[MM-29295] Fix data race in app.CreatePost() (#15880)

* Fix data race

* Better naming

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

app/post.go

@@ -320,11 +320,13 @@ func (a *App) CreatePost(post *model.Post, channel *model.Channel, triggerWebhoo
 	// might be duplicating requests.
 	a.Srv().seenPendingPostIdsCache.SetWithExpiry(post.PendingPostId, rpost.Id, PENDING_POST_IDS_CACHE_TTL)
 
+	// We make a copy of the post for the plugin hook to avoid a race condition.
+	rPostCopy := rpost.Clone()
 	if pluginsEnvironment := a.GetPluginsEnvironment(); pluginsEnvironment != nil {
 		a.Srv().Go(func() {
 			pluginContext := a.PluginContext()
 			pluginsEnvironment.RunMultiPluginHook(func(hooks plugin.Hooks) bool {
-				hooks.MessageHasBeenPosted(pluginContext, rpost)
+				hooks.MessageHasBeenPosted(pluginContext, rPostCopy)
 				return true
 			}, plugin.MessageHasBeenPostedId)
 		})