demonstration of promises breaking bugsnag requestData. Tested with node v6.9 and v8.0

index.js

const express = require("express");
const bugsnag = require("bugsnag");
const unhandledRejection = require("unhandled-rejection");

let rejectionEmitter = unhandledRejection({ timeout: 20 });
rejectionEmitter.on("unhandledRejection", (error) => {
  console.log("Promise....REJECTED!");
  bugsnag.notify(error);
});

const app = express();

bugsnag.register("SOME-KEY");

app.use(bugsnag.requestHandler);

bugsnag.onBeforeNotify((notification) => {
  console.log("callback request data: " + process.domain._bugsnagOptions);
  return false;
});

// works just fine
app.get("/", (req, res, next) => {
  console.log("GET request data: " + process.domain._bugsnagOptions);
});

// fails, but because there's no domain context when the rejection emitter
// catches the promise rejection. Regular throws don't have this problem.
app.post("/", (req, res, next) => {
  console.log("POST request data: " + process.domain._bugsnagOptions);
  new Promise((res, rej) => {
    rej(new Error("Oh no"));
  });
});

app.listen(3000);

package.json

{
  "dependencies": {
    "bugsnag": "^1.9.1",
    "express": "^4.15.3",
    "unhandled-rejection": "^1.0.0"
  }
}

post_output.log

POST request data: [object Object]
Promise....REJECTED!
/Users/delisa/Code/bugsnag/sample-apps/js/node-domain-test/index.js:18
  console.log("callback request data: " + process.domain._bugsnagOptions);
                                                        ^

TypeError: Cannot read property '_bugsnagOptions' of undefined
    at bugsnag.onBeforeNotify (/Users/delisa/Code/bugsnag/sample-apps/js/node-domain-test/index.js:18:57)
    at Notification.<anonymous> (/Users/delisa/Code/bugsnag/sample-apps/js/node-domain-test/node_modules/bugsnag/lib/notification.js:116:19)
    at Array.forEach (native)
    at Notification.deliver (/Users/delisa/Code/bugsnag/sample-apps/js/node-domain-test/node_modules/bugsnag/lib/notification.js:115:41)
    at Object.Bugsnag.notify (/Users/delisa/Code/bugsnag/sample-apps/js/node-domain-test/node_modules/bugsnag/lib/bugsnag.js:111:22)
    at process.<anonymous> (/Users/delisa/Code/bugsnag/sample-apps/js/node-domain-test/node_modules/bugsnag/lib/bugsnag.js:75:21)
    at emitOne (events.js:115:13)
    at process.emit (events.js:210:7)
    at process._fatalException (bootstrap_node.js:329:26)