Issue #3036494 by Berdir, alexpott, rgpublic, cilefen, kerasai, regilero: Race condition in ImageStyle::createDerivative()

(cherry picked from commit f9fcde32ab)

core/lib/Drupal/Core/File/FileSystem.php

@@ -535,12 +535,22 @@ class FileSystem implements FileSystemInterface {
     }
 
     if (!is_dir($directory)) {
+      if (!($options & static::CREATE_DIRECTORY)) {
+        return FALSE;
+      }
+
       // Let mkdir() recursively create directories and use the default
       // directory permissions.
-      if ($options & static::CREATE_DIRECTORY) {
-        return @$this->mkdir($directory, NULL, TRUE);
+      $success = @$this->mkdir($directory, NULL, TRUE);
+      if ($success) {
+        return TRUE;
+      }
+      // If the operation failed, check again if the directory was created
+      // by another process/server, only report a failure if not. In this case
+      // we still need to ensure the directory is writable.
+      if (!is_dir($directory)) {
+        return FALSE;
       }
-      return FALSE;
     }
 
     $writable = is_writable($directory);