Backend Plugins: Refactor backend plugin registration and start (#21452)

Moves the details of loading plugins into the backend
plugin manager from the respective plugin (datasource,
transform and renderer).

pkg/plugins/backendplugin/client.go

@@ -41,17 +41,30 @@ func newClientConfig(executablePath string, logger log.Logger, versionedPlugins
 	}
 }
 
+// LegacyStartFunc callback function called when a plugin with old plugin protocol is started.
+type LegacyStartFunc func(pluginID string, client *LegacyClient, logger log.Logger) error
+
+// StartFunc callback function called when a plugin with current plugin protocol version is started.
+type StartFunc func(pluginID string, client *Client, logger log.Logger) error
+
+// PluginStartFuncs functions called for plugin when started.
+type PluginStartFuncs struct {
+	OnLegacyStart LegacyStartFunc
+	OnStart       StartFunc
+}
+
 // PluginDescriptor descriptor used for registering backend plugins.
 type PluginDescriptor struct {
 	pluginID         string
 	executablePath   string
 	managed          bool
 	versionedPlugins map[int]plugin.PluginSet
+	startFns         PluginStartFuncs
 }
 
 // NewBackendPluginDescriptor creates a new backend plugin descriptor
 // used for registering a backend datasource plugin.
-func NewBackendPluginDescriptor(pluginID, executablePath string) PluginDescriptor {
+func NewBackendPluginDescriptor(pluginID, executablePath string, startFns PluginStartFuncs) PluginDescriptor {
 	return PluginDescriptor{
 		pluginID:       pluginID,
 		executablePath: executablePath,
@@ -65,12 +78,13 @@ func NewBackendPluginDescriptor(pluginID, executablePath string) PluginDescripto
 				"transform": &backend.TransformGRPCPlugin{},
 			},
 		},
+		startFns: startFns,
 	}
 }
 
 // NewRendererPluginDescriptor creates a new renderer plugin descriptor
 // used for registering a backend renderer plugin.
-func NewRendererPluginDescriptor(pluginID, executablePath string) PluginDescriptor {
+func NewRendererPluginDescriptor(pluginID, executablePath string, startFns PluginStartFuncs) PluginDescriptor {
 	return PluginDescriptor{
 		pluginID:       pluginID,
 		executablePath: executablePath,
@@ -80,5 +94,18 @@ func NewRendererPluginDescriptor(pluginID, executablePath string) PluginDescript
 				pluginID: &rendererV1.RendererPluginImpl{},
 			},
 		},
+		startFns: startFns,
 	}
 }
+
+// LegacyClient client for communicating with a plugin using the old plugin protocol.
+type LegacyClient struct {
+	DatasourcePlugin datasourceV1.DatasourcePlugin
+	RendererPlugin   rendererV1.RendererPlugin
+}
+
+// Client client for communicating with a plugin using the current plugin protocol.
+type Client struct {
+	BackendPlugin   backend.BackendPlugin
+	TransformPlugin backend.TransformPlugin
+}