plugin/product: improve product hooks API (#20556)

* plugin/product: improve product hooks API

* elaborate HookService.RegisterHooks method documentation

* remove unnecessary re-assigns

product/api.go

@@ -10,7 +10,6 @@ import (
 
 	"github.com/mattermost/mattermost-server/v6/app/request"
 	"github.com/mattermost/mattermost-server/v6/model"
-	"github.com/mattermost/mattermost-server/v6/plugin"
 	"github.com/mattermost/mattermost-server/v6/shared/filestore"
 )
 
@@ -108,19 +107,25 @@ type ConfigService interface {
 	SaveConfig(newCfg *model.Config, sendConfigChangeClusterMessage bool) (*model.Config, *model.Config, *model.AppError)
 }
 
-// Hooks is an interim solution for enabling plugin hooks on the multi-product architecture. After the
-// focalboard migration is completed, this API should replaced with something else that would enable a
-// product to register any hook. Currently this is added to unblock the migration.
-type Hooks interface {
-	plugin.ProductHooks
-}
-
 // HooksService is the API for adding exiting plugin hooks to the server so that they can be called as
 // they were. This Service is required to be used after the products start. Otherwise it will return an error.
 //
 // The service shall be registered via app.HooksKey service key.
 type HooksService interface {
-	RegisterHooks(productID string, hooks Hooks) error
+	// RegisterHook checks whether if the 'hooks' implements any method of plugin.Hooks methods. Rather than
+	// using the whole plugin.Hooks interface with its 20+ methods, a product can implement any exiting method
+	// of plugin.Hooks w/o requiring to declare which method they implemented or not. This is going to be
+	// checked on runtime. We have individual interfaces for each method declared in plugin.Hooks interface.
+	// Hence, while registering a product, the service will check if the product implements any of these individual
+	// interfaces. If so, a map of hook IDs that are implemented will be used to call the hooks. The method will
+	// return an error in case if there is an incorrect implementation of the any of the individual interface in runtime.
+	// Consider checking plugin.Hooks for the reference.
+	// Following methods are not allowed to be implemented in the product:
+	//  - plugin.Hooks.OnActivate
+	//  - plugin.Hooks.OnDeactivate
+	//  - plugin.Hooks.Implemented
+	//  - plugin.Hooks.ServeHTTP
+	RegisterHooks(productID string, hooks any) error
 }
 
 // FilestoreService is the API for accessing the file store.