MDL-58486 user_prefs: always update $USER cache when applicable

set_user_preference and unset_user_preference let you pass the user to
set preferences for as an id. Previously, if you passed $USER->id there,
those methods did not update the Cache in $USER, which was surprising,
and not easy to debug.

Now, we always update the preference cache in $USER if the preference is
being changed for the current user.

lib/moodlelib.php

@@ -1896,6 +1896,9 @@ function set_user_preference($name, $value, $user = null) {
 
     // Update value in cache.
     $user->preference[$name] = $value;
+    if ($user !== $USER && $user->id == $USER->id) {
+        $USER->preference[$name] = $value;
+    }
 
     // Set reload flag for other sessions.
     mark_user_preferences_changed($user->id);
@@ -1965,6 +1968,9 @@ function unset_user_preference($name, $user = null) {
 
     // Delete the preference from cache.
     unset($user->preference[$name]);
+    if ($user !== $USER && $user->id == $USER->id) {
+        unset($USER->preference[$name]);
+    }
 
     // Set reload flag for other sessions.
     mark_user_preferences_changed($user->id);

lib/tests/moodlelib_test.php

@@ -1170,6 +1170,26 @@ class core_moodlelib_testcase extends advanced_testcase {
         }
     }
 
+    public function test_set_user_preference_for_current_user() {
+        global $USER;
+        $this->resetAfterTest();
+        $this->setAdminUser();
+
+        set_user_preference('test_pref', 2);
+        set_user_preference('test_pref', 1, $USER->id);
+        $this->assertEquals(1, get_user_preferences('test_pref'));
+    }
+
+    public function test_unset_user_preference_for_current_user() {
+        global $USER;
+        $this->resetAfterTest();
+        $this->setAdminUser();
+
+        set_user_preference('test_pref', 1);
+        unset_user_preference('test_pref', $USER->id);
+        $this->assertNull(get_user_preferences('test_pref'));
+    }
+
     public function test_get_extra_user_fields() {
         global $CFG, $USER, $DB;
         $this->resetAfterTest();