How to set a unique hash_salt for an individual site

Environment

• Multi-site
• Acquia Cloud Site Factory

Objective

In certain scenarios, if your application is using a multisite setup (like Acquia Site Factory), non-specific "PluginNotFound" or "Module not found" errors can show up in the logs.

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  Drupal\Component\Plugin\Exception\PluginNotFoundException: "The {...} plugin does not exist. Valid plugin IDs...

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  Drupal\Core\Extension\Exception\UnknownExtensionException: The module {...} does not exist. in {...}/core/lib/Drupal/Core/Extension/ExtensionList.php {...}

This happens because Drupal keeps track of available resources (modules, plugins, etc.) in shared cache backends (like memcache or APCu) that use the hash_salt value to identify the multisite instance. If this hash_salt isn't unique for each site, it can cause one multisite's settings to "bleed" into the other site(s).

Resolution

• Confirm which sites are not using a unique hash: How to find out if your Application is using unique hash salts between sites
• Update settings for unique hash_salts
  • Acquia Cloud Multi-Site
    • ​​​​​​​Add this line to the end of all your sites' settings.php file:
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      $settings['hash_salt'] = hash('sha256', $app_root . '/' . $site_path);

  • Site Factory 
    • ​​​​​​​Add a variation of the below code to your post-settings-php hooks: https://docs.acquia.com/site-factory/extend/hooks/settings-php/
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      $settings['hash_salt'] = hash('sha256', $site_settings['site'] . $site_settings['env'] . $site_settings['conf']['acsf_db_name']);​​​​​​​