promise: warn on unhandled rejections

Log unhandled promise rejections with a guid and emit
a process warning. When rejection is eventually handled,
emit a secondary warning.

PR-URL: https://github.com/nodejs/node/pull/8217
Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

lib/internal/process/promises.js

@@ -2,7 +2,9 @@
 
 const promiseRejectEvent = process._promiseRejectEvent;
 const hasBeenNotifiedProperty = new WeakMap();
+const promiseToGuidProperty = new WeakMap();
 const pendingUnhandledRejections = [];
+let lastPromiseId = 1;
 
 exports.setup = setupPromises;
 
@@ -18,6 +20,7 @@ function setupPromises(scheduleMicrotasks) {
 
   function unhandledRejection(promise, reason) {
     hasBeenNotifiedProperty.set(promise, false);
+    promiseToGuidProperty.set(promise, lastPromiseId++);
     addPendingUnhandledRejection(promise, reason);
   }
 
@@ -25,9 +28,17 @@ function setupPromises(scheduleMicrotasks) {
     var hasBeenNotified = hasBeenNotifiedProperty.get(promise);
     if (hasBeenNotified !== undefined) {
       hasBeenNotifiedProperty.delete(promise);
+      const uid = promiseToGuidProperty.get(promise);
+      promiseToGuidProperty.delete(promise);
       if (hasBeenNotified === true) {
         process.nextTick(function() {
-          process.emit('rejectionHandled', promise);
+          if (!process.emit('rejectionHandled', promise)) {
+            const warning = new Error('Promise rejection was handled ' +
+                                      `asynchronously (rejection id: ${uid})`);
+            warning.name = 'PromiseRejectionHandledWarning';
+            warning.id = uid;
+            process.emitWarning(warning);
+          }
         });
       }
 
@@ -35,15 +46,19 @@ function setupPromises(scheduleMicrotasks) {
   }
 
   function emitPendingUnhandledRejections() {
-    var hadListeners = false;
+    let hadListeners = false;
     while (pendingUnhandledRejections.length > 0) {
-      var promise = pendingUnhandledRejections.shift();
+      const promise = pendingUnhandledRejections.shift();
-      var reason = pendingUnhandledRejections.shift();
+      const reason = pendingUnhandledRejections.shift();
       if (hasBeenNotifiedProperty.get(promise) === false) {
         hasBeenNotifiedProperty.set(promise, true);
+        const uid = promiseToGuidProperty.get(promise);
         if (!process.emit('unhandledRejection', reason, promise)) {
-          // Nobody is listening.
+          const warning = new Error('Unhandled promise rejection ' +
-          // TODO(petkaantonov) Take some default action, see #830
+                                    `(rejection id: ${uid}): ${reason}`);
+          warning.name = 'UnhandledPromiseRejectionWarning';
+          warning.id = uid;
+          process.emitWarning(warning);
         } else {
           hadListeners = true;
         }

test/parallel/test-promises-warning-on-unhandled-rejection.js

@@ -0,0 +1,26 @@
+// Flags: --no-warnings
+'use strict';
+
+// Test that warnings are emitted when a Promise experiences an uncaught
+// rejection, and then again if the rejection is handled later on.
+
+const common = require('../common');
+const assert = require('assert');
+
+var b = 0;
+
+process.on('warning', common.mustCall((warning) => {
+  switch (b++) {
+    case 0:
+      assert.strictEqual(warning.name, 'UnhandledPromiseRejectionWarning');
+      assert(/Unhandled promise rejection/.test(warning.message));
+      break;
+    case 1:
+      assert.strictEqual(warning.name, 'PromiseRejectionHandledWarning');
+      assert(/Promise rejection was handled asynchronously/
+        .test(warning.message));
+  }
+}, 2));
+
+const p = Promise.reject('This was rejected');
+setImmediate(common.mustCall(() => p.catch(() => {})));