debug() may trigger fatal error "Serialization of 'Closure' is not allowed"

[leonstr]

When local_o365\debug() is called and debugmode is enabled an api_call_failed event is triggered containing the results of debug_backtrace():

    public static function debug($message, $where = '', $debugdata = null) {
            ⋮
            $backtrace = debug_backtrace();
            $otherdata = [
                'other' => [
                    ⋮
                    'backtrace' => $backtrace,
                ],
            ];
            $event = api_call_failed::create($otherdata);
            $event->trigger();
            ⋮
    }

However, when Moodle core's tool_log\helper\buffered_writer->write() tries to write this event it may fail with a PHP fatal error: Exception detected when logging event \local_o365\event\api_call_failed in logstore_standard: Serialization of 'Closure' is not allowed.

This is because the result of debug_backtrace() contains functions in the backtrace with their parameters, and these parameters may be closures which are not serializable. The following code demonstrates the problem:

function debug($f) {
    $backtrace = debug_backtrace();
    echo serialize($backtrace);
}   

$f = function () {
    return "test";
};  
    
debug($f);

Running this gets PHP Fatal error: Uncaught Exception: Serialization of 'Closure' is not allowed.

Obviously this is confusing because debug() is called to log some error state, but instead a completely unrelated error occurs.

This error does not occur if debug_backtrace() is called with $options = DEBUG_BACKTRACE_PROVIDE_OBJECT|DEBUG_BACKTRACE_IGNORE_ARGS, but then the backtrace does not contain function parameters which may be useful information when debugging.