Adding 'MessageHasBeenDeleted' hook to plugins (#22476)

* Adding 'MessageHasBeenDeleted' hook to plugins

* Remove debug logs

* Add unit test

* Bumping the required version

* Fix CI problems

* Increasing the minimum version

* go fmt

---------

Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>
Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/app/plugin_hooks_test.go

@@ -429,6 +429,50 @@ func TestHookMessageHasBeenUpdated(t *testing.T) {
 	require.Nil(t, err)
 }
 
+func TestHookMessageHasBeenDeleted(t *testing.T) {
+	th := Setup(t).InitBasic()
+	defer th.TearDown()
+
+	var mockAPI plugintest.API
+	mockAPI.On("LoadPluginConfiguration", mock.Anything).Return(nil)
+	mockAPI.On("LogDebug", "message").Return(nil).Times(1)
+
+	tearDown, _, _ := SetAppEnvironmentWithPlugins(t,
+		[]string{
+			`
+		package main
+
+		import (
+			"github.com/mattermost/mattermost/server/public/plugin"
+			"github.com/mattermost/mattermost/server/public/model"
+		)
+
+		type MyPlugin struct {
+			plugin.MattermostPlugin
+		}
+
+		func (p *MyPlugin) MessageHasBeenDeleted(c *plugin.Context, post *model.Post) {
+			p.API.LogDebug(post.Message)
+		}
+
+		func main() {
+			plugin.ClientMain(&MyPlugin{})
+		}
+	`}, th.App, func(*model.Manifest) plugin.API { return &mockAPI })
+	defer tearDown()
+
+	post := &model.Post{
+		UserId:    th.BasicUser.Id,
+		ChannelId: th.BasicChannel.Id,
+		Message:   "message",
+		CreateAt:  model.GetMillis() - 10000,
+	}
+	_, err := th.App.CreatePost(th.Context, post, th.BasicChannel, false, true)
+	require.Nil(t, err)
+	_, err = th.App.DeletePost(th.Context, post.Id, th.BasicUser.Id)
+	require.Nil(t, err)
+}
+
 func TestHookFileWillBeUploaded(t *testing.T) {
 	t.Run("rejected", func(t *testing.T) {
 		th := Setup(t).InitBasic()

server/channels/app/post.go

@@ -1339,6 +1339,15 @@ func (a *App) DeletePost(c request.CTX, postID, deleteByID string) (*model.Post,
 		a.deleteFlaggedPosts(post.Id)
 	})
 
+	pluginPost := post.ForPlugin()
+	pluginContext := pluginContext(c)
+	a.Srv().Go(func() {
+		a.ch.RunMultiHook(func(hooks plugin.Hooks) bool {
+			hooks.MessageHasBeenDeleted(pluginContext, pluginPost)
+			return true
+		}, plugin.MessageHasBeenDeletedID)
+	})
+
 	a.Srv().Go(func() {
 		if err = a.RemoveNotifications(c, post, channel); err != nil {
 			a.Log().Error("DeletePost failed to delete notification", mlog.Err(err))