MDL-81480 core: Sets user-level session storage functions

Also added @runInSeparateProcess on each session Redis cluster test to let the test run
in a separate process to avoid the error "the headers are not already sent" by PHPUnit.

lang/en/error.php

@@ -511,6 +511,7 @@ $string['privatefilesupload'] = 'Uploading directly to user private files area i
 $string['processingstops'] = 'Processing stops here. Remaining records ignored.';
 $string['querystringcannotbeempty'] = 'The query string cannot be empty.';
 $string['redirecterrordetected'] = 'Unsupported redirect detected, script execution terminated';
+$string['redissessionhandlerproblem'] = 'Setting up of redis session failed. Please notify the server administrator.';
 $string['refoundto'] = 'Can be refunded to {$a}';
 $string['refoundtoorigi'] = 'Refunded to original amount: {$a}';
 $string['registrationdisabled'] = 'Registration is disabled on this site';

lib/classes/session/redis.php

@@ -213,6 +213,11 @@ class redis extends handler implements SessionHandlerInterface {
             throw new exception('sessionhandlerproblem', 'error', '', null, 'redis extension version must be at least 2.0');
         }
 
+        $result = session_set_save_handler($this);
+        if (!$result) {
+            throw new exception('redissessionhandlerproblem', 'error');
+        }
+
         $encrypt = (bool) ($this->sslopts ?? false);
         // Set Redis server(s).
         $trimmedservers = [];

lib/tests/session_redis_cluster_test.php

@@ -68,6 +68,7 @@ class session_redis_cluster_test extends \advanced_testcase {
      *
      * It covers the behavior of session read and write operations under different compression configurations.
      *
+     * @runInSeparateProcess
      * @covers ::read
      * @covers ::write
      */
@@ -85,6 +86,7 @@ class session_redis_cluster_test extends \advanced_testcase {
      * It sets up the environment to simulate multiple failed connection attempts and
      * checks if the expected exception message is received.
      *
+     * @runInSeparateProcess
      * @covers ::init
      */
     public function test_exception_when_connection_attempts_exceeded(): void {