promise: better stack traces for --trace-warnings

Give better stack traces for `PromiseRejectionHandledWarning`
and `UnhandledPromiseRejectionWarning`s.

For `PromiseRejectionHandledWarning`, when it is likely that there
is an `Error` object generated, it is created early to provide a
proper stack trace.

For `UnhandledPromiseRejectionWarning`, the stack trace of the
underlying error object is used, if possible.

Fixes: https://github.com/nodejs/node/issues/9523
PR-URL: https://github.com/nodejs/node/pull/9525
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

lib/internal/process/promises.js

@@ -8,6 +8,11 @@ let lastPromiseId = 1;
 
 exports.setup = setupPromises;
 
+function getAsynchronousRejectionWarningObject(uid) {
+  return new Error('Promise rejection was handled ' +
+                   `asynchronously (rejection id: ${uid})`);
+}
+
 function setupPromises(scheduleMicrotasks) {
   process._setupPromises(function(event, promise, reason) {
     if (event === promiseRejectEvent.unhandled)
@@ -31,10 +36,15 @@ function setupPromises(scheduleMicrotasks) {
       const uid = promiseToGuidProperty.get(promise);
       promiseToGuidProperty.delete(promise);
       if (hasBeenNotified === true) {
+        let warning = null;
+        if (!process.listenerCount('rejectionHandled')) {
+          // Generate the warning object early to get a good stack trace.
+          warning = getAsynchronousRejectionWarningObject(uid);
+        }
         process.nextTick(function() {
           if (!process.emit('rejectionHandled', promise)) {
-            const warning = new Error('Promise rejection was handled ' +
-                                      `asynchronously (rejection id: ${uid})`);
+            if (warning === null)
+              warning = getAsynchronousRejectionWarningObject(uid);
             warning.name = 'PromiseRejectionHandledWarning';
             warning.id = uid;
             process.emitWarning(warning);
@@ -50,6 +60,9 @@ function setupPromises(scheduleMicrotasks) {
                               `(rejection id: ${uid}): ${reason}`);
     warning.name = 'UnhandledPromiseRejectionWarning';
     warning.id = uid;
+    if (reason instanceof Error) {
+      warning.stack = reason.stack;
+    }
     process.emitWarning(warning);
     if (!deprecationWarned) {
       deprecationWarned = true;

test/message/unhandled_promise_trace_warnings.js

@@ -0,0 +1,5 @@
+// Flags: --trace-warnings
+'use strict';
+require('../common');
+const p = Promise.reject(new Error('This was rejected'));
+setImmediate(() => p.catch(() => {}));

test/message/unhandled_promise_trace_warnings.out

@@ -0,0 +1,31 @@
+(node:*) Error: This was rejected
+    at * (*test*message*unhandled_promise_trace_warnings.js:*)
+    at *
+    at *
+    at *
+    at *
+    at *
+    at *
+    at *
+    at *
+    at *
+(node:*) DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
+    at *
+    at *
+    at *
+    at *
+    at *
+    at *
+    at *
+    at *
+    at *
+(node:*) PromiseRejectionHandledWarning: Promise rejection was handled asynchronously (rejection id: 1)
+    at getAsynchronousRejectionWarningObject (internal/process/promises.js:*)
+    at rejectionHandled (internal/process/promises.js:*)
+    at *
+    at Promise.then (native)
+    at Promise.catch (native)
+    at Immediate.setImmediate (*test*message*unhandled_promise_trace_warnings.js:*)
+    at *
+    at *
+    at *