unpack prepacked plugins on plugins initialization (#9149)

* unpack prepackaged plugins on plugins initialization

* leverage utils.FindDir

app/plugin.go

@@ -6,11 +6,13 @@ package app
 import (
 	"net/http"
 	"os"
+	"path/filepath"
 	"strings"
 
 	"github.com/mattermost/mattermost-server/mlog"
 	"github.com/mattermost/mattermost-server/model"
 	"github.com/mattermost/mattermost-server/plugin"
+	"github.com/mattermost/mattermost-server/utils"
 )
 
 func (a *App) SyncPluginsActiveState() {
@@ -101,6 +103,25 @@ func (a *App) InitPlugins(pluginDir, webappPluginDir string) {
 		a.Plugins = env
 	}
 
+	prepackagedPluginsDir, found := utils.FindDir("prepackaged_plugins")
+	if found {
+		if err := filepath.Walk(prepackagedPluginsDir, func(walkPath string, info os.FileInfo, err error) error {
+			if !strings.HasSuffix(walkPath, ".tar.gz") {
+				return nil
+			}
+
+			if fileReader, err := os.Open(walkPath); err != nil {
+				mlog.Error("Failed to open prepackaged plugin", mlog.Err(err), mlog.String("path", walkPath))
+			} else if _, err := a.InstallPlugin(fileReader, true); err != nil {
+				mlog.Error("Failed to unpack prepackaged plugin", mlog.Err(err), mlog.String("path", walkPath))
+			}
+
+			return nil
+		}); err != nil {
+			mlog.Error("Failed to complete unpacking prepackaged plugins", mlog.Err(err))
+		}
+	}
+
 	// Sync plugin active state when config changes. Also notify plugins.
 	a.RemoveConfigListener(a.PluginConfigListenerId)
 	a.PluginConfigListenerId = a.AddConfigListener(func(*model.Config, *model.Config) {