MDL-75381 gradereport_grader: ensure valid paging preference value.

Set type of the report paging setting to integer, to ensure usage
of it is predictable. Unsupported operated type errors were thrown
on PHP8.0 when it's value contained a string or was empty.

grade/report/grader/lib.php

@@ -1994,7 +1994,7 @@ public function get_sort_arrows(array $extrafields = array()) {
      *
      * @return int The maximum number of students to display per page
      */
-    public function get_students_per_page() {
-        return $this->get_pref('studentsperpage');
+    public function get_students_per_page(): int {
+        return (int) $this->get_pref('studentsperpage');
     }
 }

grade/report/grader/settings.php

@@ -33,7 +33,7 @@
 
     /// Add settings for this module to the $settings object (it's already defined)
     $settings->add(new admin_setting_configtext('grade_report_studentsperpage', get_string('studentsperpage', 'grades'),
-                                            get_string('studentsperpage_help', 'grades'), 100));
+                                            get_string('studentsperpage_help', 'grades'), 100, PARAM_INT));
 
     $settings->add(new admin_setting_configcheckbox('grade_report_showonlyactiveenrol', get_string('showonlyactiveenrol', 'grades'),
                                                 get_string('showonlyactiveenrol_help', 'grades'), 1));

grade/tests/report_graderlib_test.php

@@ -29,6 +29,7 @@
  * Tests grade_report_grader (the grader report)
  *
  * @package  core_grades
+ * @covers   \grade_report_grader
  * @category test
  * @copyright 2012 Andrew Davis
  * @license  http://www.gnu.org/copyleft/gpl.html GNU Public License
@@ -518,6 +519,47 @@ public function test_get_right_rows() {
         $this->assertCount(3, $result);
     }
 
+    /**
+     * Test loading report users when per page preferences are set
+     */
+    public function test_load_users_paging_preference(): void {
+        $this->resetAfterTest();
+
+        $course = $this->getDataGenerator()->create_course();
+
+        // The report users will default to sorting by their lastname.
+        $user1 = $this->getDataGenerator()->create_and_enrol($course, 'student', ['lastname' => 'Apple']);
+        $user2 = $this->getDataGenerator()->create_and_enrol($course, 'student', ['lastname' => 'Banana']);
+        $user3 = $this->getDataGenerator()->create_and_enrol($course, 'student', ['lastname' => 'Carrot']);
+
+        // Set to empty string.
+        $report = $this->create_report($course);
+        $report->set_pref('studentsperpage', '');
+        $users = $report->load_users();
+        $this->assertEquals([$user1->id, $user2->id, $user3->id], array_column($users, 'id'));
+
+        // Set to valid value.
+        $report = $this->create_report($course);
+        $report->set_pref('studentsperpage', 2);
+        $users = $report->load_users();
+        $this->assertEquals([$user1->id, $user2->id], array_column($users, 'id'));
+    }
+
+    /**
+     * Test getting students per page report preference
+     */
+    public function test_get_students_per_page(): void {
+        $this->resetAfterTest();
+
+        $course = $this->getDataGenerator()->create_course();
+
+        $report = $this->create_report($course);
+        $report->set_pref('studentsperpage', 10);
+
+        $perpage = $report->get_students_per_page();
+        $this->assertSame(10, $perpage);
+    }
+
     private function create_grade_category($course) {
         static $cnt = 0;
         $cnt++;