Move some utils into middleware/utils

server/middleware/utils/createMutex.js

@@ -0,0 +1,21 @@
+function createMutex(doWork) {
+  const mutex = {}
+
+  return function (key, payload, callback) {
+    if (mutex[key]) {
+      mutex[key].push(callback)
+    } else {
+      mutex[key] = [ function () {
+        delete mutex[key]
+      }, callback ]
+
+      doWork(payload, function (error, value) {
+        mutex[key].forEach(function (callback) {
+          callback(error, value)
+        })
+      })
+    }
+  }
+}
+
+module.exports = createMutex