process: refactor unhandledRejection logic

This commit prevents a deprecation warning from being emitted
if the unhandledRejection event was actually handled.

PR-URL: #28540
Fixes: #28539
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Masashi Hirano <shisama07@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

lib/internal/process/promises.js

@@ -199,10 +199,12 @@ function processPromiseRejections() {
       }
       case kDefaultUnhandledRejections: {
         const handled = process.emit('unhandledRejection', reason, promise);
-        if (!handled) emitUnhandledRejectionWarning(uid, reason);
-        if (!deprecationWarned) {
-          emitDeprecationWarning();
-          deprecationWarned = true;
+        if (!handled) {
+          emitUnhandledRejectionWarning(uid, reason);
+          if (!deprecationWarned) {
+            emitDeprecationWarning();
+            deprecationWarned = true;
+          }
         }
         break;
       }

test/parallel/test-promise-handled-rejection-no-warning.js

@@ -0,0 +1,8 @@
+'use strict';
+const common = require('../common');
+
+// This test verifies that DEP0018 does not occur when rejections are handled.
+common.disableCrashOnUnhandledRejection();
+process.on('warning', common.mustNotCall());
+process.on('unhandledRejection', common.mustCall());
+Promise.reject(new Error());