.hooks.OnDeactivate()
+	if h.remoteAPI != nil {
+		h.remoteAPI.Close()
+		h.remoteAPI = nil
+	}
+	return err
+}
+
+type RemoteHooks struct {
+	client    *rpc.Client
+	muxer     *Muxer
+	apiCloser io.Closer
+}
+
+func ServeHooks(hooks plugin.Hooks, conn io.ReadWriteCloser, muxer *Muxer) {
+	server := rpc.NewServer()
+	server.Register(&LocalHooks{
+		hooks: hooks,
+		muxer: muxer,
+	})
+	server.ServeConn(conn)
+}
+
+var _ plugin.Hooks = (*RemoteHooks)(nil)
+
+func (h *RemoteHooks) OnActivate(api plugin.API) error {
+	id, stream := h.muxer.Serve()
+	if h.apiCloser != nil {
+		h.apiCloser.Close()
+	}
+	h.apiCloser = stream
+	go ServeAPI(api, stream, h.muxer)
+	return h.client.Call("LocalHooks.OnActivate", id, nil)
+}
+
+func (h *RemoteHooks) OnDeactivate() error {
+	return h.client.Call("LocalHooks.OnDeactivate", struct{}{}, nil)
+}
+
+func (h *RemoteHooks) Close() error {
+	if h.apiCloser != nil {
+		h.apiCloser.Close()
+	}
+	return h.client.Close()
+}
+
+func ConnectHooks(conn io.ReadWriteCloser, muxer *Muxer) *RemoteHooks {
+	return &RemoteHooks{
+		client: rpc.NewClient(conn),
+		muxer:  muxer,
+	}
+}