archwiki: ensure that MediaWiki file cache is invalidated on each MediaWiki update

We've hit this problem at least twice in the past, last time with the new
Vector skin update. With this change it should not be necessary to do
anything manually after an update.

roles/archwiki/handlers/main.yml

@@ -15,3 +15,12 @@
 # reference: https://stackoverflow.com/a/6896903
 - name: purge nginx cache
   command: find /var/lib/nginx/cache -type f -delete
+
+# The MediaWiki file cache can be invalidated by deleting the files in the
+# cache, by setting $wgCacheEpoch to the current time, or by touching the
+# LocalSettings.php file (thanks to $wgInvalidateCacheOnLocalSettingsChange
+# being set to true). References:
+# - https://www.mediawiki.org/wiki/Manual:File_cache
+# - https://www.mediawiki.org/wiki/Manual:$wgInvalidateCacheOnLocalSettingsChange
+- name: invalidate MediaWiki file cache
+  file: path="{{ archwiki_dir }}/public/LocalSettings.php" state=touch

roles/archwiki/tasks/main.yml

@@ -60,8 +60,10 @@
   become_user: "{{ archwiki_user }}"
   notify:
     - run wiki updatescript
-    # purge the nginx cache to make sure clients get the updated assets
+    # purge the nginx cache and MediaWiki file cache to make sure clients get updated assets
+    # as well as freshly rendered pages using the new assets
     - purge nginx cache
+    - invalidate MediaWiki file cache
 
 - name: configure archwiki
   template: src=LocalSettings.php.j2 dest="{{ archwiki_dir }}/public/LocalSettings.php" owner="{{ archwiki_user }}" group="{{ archwiki_user }}" mode=0640