v6.x backport: promise: better stack traces for --trace-warnings

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);
@@ -58,6 +68,9 @@ function setupPromises(scheduleMicrotasks) {
                                     `(rejection id: ${uid}): ${reason}`);
           warning.name = 'UnhandledPromiseRejectionWarning';
           warning.id = uid;
+          if (reason instanceof Error) {
+            warning.stack = reason.stack;
+          }
           process.emitWarning(warning);
         } else {
           hadListeners = 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 +1,21 @@
+(node:*) Error: This was rejected
+    at * (*test*message*unhandled_promise_trace_warnings.js:*)
+    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 *