Stop shutdown handler outputting in child processes

src/Runner/ShutdownHandler.php

@@ -10,6 +10,7 @@
 namespace PHPUnit\Runner;
 
 use const PHP_EOL;
+use function getmypid;
 use function register_shutdown_function;
 use function rtrim;
 
@@ -42,13 +43,14 @@ private static function register(): void
         }
 
         self::$registered = true;
+        $pid              = getmypid();
 
         register_shutdown_function(
-            static function (): void
+            static function () use ($pid): void
             {
                 $message = rtrim(self::$message);
 
-                if ($message === '') {
+                if ($message === '' || $pid !== getmypid()) {
                     return;
                 }
 

tests/end-to-end/_files/ChildProcessTest.php

@@ -0,0 +1,30 @@
+<?php declare(strict_types=1);
+/*
+ * This file is part of PHPUnit.
+ *
+ * (c) Sebastian Bergmann <sebastian@phpunit.de>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+namespace PHPUnit\TestFixture;
+
+use function pcntl_fork;
+use function pcntl_wait;
+use PHPUnit\Framework\TestCase;
+
+final class ChildProcessTest extends TestCase
+{
+    public function testChildProcessOutput(): void
+    {
+        $child = pcntl_fork();
+        $this->assertGreaterThan(-1, $child);
+
+        if ($child) {
+            pcntl_wait($child);
+            $this->assertTrue(true);
+        } else {
+            exit(0);
+        }
+    }
+}

tests/end-to-end/cli/child-process-output.phpt

@@ -0,0 +1,26 @@
+--TEST--
+ShutdownHandler does not output when child process exits
+--SKIPIF--
+<?php declare(strict_types=1);
+if (!extension_loaded('pcntl')) {
+    print 'skip: Extension pcntl must be loaded.';
+}
+--FILE--
+<?php declare(strict_types=1);
+$_SERVER['argv'][] = '--do-not-cache-result';
+$_SERVER['argv'][] = '--no-configuration';
+$_SERVER['argv'][] = __DIR__ . '/../_files/ChildProcessTest.php';
+
+require __DIR__ . '/../../bootstrap.php';
+
+(new PHPUnit\TextUI\Application)->run($_SERVER['argv']);
+--EXPECTF--
+PHPUnit %s by Sebastian Bergmann and contributors.
+
+Runtime: %s
+
+.                                                                   1 / 1 (100%)
+
+Time: %s, Memory: %s
+
+OK (1 test, 2 assertions)