Plugins: Move backend plugin manager to service (#21474)

Moves backend plugin manager to service instead of
global functions in backendplugin package.

Closes #20053

pkg/plugins/backendplugin/backend_plugin.go

@@ -0,0 +1,98 @@
+package backendplugin
+
+import (
+	"context"
+	"errors"
+
+	datasourceV1 "github.com/grafana/grafana-plugin-model/go/datasource"
+	rendererV1 "github.com/grafana/grafana-plugin-model/go/renderer"
+	backend "github.com/grafana/grafana-plugin-sdk-go/backend"
+	"github.com/grafana/grafana/pkg/infra/log"
+	plugin "github.com/hashicorp/go-plugin"
+)
+
+// BackendPlugin a registered backend plugin.
+type BackendPlugin struct {
+	id             string
+	executablePath string
+	managed        bool
+	clientFactory  func() *plugin.Client
+	client         *plugin.Client
+	logger         log.Logger
+	startFns       PluginStartFuncs
+}
+
+func (p *BackendPlugin) start(ctx context.Context) error {
+	p.client = p.clientFactory()
+	rpcClient, err := p.client.Client()
+	if err != nil {
+		return err
+	}
+
+	var legacyClient *LegacyClient
+	var client *Client
+
+	if p.client.NegotiatedVersion() > 1 {
+		rawBackend, err := rpcClient.Dispense("backend")
+		if err != nil {
+			return err
+		}
+
+		rawTransform, err := rpcClient.Dispense("transform")
+		if err != nil {
+			return err
+		}
+
+		client = &Client{}
+		if rawBackend != nil {
+			if plugin, ok := rawBackend.(backend.BackendPlugin); ok {
+				client.BackendPlugin = plugin
+			}
+		}
+
+		if rawTransform != nil {
+			if plugin, ok := rawTransform.(backend.TransformPlugin); ok {
+				client.TransformPlugin = plugin
+			}
+		}
+	} else {
+		raw, err := rpcClient.Dispense(p.id)
+		if err != nil {
+			return err
+		}
+
+		legacyClient = &LegacyClient{}
+		if plugin, ok := raw.(datasourceV1.DatasourcePlugin); ok {
+			legacyClient.DatasourcePlugin = plugin
+		}
+
+		if plugin, ok := raw.(rendererV1.RendererPlugin); ok {
+			legacyClient.RendererPlugin = plugin
+		}
+	}
+
+	if legacyClient == nil && client == nil {
+		return errors.New("no compatible plugin implementation found")
+	}
+
+	if legacyClient != nil && p.startFns.OnLegacyStart != nil {
+		if err := p.startFns.OnLegacyStart(p.id, legacyClient, p.logger); err != nil {
+			return err
+		}
+	}
+
+	if client != nil && p.startFns.OnStart != nil {
+		if err := p.startFns.OnStart(p.id, client, p.logger); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+func (p *BackendPlugin) stop() error {
+	if p.client != nil {
+		p.client.Kill()
+	}
+	return nil
+}