Issue #3364748: Add test coverage for the permissions handling that SiteConfigurationExcluder does for `sites/default`

package_manager/tests/src/Kernel/PathExcluder/SiteConfigurationExcluderTest.php

@@ -80,6 +80,26 @@ class SiteConfigurationExcluderTest extends PackageManagerKernelTestBase {
     }
   }
 
+  /**
+   * Tests that `sites/default` is made writable in the stage directory.
+   */
+  public function testDefaultSiteDirectoryPermissions(): void {
+    $project_root = $this->container->get(PathLocator::class)
+      ->getProjectRoot();
+    $live_dir = $project_root . '/sites/default';
+    chmod($live_dir, 0555);
+    $this->assertDirectoryIsNotWritable($live_dir);
+
+    $stage = $this->createStage();
+    $stage->create();
+    $staged_dir = $stage->getStageDirectory() . '/sites/default';
+    $this->assertDirectoryIsWritable($staged_dir);
+
+    $stage->require(['ext-json:*']);
+    $stage->apply();
+    $this->assertDirectoryIsNotWritable($live_dir);
+  }
+
 }
 
 /**