MDL-71401 core_completion: Fix completion status caching

When an automatic completion condition may have had its state change,
we now unset the cached value for the user's completion in the relevant
activity, so up-to-date values are re-fetched and available to students.
The previous behaviour was that custom conditions would remain cached
until the activity reached overall completion.

lib/completionlib.php

@@ -599,6 +599,24 @@ class completion_info {
             }
         }
 
+        // Default to current user if one is not provided.
+        if ($userid == 0) {
+            $userid = $USER->id;
+        }
+
+        // Delete the cm's cached completion data for this user if automatic completion is enabled.
+        // This ensures any changes to the status of individual completion conditions in the activity will be fetched.
+        if ($cm->completion == COMPLETION_TRACKING_AUTOMATIC) {
+            $completioncache = cache::make('core', 'completion');
+            $completionkey = $userid . '_' . $this->course->id;
+            $completiondata = $completioncache->get($completionkey);
+
+            if ($completiondata !== false) {
+                unset($completiondata[$cm->id]);
+                $completioncache->set($completionkey, $completiondata);
+            }
+        }
+
         // Get current value of completion state and do nothing if it's same as
         // the possible result of this change. If the change is to COMPLETE and the
         // current value is one of the COMPLETE_xx subtypes, ignore that as well
@@ -634,7 +652,7 @@ class completion_info {
             $newstate = $this->internal_get_state($cm, $userid, $current);
         }
 
-        // If changed, update
+        // If the overall completion state has changed, update it in the cache.
         if ($newstate != $current->completionstate) {
             $current->completionstate = $newstate;
             $current->timemodified    = time();