From 7e3e2077f09414621f4ae9a62af353b9098a0d01 Mon Sep 17 00:00:00 2001
From: Mi Jia Looi <looimijia@gmail.com>
Date: Wed, 2 Jul 2025 12:58:08 +1000
Subject: [PATCH] MDL-73845 enrol_database: Add lock to prevent race condition

---
 enrol/database/db/upgrade.php | 51 +++++++++++++++++++++++++++++++++++
 enrol/database/lib.php        | 33 +++++++++++++++++++++--
 enrol/database/version.php    |  2 +-
 3 files changed, 83 insertions(+), 3 deletions(-)

diff --git a/enrol/database/db/upgrade.php b/enrol/database/db/upgrade.php
index b30cb9418b3..e1b606ba6cc 100644
--- a/enrol/database/db/upgrade.php
+++ b/enrol/database/db/upgrade.php
@@ -23,6 +23,7 @@
  */
 
 function xmldb_enrol_database_upgrade($oldversion) {
+    global $DB;
     // Automatically generated Moodle v4.1.0 release upgrade line.
     // Put any upgrade step following this.
 
@@ -38,5 +39,55 @@ function xmldb_enrol_database_upgrade($oldversion) {
     // Automatically generated Moodle v4.5.0 release upgrade line.
     // Put any upgrade step following this.
 
+    if ($oldversion < 2024100701) {
+        // Remove duplicated enrolment records, keeping only the earliest records.
+        $transaction = $DB->start_delegated_transaction();
+        $courses = $DB->get_records_sql(
+            "
+             SELECT courseid
+               FROM {enrol}
+              WHERE enrol = 'database'
+           GROUP BY courseid
+             HAVING COUNT(*) > 1"
+        );
+        foreach ($courses as $course) {
+            $instances = $DB->get_records('enrol', ['enrol' => 'database', 'courseid' => $course->courseid], 'id ASC');
+            $idtokeep = array_key_first($instances);
+            $idstodelete = array_slice(array_keys($instances), 1);
+            [$insql, $inparams] = $DB->get_in_or_equal($idstodelete, SQL_PARAMS_NAMED);
+
+            // Migrate enrolments where possible.
+            $DB->execute(
+                "
+                UPDATE {user_enrolments} ue
+                   SET enrolid = :idtokeep1
+                 WHERE ue.enrolid $insql
+                   AND NOT EXISTS (
+                       SELECT 1
+                         FROM {user_enrolments} ue2
+                        WHERE ue2.userid  = ue.userid
+                          AND ue2.enrolid = :idtokeep2)",
+                array_merge($inparams, ['idtokeep1' => $idtokeep, 'idtokeep2' => $idtokeep])
+            );
+
+            $DB->delete_records_select('user_enrolments', "enrolid $insql", $inparams);
+
+            // Migrate role assignments.
+            $DB->execute(
+                "
+                UPDATE {role_assignments}
+                   SET itemid = :idtokeep
+                 WHERE component = :component
+                   AND itemid $insql",
+                array_merge($inparams, ['component' => 'enrol_database', 'idtokeep' => $idtokeep])
+            );
+            $DB->delete_records_select('role_assignments', "itemid $insql", $inparams);
+
+            $DB->delete_records_list('enrol', 'id', $idstodelete);
+        }
+        $transaction->allow_commit();
+        upgrade_plugin_savepoint(true, 2024100701, 'enrol', 'database');
+    }
+
     return true;
 }
diff --git a/enrol/database/lib.php b/enrol/database/lib.php
index ec3d8d65e64..5b2173189f1 100644
--- a/enrol/database/lib.php
+++ b/enrol/database/lib.php
@@ -186,8 +186,37 @@ class enrol_database_plugin extends enrol_plugin {
                         continue;
                     }
 
-                    $enrolid = $this->add_instance($course);
-                    $instances[$course->id] = $DB->get_record('enrol', array('id'=>$enrolid));
+                    $timeout = 5;
+                    $locktype = 'enrol_database_user_enrolments';
+                    $resource = 'course:' . $course->id;
+                    $lockfactory = \core\lock\lock_config::get_lock_factory($locktype);
+                    if ($lock = $lockfactory->get_lock($resource, $timeout)) {
+                        try {
+                            $instance = $DB->get_record('enrol', ['enrol' => 'database', 'courseid' => $course->id]);
+                            if (!$instance) {
+                                $enrolid = $this->add_instance($course);
+                                $instance = $DB->get_record('enrol', ['id' => $enrolid]);
+                            }
+                        } finally {
+                            $lock->release();
+                        }
+                    } else {
+                        // Attempt to reuse an existing record added by another process during race condition.
+                        if ($instance = $DB->get_record('enrol', ['enrol' => 'database', 'courseid' => $course->id])) {
+                            $instances[$course->id] = $instance;
+                            continue;
+                        } else {
+                            // Give up.
+                            throw new moodle_exception(
+                                'locktimeout',
+                                'enrol_database',
+                                '',
+                                null,
+                                'Could not create database enrolment instance for course ' . $course->id
+                            );
+                        }
+                    }
+                    $instances[$course->id] = $instance;
                 }
             }
             $rs->Close();
diff --git a/enrol/database/version.php b/enrol/database/version.php
index 08a673e8be6..da6107c7df7 100644
--- a/enrol/database/version.php
+++ b/enrol/database/version.php
@@ -24,6 +24,6 @@
 
 defined('MOODLE_INTERNAL') || die();
 
-$plugin->version   = 2024100700;        // The current plugin version (Date: YYYYMMDDXX).
+$plugin->version   = 2024100701;        // The current plugin version (Date: YYYYMMDDXX).
 $plugin->requires  = 2024100100;        // Requires this Moodle version.
 $plugin->component = 'enrol_database';  // Full name of the plugin (used for diagnostics)