MM-10249 Adding plugin ability to intercept posts before they reach the DB. (#8791)

* Adding plugin ability to intercept posts before they reach the DB.

* s/envoked/invoked/

app/plugin.go

@@ -661,3 +661,7 @@ func (a *App) ExecutePluginCommand(args *model.CommandArgs) (*model.Command, *mo
 	}
 	return nil, nil, nil
 }
+
+func (a *App) PluginsReady() bool {
+	return a.PluginEnv != nil && *a.Config().PluginSettings.Enable
+}

app/post.go

@@ -160,6 +160,14 @@ func (a *App) CreatePost(post *model.Post, channel *model.Channel, triggerWebhoo
 		return nil, err
 	}
 
+	if a.PluginsReady() {
+		if newPost, rejectionReason := a.PluginEnv.Hooks().MessageWillBePosted(post); newPost == nil {
+			return nil, model.NewAppError("createPost", "Post rejected by plugin. "+rejectionReason, nil, "", http.StatusBadRequest)
+		} else {
+			post = newPost
+		}
+	}
+
 	var rpost *model.Post
 	if result := <-a.Srv.Store.Post().Save(post); result.Err != nil {
 		return nil, result.Err
@@ -167,6 +175,12 @@ func (a *App) CreatePost(post *model.Post, channel *model.Channel, triggerWebhoo
 		rpost = result.Data.(*model.Post)
 	}
 
+	if a.PluginsReady() {
+		a.Go(func() {
+			a.PluginEnv.Hooks().MessageHasBeenPosted(rpost)
+		})
+	}
+
 	esInterface := a.Elasticsearch
 	if esInterface != nil && *a.Config().ElasticsearchSettings.EnableIndexing {
 		a.Go(func() {
@@ -371,11 +385,25 @@ func (a *App) UpdatePost(post *model.Post, safeUpdate bool) (*model.Post, *model
 		return nil, err
 	}
 
+	if a.PluginsReady() {
+		if pluginModifiedPost, rejectionReason := a.PluginEnv.Hooks().MessageWillBeUpdated(newPost, oldPost); pluginModifiedPost == nil {
+			return nil, model.NewAppError("createPost", "Post rejected by plugin. "+rejectionReason, nil, "", http.StatusBadRequest)
+		} else {
+			newPost = pluginModifiedPost
+		}
+	}
+
 	if result := <-a.Srv.Store.Post().Update(newPost, oldPost); result.Err != nil {
 		return nil, result.Err
 	} else {
 		rpost := result.Data.(*model.Post)
 
+		if a.PluginsReady() {
+			a.Go(func() {
+				a.PluginEnv.Hooks().MessageHasBeenUpdated(newPost, oldPost)
+			})
+		}
+
 		esInterface := a.Elasticsearch
 		if esInterface != nil && *a.Config().ElasticsearchSettings.EnableIndexing {
 			a.Go(func() {