Fix ERR_HTTP_HEADERS_ALREADY_SENT errors that pop up in the logs

lib/backend/api/index.js

@@ -6,6 +6,29 @@ const auth = require('@screeps/backend/lib/game/api/auth')
 const authlib = require('@screeps/backend/lib/authlib')
 const _ = require('lodash')
 
+/**
+ * Like legacy `q-json-response`, envelopes success as `{ ok: 1, ...data }`.
+ * Unlike it, does NOT call next() after responding: our routes shadow vanilla
+ * backend routes at the same paths, and falling through would execute the
+ * vanilla handler on an already-answered request (double side effects, and
+ * ERR_HTTP_HEADERS_SENT crashes in its tokenAuth).
+ */
+function jsonResponse (fn) {
+  return function (request, response) {
+    Promise.resolve()
+      .then(() => fn(request, response))
+      .then((data) => {
+        response.json(_.extend({ ok: 1 }, data))
+      })
+      .catch((error) => {
+        if (error.stack) console.error(error.stack)
+        if (!response.headersSent) {
+          response.json({ error: String(error) })
+        }
+      })
+  }
+}
+
 module.exports = function (config) {
   const { common } = config
   const { db, env, pubsub } = common.storage