fix(xo-server/worker): sync the remote handlers

Fixes #3739

packages/xo-server/src/xo-mixins/workers/worker.js

@@ -12,16 +12,18 @@ global.Promise = require('bluebird')
 // $FlowFixMe
 const config: Object = JSON.parse(process.env.XO_CONFIG)
 
-export function mergeVhd(
+export async function mergeVhd(
   parentRemote: Remote,
   parentPath: string,
   childRemote: Remote,
   childPath: string
 ) {
-  return mergeVhd_(
-    getHandler(parentRemote, config.remoteOptions),
-    parentPath,
-    getHandler(childRemote, config.remoteOptions),
-    childPath
-  )
+  const parentHandler = getHandler(parentRemote, config.remoteOptions)
+  const childHandler = getHandler(childRemote, config.remoteOptions)
+
+  // DO NOT forget the handlers as it they are still in use in the main process
+  await parentHandler.sync()
+  await childHandler.sync()
+
+  return mergeVhd_(parentHandler, parentPath, childHandler, childPath)
 }