MM-12974: guard app plugins with mutex (#9818)

* guard app plugins with mutex

Shutting down the app could race with a goroutine that uses the plugins environment, since we shut down the plugins first before cleaning up goroutines.

* fix go vet issues

app/plugin_api_test.go

@@ -41,7 +41,7 @@ func setupPluginApiTest(t *testing.T, pluginCode string, pluginManifest string,
 	require.NotNil(t, manifest)
 	require.True(t, activated)
 
-	app.Srv.Plugins = env
+	app.SetPluginsEnvironment(env)
 }
 
 func TestPluginAPIUpdateUserStatus(t *testing.T) {
@@ -206,7 +206,7 @@ func TestPluginAPILoadPluginConfiguration(t *testing.T) {
 			}
 		]
 	}}`, "testloadpluginconfig", th.App)
-	hooks, err := th.App.Srv.Plugins.HooksForPlugin("testloadpluginconfig")
+	hooks, err := th.App.GetPluginsEnvironment().HooksForPlugin("testloadpluginconfig")
 	assert.NoError(t, err)
 	_, ret := hooks.MessageWillBePosted(nil, nil)
 	assert.Equal(t, "str32true", ret)
@@ -280,7 +280,7 @@ func TestPluginAPILoadPluginConfigurationDefaults(t *testing.T) {
 			}
 		]
 	}}`, "testloadpluginconfig", th.App)
-	hooks, err := th.App.Srv.Plugins.HooksForPlugin("testloadpluginconfig")
+	hooks, err := th.App.GetPluginsEnvironment().HooksForPlugin("testloadpluginconfig")
 	assert.NoError(t, err)
 	_, ret := hooks.MessageWillBePosted(nil, nil)
 	assert.Equal(t, "override35true", ret)
@@ -377,9 +377,9 @@ func TestPluginAPIGetPlugins(t *testing.T) {
 		require.True(t, activated)
 		pluginManifests = append(pluginManifests, manifest)
 	}
-	th.App.Srv.Plugins = env
+	th.App.SetPluginsEnvironment(env)
 
-	// Decative the last one for testing
+	// Decativate the last one for testing
 	sucess := env.Deactivate(pluginIDs[len(pluginIDs)-1])
 	require.True(t, sucess)