Issue #3231992: Make the staged sites/default writable before cleaning

src/ComposerStager/Cleaner.php

+<?php
+
+namespace Drupal\automatic_updates\ComposerStager;
+
+use Drupal\automatic_updates\Locator;
+use PhpTuf\ComposerStager\Domain\CleanerInterface;
+use PhpTuf\ComposerStager\Domain\Output\ProcessOutputCallbackInterface;
+use Symfony\Component\Filesystem\Filesystem;
+
+/**
+ * Defines a cleaner service that makes the staged site directory writable.
+ */
+class Cleaner implements CleanerInterface {
+
+  /**
+   * The decorated cleaner service.
+   *
+   * @var \PhpTuf\ComposerStager\Domain\CleanerInterface
+   */
+  protected $decorated;
+
+  /**
+   * The current site path, without leading or trailing slashes.
+   *
+   * @var string
+   */
+  protected $sitePath;
+
+  /**
+   * The locator service.
+   *
+   * @var \Drupal\automatic_updates\Locator
+   */
+  protected $locator;
+
+  /**
+   * @param \PhpTuf\ComposerStager\Domain\CleanerInterface $decorated
+   *   The decorated cleaner service.
+   * @param string $site_path
+   *   The current site path (e.g., 'sites/default'), without leading or
+   *   trailing slashes.
+   * @param \Drupal\automatic_updates\Locator $locator
+   *   The locator service.
+   */
+  public function __construct(CleanerInterface $decorated, string $site_path, Locator $locator) {
+    $this->decorated = $decorated;
+    $this->sitePath = $site_path;
+    $this->locator = $locator;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function clean(string $stagingDir, ?ProcessOutputCallbackInterface $callback, ?int $timeout = 120): void {
+    // Ensure that the staged site path is writable.
+    $site_dir = implode(DIRECTORY_SEPARATOR, [
+      $stagingDir,
+      $this->locator->getProjectRoot() ?: '.',
+      $this->sitePath,
+    ]);
+
+    if ($this->directoryExists($site_dir)) {
+      (new Filesystem())->chmod($site_dir, 0777);
+    }
+    $this->decorated->clean($stagingDir, $callback, $timeout);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function directoryExists(string $stagingDir): bool {
+    return $this->decorated->directoryExists($stagingDir);
+  }
+
+}