process: change default --unhandled-rejections=throw

This is a semver-major change that resolves DEP0018.

All users that have set an unhandledRejection hook or set a non-default
value for the --unhandled-rejections flag will see no change in behavior
after this change.

Refs: https://nodejs.org/dist/latest/docs/api/deprecations.html#deprecations_dep0018_unhandled_promise_rejections

PR-URL: #33021
Fixes: #20392
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Mary Marchini <oss@mmarchini.me>
Reviewed-By: Shelley Vohr <codebytere@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>

Author: dfabulich

doc/api/cli.md

@@ -1007,15 +1007,11 @@ added:
  - v10.17.0
 -->
 
-By default all unhandled rejections trigger a warning plus a deprecation warning
-for the very first unhandled rejection in case no [`unhandledRejection`][] hook
-is used.
-
 Using this flag allows to change what should happen when an unhandled rejection
 occurs. One of the following modes can be chosen:
 
 * `throw`: Emit [`unhandledRejection`][]. If this hook is not set, raise the
-  unhandled rejection as an uncaught exception.
+  unhandled rejection as an uncaught exception. This is the default.
 * `strict`: Raise the unhandled rejection as an uncaught exception.
 * `warn`: Always trigger a warning, no matter if the [`unhandledRejection`][]
   hook is set or not but do not print the deprecation warning.

lib/internal/process/promises.js

@@ -57,11 +57,6 @@ const kThrowUnhandledRejections = 3;
 
 const kWarnWithErrorCodeUnhandledRejections = 4;
 
-// --unhandled-rejections is unset:
-// Emit 'unhandledRejection', if it's unhandled, emit
-// 'UnhandledPromiseRejectionWarning', then emit deprecation warning.
-const kDefaultUnhandledRejections = 5;
-
 let unhandledRejectionsMode;
 
 function setHasRejectionToWarn(value) {
@@ -86,7 +81,7 @@ function getUnhandledRejectionsMode() {
     case 'warn-with-error-code':
       return kWarnWithErrorCodeUnhandledRejections;
     default:
-      return kDefaultUnhandledRejections;
+      return kThrowUnhandledRejections;
   }
 }
 
@@ -175,15 +170,6 @@ function emitUnhandledRejectionWarning(uid, reason) {
   process.emitWarning(warning);
 }
 
-let deprecationWarned = false;
-function emitDeprecationWarning() {
-  process.emitWarning(
-    '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.',
-    'DeprecationWarning', 'DEP0018');
-}
-
 // If this method returns true, we've executed user code or triggered
 // a warning to be emitted which requires the microtask and next tick
 // queues to be drained again.
@@ -241,17 +227,6 @@ function processPromiseRejections() {
         }
         break;
       }
-      case kDefaultUnhandledRejections: {
-        const handled = process.emit('unhandledRejection', reason, promise);
-        if (!handled) {
-          emitUnhandledRejectionWarning(uid, reason);
-          if (!deprecationWarned) {
-            emitDeprecationWarning();
-            deprecationWarned = true;
-          }
-        }
-        break;
-      }
     }
     maybeScheduledTicksOrMicrotasks = true;
   }

test/common/index.js

@@ -599,11 +599,8 @@ function getBufferSources(buf) {
   return [...getArrayBufferViews(buf), new Uint8Array(buf).buffer];
 }
 
-// Crash the process on unhandled rejections.
-const crashOnUnhandledRejection = (err) => { throw err; };
-process.on('unhandledRejection', crashOnUnhandledRejection);
 function disableCrashOnUnhandledRejection() {
-  process.removeListener('unhandledRejection', crashOnUnhandledRejection);
+  process.on('unhandledRejection', () => {});
 }
 
 function getTTYfd() {

test/es-module/test-esm-cjs-load-error-note.mjs

@@ -63,8 +63,7 @@ pImport2.stderr.on('data', (data) => {
   pImport2Stderr += data;
 });
 pImport2.on('close', mustCall((code) => {
-  // As this exits normally we expect 0
-  assert.strictEqual(code, 0);
+  assert.strictEqual(code, expectedCode);
   assert.ok(!pImport2Stderr.includes(expectedNote),
             `${expectedNote} must not be included in ${pImport2Stderr}`);
 }));
@@ -94,15 +93,15 @@ pImport4.on('close', mustCall((code) => {
             `${expectedNote} not found in ${pImport4Stderr}`);
 }));
 
-// Must exit with zero and show note
+// Must exit non-zero and show note
 const pImport5 = spawn(process.execPath, [Import5]);
 let pImport5Stderr = '';
 pImport5.stderr.setEncoding('utf8');
 pImport5.stderr.on('data', (data) => {
   pImport5Stderr += data;
 });
 pImport5.on('close', mustCall((code) => {
-  assert.strictEqual(code, 0);
+  assert.strictEqual(code, expectedCode);
   assert.ok(!pImport5Stderr.includes(expectedNote),
             `${expectedNote} must not be included in ${pImport5Stderr}`);
 }));

test/message/promise_unhandled_warn_with_error.js

@@ -1,10 +1,8 @@
 // Flags: --unhandled-rejections=warn-with-error-code
 'use strict';
 
-const common = require('../common');
+require('../common');
 const assert = require('assert');
 
-common.disableCrashOnUnhandledRejection();
-
 Promise.reject(new Error('alas'));
 process.on('exit', assert.strictEqual.bind(null, 1));

test/message/unhandled_promise_trace_warnings.js

@@ -1,6 +1,5 @@
-// Flags: --trace-warnings
+// Flags: --trace-warnings --unhandled-rejections=warn
 'use strict';
-const common = require('../common');
-common.disableCrashOnUnhandledRejection();
+require('../common');
 const p = Promise.reject(new Error('This was rejected'));
 setImmediate(() => p.catch(() => {}));

test/message/unhandled_promise_trace_warnings.out

@@ -17,10 +17,6 @@
     at *
     at *
     at *
-(node:*) [DEP0018] 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 *
 (node:*) PromiseRejectionHandledWarning: Promise rejection was handled asynchronously (rejection id: 1)
     at handledRejection (internal/process/promises.js:*)
     at promiseRejectHandler (internal/process/promises.js:*)

test/parallel/test-promise-unhandled-default.js

@@ -0,0 +1,52 @@
+'use strict';
+
+const common = require('../common');
+const Countdown = require('../common/countdown');
+const assert = require('assert');
+
+// Verify that unhandled rejections always trigger uncaught exceptions instead
+// of triggering unhandled rejections.
+
+const err1 = new Error('One');
+const err2 = new Error(
+  'This error originated either by throwing ' +
+  'inside of an async function without a catch block, or by rejecting a ' +
+  'promise which was not handled with .catch(). The promise rejected with the' +
+  ' reason "null".'
+);
+err2.code = 'ERR_UNHANDLED_REJECTION';
+Object.defineProperty(err2, 'name', {
+  value: 'UnhandledPromiseRejection',
+  writable: true,
+  configurable: true
+});
+
+const errors = [err1, err2];
+const identical = [true, false];
+
+const ref = new Promise(() => {
+  throw err1;
+});
+// Explicitly reject `null`.
+Promise.reject(null);
+
+process.on('warning', common.mustNotCall('warning'));
+// If we add an unhandledRejection handler, the exception won't be thrown
+// process.on('unhandledRejection', common.mustCall(2));
+process.on('rejectionHandled', common.mustNotCall('rejectionHandled'));
+process.on('exit', assert.strictEqual.bind(null, 0));
+
+const timer = setTimeout(() => console.log(ref), 1000);
+
+const counter = new Countdown(2, () => {
+  clearTimeout(timer);
+});
+
+process.on('uncaughtException', common.mustCall((err, origin) => {
+  counter.dec();
+  assert.strictEqual(origin, 'unhandledRejection', err);
+  const knownError = errors.shift();
+  assert.deepStrictEqual(err, knownError);
+  // Check if the errors are reference equal.
+  assert(identical.shift() ? err === knownError : err !== knownError);
+}, 2));

test/parallel/test-promise-unhandled-throw.js

@@ -5,8 +5,6 @@ const common = require('../common');
 const Countdown = require('../common/countdown');
 const assert = require('assert');
 
-common.disableCrashOnUnhandledRejection();
-
 // Verify that unhandled rejections always trigger uncaught exceptions instead
 // of triggering unhandled rejections.
 

test/parallel/test-promise-unhandled-warn-no-hook.js

@@ -3,10 +3,7 @@
 
 const common = require('../common');
 
-common.disableCrashOnUnhandledRejection();
-
-// Verify that ignoring unhandled rejection works fine and that no warning is
-// logged.
+// Verify that --unhandled-rejections=warn works fine
 
 new Promise(() => {
   throw new Error('One');