Plugins: Add inititialization stage to plugin loader pipeline (#72667)

* first pass

* migrate tests

* simplify

* fix comments

* fix linter

* nil checks

* remove comment

pkg/plugins/manager/pipeline/initialization/steps.go

@@ -0,0 +1,90 @@
+package initialization
+
+import (
+	"context"
+	"errors"
+
+	"github.com/grafana/grafana/pkg/plugins"
+	"github.com/grafana/grafana/pkg/plugins/envvars"
+	"github.com/grafana/grafana/pkg/plugins/log"
+	"github.com/grafana/grafana/pkg/plugins/manager/registry"
+)
+
+// BackendClientInit implements an InitializeFunc for initializing a backend plugin process.
+//
+// It uses the envvars.Provider to retrieve the environment variables required for the plugin and the plugins.BackendFactoryProvider
+// to get fetch backend factory, which is used to form a connection to the backend plugin process.
+//
+// Note: This step does not start the backend plugin process.
+type BackendClientInit struct {
+	envVarProvider  envvars.Provider
+	backendProvider plugins.BackendFactoryProvider
+	log             log.Logger
+}
+
+// NewBackendClientInitStep returns a new InitializeFunc for registering a backend plugin process.
+func NewBackendClientInitStep(envVarProvider envvars.Provider,
+	backendProvider plugins.BackendFactoryProvider) InitializeFunc {
+	return newBackendProcessRegistration(envVarProvider, backendProvider).Initialize
+}
+
+func newBackendProcessRegistration(envVarProvider envvars.Provider,
+	backendProvider plugins.BackendFactoryProvider) *BackendClientInit {
+	return &BackendClientInit{
+		backendProvider: backendProvider,
+		envVarProvider:  envVarProvider,
+		log:             log.New("plugins.backend.registration"),
+	}
+}
+
+// Initialize will initialize a backend plugin client, if the plugin is a backend plugin.
+func (b *BackendClientInit) Initialize(ctx context.Context, p *plugins.Plugin) (*plugins.Plugin, error) {
+	if p.Backend {
+		backendFactory := b.backendProvider.BackendFactory(ctx, p)
+		if backendFactory == nil {
+			return nil, errors.New("could not find backend factory for plugin")
+		}
+
+		env, err := b.envVarProvider.Get(ctx, p)
+		if err != nil {
+			return nil, err
+		}
+		if backendClient, err := backendFactory(p.ID, p.Logger(), env); err != nil {
+			return nil, err
+		} else {
+			p.RegisterClient(backendClient)
+		}
+	}
+	return p, nil
+}
+
+// PluginRegistration implements an InitializeFunc for registering a plugin with the plugin registry.
+type PluginRegistration struct {
+	pluginRegistry registry.Service
+	log            log.Logger
+}
+
+// NewPluginRegistrationStep returns a new InitializeFunc for registering a plugin with the plugin registry.
+func NewPluginRegistrationStep(pluginRegistry registry.Service) InitializeFunc {
+	return newPluginRegistration(pluginRegistry).Initialize
+}
+
+func newPluginRegistration(pluginRegistry registry.Service) *PluginRegistration {
+	return &PluginRegistration{
+		pluginRegistry: pluginRegistry,
+		log:            log.New("plugins.registration"),
+	}
+}
+
+// Initialize registers the plugin with the plugin registry.
+func (r *PluginRegistration) Initialize(ctx context.Context, p *plugins.Plugin) (*plugins.Plugin, error) {
+	if err := r.pluginRegistry.Add(ctx, p); err != nil {
+		r.log.Error("Could not register plugin", "pluginID", p.ID, "err", err)
+		return nil, errors.New("could not register plugin")
+	}
+	if !p.IsCorePlugin() {
+		r.log.Info("Plugin registered", "pluginID", p.ID)
+	}
+
+	return p, nil
+}