process: emit unhandled warning immediately

PR-URL: https://github.com/nodejs/node/pull/24632
Fixes: https://github.com/nodejs/node/issues/24209
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>

lib/internal/process/promises.js

@@ -108,7 +108,7 @@ function emitPromiseRejectionWarnings() {
     }
   }
 
-  let hadListeners = false;
+  let maybeScheduledTicks = false;
   let len = pendingUnhandledRejections.length;
   while (len--) {
     const promise = pendingUnhandledRejections.shift();
@@ -118,10 +118,9 @@ function emitPromiseRejectionWarnings() {
       const { reason, uid } = promiseInfo;
       if (!process.emit('unhandledRejection', reason, promise)) {
         emitWarning(uid, reason);
-      } else {
-        hadListeners = true;
       }
+      maybeScheduledTicks = true;
     }
   }
-  return hadListeners || pendingUnhandledRejections.length !== 0;
+  return maybeScheduledTicks || pendingUnhandledRejections.length !== 0;
 }

test/parallel/test-promises-unhandled-rejections.js

@@ -699,3 +699,22 @@ asyncTest('Rejected promise inside unhandledRejection allows nextTick loop' +
     process.nextTick(() => promise.catch(() => done()));
   });
 });
+
+asyncTest(
+  'Unhandled promise rejection emits a warning immediately',
+  function(done) {
+    clean();
+    Promise.reject(0);
+    const { emitWarning } = process;
+    process.emitWarning = common.mustCall((...args) => {
+      if (timer) {
+        clearTimeout(timer);
+        timer = null;
+        done();
+      }
+      emitWarning(...args);
+    }, 2);
+
+    let timer = setTimeout(common.mustNotCall(), 10000);
+  },
+);