Add fetch* utils

server/utils/createMutex.js

@@ -1,7 +1,20 @@
-function createMutex(doWork) {
+const invariant = require("invariant");
+
+function defaultCreateKey(payload) {
+  return payload;
+}
+
+function createMutex(doWork, createKey = defaultCreateKey) {
   const mutex = Object.create(null);
 
-  return (key, payload, callback) => {
+  return (payload, callback) => {
+    const key = createKey(payload);
+
+    invariant(
+      typeof key === "string",
+      "Mutex needs a string key; please provide a createKey function that returns a string"
+    );
+
     if (mutex[key]) {
       mutex[key].push(callback);
     } else {