Disable Xdebug in subprocesses when it is not used

src/Runner/PHPT/PhptTestCase.php

@@ -906,8 +906,6 @@ private function settings(bool $collectCoverage): array
         if (extension_loaded('xdebug')) {
             if ($collectCoverage) {
                 $settings[] = 'xdebug.mode=coverage';
-            } else {
-                $settings[] = 'xdebug.mode=off';
             }
         }
 

src/Util/PHP/DefaultJobRunner.php

@@ -27,6 +27,7 @@
 use function tempnam;
 use function trim;
 use function unlink;
+use PHPUnit\Runner\CodeCoverage;
 use SebastianBergmann\Environment\Runtime;
 
 /**
@@ -184,6 +185,14 @@ private function buildCommand(Job $job, ?string $file): array
                     array_keys($xdebugSettings),
                 ),
             );
+
+            // disable xdebug if not required to reduce xdebug performance overhead in subprocesses
+            if (
+                !CodeCoverage::instance()->isActive() &&
+                xdebug_is_debugger_active() === false
+            ) {
+                $phpSettings['xdebug.mode'] = 'xdebug.mode=off';
+            }
         }
 
         $command = array_merge($command, $this->settingsToParameters($phpSettings));

tests/end-to-end/event/_files/XdebugIsDisabled.php

@@ -0,0 +1,23 @@
+<?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\Event;
+
+use function extension_loaded;
+use function ini_get;
+use PHPUnit\Framework\TestCase;
+
+final class XdebugIsDisabled extends TestCase
+{
+    public function testOne(): void
+    {
+        $this->assertTrue(extension_loaded('xdebug'));
+        $this->assertSame('', (string) ini_get('xdebug.mode'));
+    }
+}

tests/end-to-end/event/process-isolation-disable-xdebug.phpt

@@ -0,0 +1,38 @@
+--TEST--
+Subprocesses auto-disable xdebug when no debugger is attached.
+--SKIPIF--
+<?php declare(strict_types=1);
+if (!extension_loaded('xdebug')) {
+    print 'skip: Extension xdebug must be loaded.';
+} elseif (xdebug_is_debugger_active() === true) {
+    print 'skip: Debugger must not be attached.';
+}
+
+--FILE--
+<?php declare(strict_types=1);
+$_SERVER['argv'][] = '--do-not-cache-result';
+$_SERVER['argv'][] = '--no-configuration';
+$_SERVER['argv'][] = '--process-isolation';
+$_SERVER['argv'][] = '--debug';
+$_SERVER['argv'][] = __DIR__ . '/_files/XdebugIsDisabled.php';
+
+require __DIR__ . '/../../bootstrap.php';
+
+(new PHPUnit\TextUI\Application)->run($_SERVER['argv']);
+--EXPECTF--
+PHPUnit Started (PHPUnit %s using %s)
+Test Runner Configured
+Event Facade Sealed
+Test Suite Loaded (1 test)
+Test Runner Started
+Test Suite Sorted
+Test Runner Execution Started (1 test)
+Test Suite Started (PHPUnit\TestFixture\Event\XdebugIsDisabled, 1 test)
+Test Preparation Started (PHPUnit\TestFixture\Event\XdebugIsDisabled::testOne)
+Test Prepared (PHPUnit\TestFixture\Event\XdebugIsDisabled::testOne)
+Test Passed (PHPUnit\TestFixture\Event\XdebugIsDisabled::testOne)
+Test Finished (PHPUnit\TestFixture\Event\XdebugIsDisabled::testOne)
+Test Suite Finished (PHPUnit\TestFixture\Event\XdebugIsDisabled, 1 test)
+Test Runner Execution Finished
+Test Runner Finished
+PHPUnit Finished (Shell Exit Code: 0)