MDL-67594 core_lock: Modify postgres get_lock() to avoid recursion

lib/classes/lock/postgres_lock_factory.php

@@ -118,15 +118,15 @@ class postgres_lock_factory implements lock_factory {
     }
 
     /**
-     * Multiple locks for the same resource can be held by a single process.
+     * Multiple locks for the same resource can NOT be held by a single process.
      *
      * @deprecated since Moodle 4.0.
-     * @return boolean - Defer to the DB driver.
+     * @return boolean - false.
      */
     public function supports_recursion() {
         debugging('The function supports_recursion() is deprecated, please do not use it anymore.',
             DEBUG_DEVELOPER);
-        return true;
+        return false;
     }
 
     /**
@@ -149,6 +149,7 @@ class postgres_lock_factory implements lock_factory {
 
     /**
      * Create and get a lock
+     *
      * @param string $resource - The identifier for the lock. Should use frankenstyle prefix.
      * @param int $timeout - The number of seconds to wait for a lock before giving up.
      * @param int $maxlifetime - Unused by this lock type.
@@ -159,8 +160,14 @@ class postgres_lock_factory implements lock_factory {
 
         $token = $this->get_index_from_key($this->type . '_' . $resource);
 
-        $params = array('locktype' => $this->dblockid,
-                        'token' => $token);
+        if (isset($this->openlocks[$token])) {
+            return false;
+        }
+
+        $params = [
+            'locktype' => $this->dblockid,
+            'token' => $token
+        ];
 
         $locked = false;