MDL-43721 Assign + groups: Improve performance of assign grading table

Add a function to the groups lib to filter a list of users down
to the ones who can see the module. Required because calling
groups_course_module_visible() for a list of users is too slow
and we shouldn't spread group logic outside of grouplib.php.

Using it in the assign grading table reduces DB queries from 6198/1 to 256/3.

This is 12secs down to 2.5secs.

lib/grouplib.php

@@ -848,6 +848,77 @@ function groups_get_activity_allowed_groups($cm,$userid=0) {
     }
 }
 
+/**
+ * Filter a user list and return only the users that can see the course module based on
+ * groups/permissions etc. It is assumed that the users are pre-filtered to those who are enrolled in the course.
+ *
+ * @category group
+ * @param stdClass $cm The course module
+ * @param array $users An array of users, indexed by userid
+ * @return array A filtered list of users that can see the module, indexed by userid.
+ */
+function groups_filter_users_by_course_module_visible($cm, $users) {
+    global $CFG, $DB;
+
+    if (empty($CFG->enablegroupmembersonly)) {
+        return $users;
+    }
+    if (empty($cm->groupmembersonly)) {
+        return $users;
+    }
+    list($usql, $uparams) = $DB->get_in_or_equal(array_keys($users), SQL_PARAMS_NAMED, 'userid', true);
+
+    // Group membership sub-query.
+    if ($cm->groupingid) {
+        // Find out if member of any group in selected activity grouping.
+        $igsql = "SELECT gm.userid
+                  FROM {groups_members} gm
+                  LEFT JOIN {groupings_groups} gg
+                  ON gm.groupid = gg.groupid
+                  WHERE gm.userid $usql AND gg.groupingid = :groupingid";
+        $igparams = array_merge($uparams, array('groupingid' => $cm->groupingid));
+
+    } else {
+        // No grouping used - check all groups in course.
+        $igsql = "SELECT gm.userid
+                  FROM {groups_members} gm
+                  LEFT JOIN {groups} g
+                  ON gm.groupid = g.id
+                  WHERE gm.userid $usql AND g.courseid = :courseid";
+        $igparams = array_merge($uparams, array('courseid' => $cm->course));
+    }
+
+    $context = context_module::instance($cm->id);
+
+    // Get the list of users in a valid group.
+    $usersingroup = $DB->get_records_sql($igsql, $igparams);
+    if (!$usersingroup) {
+        $usersingroup = array();
+    } else {
+        $usersingroup = array_keys($usersingroup);
+    }
+
+    // Get the list of users who can access all groups.
+    list($accessallgroupssql, $accessallgroupsparams) = get_enrolled_sql($context, 'moodle/site:accessallgroups');
+
+    $userswithaccessallgroups = $DB->get_records_sql($accessallgroupssql, $accessallgroupsparams);
+    if (!$userswithaccessallgroups) {
+        $userswithaccessallgroups = array();
+    } else {
+        $userswithaccessallgroups = array_keys($userswithaccessallgroups);
+    }
+
+    // Explaining this array mangling:
+    // $users is an array of $user[$userid] => stdClass etc
+    // $userswithaccessallgroups is an array of $userswithaccessallgroups[random int] = $userid
+    // $usersingroup is an array of $usersingroup[random int] = $userid
+    // so - the inner array_merge combines the values of the 2 arrays disregarding the keys (because they are ints)
+    // this is then flipped so the values become the keys (and the values are now nonsense)
+    // this is then intersected with the users array only by looking at the keys - this
+    // returns only the users from the original array that had a value in one of the two $usersxxx lists.
+    return array_intersect_key($users, array_flip(array_merge($userswithaccessallgroups, $usersingroup)));
+}
+
 /**
  * Determine if a course module is currently visible to a user
  *

mod/assign/locallib.php

@@ -131,6 +131,9 @@ class assign {
     /** @var array list of suspended user IDs in form of ([id1] => id1) */
     public $susers = null;
 
+    /** @var array cached list of participants for this assignment. The cache key will be group, showactive and the context id */
+    private $participants = array();
+
     /**
      * Constructor for the base assign class.
      *
@@ -1292,22 +1295,26 @@ class assign {
      * @return array List of user records
      */
     public function list_participants($currentgroup, $idsonly) {
-        if ($idsonly) {
+        $key = $this->context->id . '-' . $currentgroup . '-' . $this->show_only_active_users();
-            $users = get_enrolled_users($this->context, 'mod/assign:submit', $currentgroup, 'u.id', null, null, null,
+        if (!isset($this->participants[$key])) {
-                    $this->show_only_active_users());
-        } else {
             $users = get_enrolled_users($this->context, 'mod/assign:submit', $currentgroup, 'u.*', null, null, null,
                     $this->show_only_active_users());
+
+            $cm = $this->get_course_module();
+            $users = groups_filter_users_by_course_module_visible($cm, $users);
+
+            $this->participants[$key] = $users;
         }
 
-        $cm = $this->get_course_module();
+        if ($idsonly) {
-        foreach ($users as $userid => $user) {
+            $idslist = array();
-            if (!groups_course_module_visible($cm, $userid)) {
+            foreach ($this->participants[$key] as $id => $user) {
-                unset($users[$userid]);
+                $idslist[$id] = new stdClass();
+                $idslist[$id]->id = $id;
             }
+            return $idslist;
         }
-
+        return $this->participants[$key];
-        return $users;
     }
 
     /**