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Issue #1934508 by jwilson3, clemens.tolboom, wesleydv, matglas86, longwave, sidharthap, superspring, MegaChriz: Make cache clear affect logo and favicon.
Problem/Motivation
Drupal has a great piece of functionality for front-end developers to force browsers to reload css and javascript. However, this same courtesy is not extended to the favicon and the logo image.
Developers using a git/svn based deployment process cannot overwrite the logo.png or favicon.ico files in a theme, deploy into a production environment, and expect the site to display the updated files because browsers that have already visited the site will pull these files from their cache.
From a front-end developer perspective, theme_get_setting does not provide any clean method to alter the data in order to add a cache-busting query string from a custom module or theme, thus the only way to ensure all browsers get a new version of these files is to either specify a separate path to a different logo or favicon file or upload a file with a different name. This can be done only via the Appearance page in the Drupal administrative user interface or by writing an update script in a custom module.
Proposed resolution
The proposed solution here appends the standard Drupal browser cache-busting string (css_js_query_string) to the logo and favicon URLs generated in theme_get_setting.
Remaining tasks
None.
User interface changes
No visible changes out of the box. But, because we've added query strings to the logo and to the favicon URLs, a net positive effect on rendering will occur when a site administrator updates the favicon or logo files on the server. A subsequent cache clear will guarantee all visitors will see the new file instead of the old file from cache.
API changes
The Drupal CSS/JS query-string internal variable has been renamed from css_js_query_string to drupal_asset_query_string, and the internal function _drupal_flush_css_js() to _drupal_flush_assets(), because both affect not only CSS and JS anymore.