How to debug Magento 2.0 LESS processor (Oyejorge)

Development
, ,
1

Magento 2.0 uses Oyejorge LESS processor to compile stylesheets from Less to CSS.
Today I encountered a bug in Oyejorge LESS processor which leads to segmentation fault in PHP interpreter.
Google Chrome browser returns ERR_CONNECTION_RESET error code.
wget return connection reset by peer message.
I fixed this bug, but I want to save the way to fix similar bugs for future.

File: vendor/oyejorge/less.php/lib/Less/Tree/Ruleset.php
Method: evalImports
Add temporary logging to it:

function evalImports($env) {
    static $deepLevel = -1;
    /** @var Magento\Framework\Logger\Monolog $logger */
    static $logger = \Magento\Framework\App\ObjectManager::getInstance()->get('Psr\Log\LoggerInterface');
    $deepLevel++;
    $logger->debug(str_repeat("\t", $deepLevel) . 'evalImports BEGIN');
    $rules_len = count($this->rules);
    for($i=0; $i < $rules_len; $i++){
        $rule = $this->rules[$i];

        if( $rule instanceof Less_Tree_Import ) {
            $logger->debug(str_repeat("\t", $deepLevel) . 'BEGIN RULE: ' . $rule->path->value);
            $deepLevel++;
            $rules = $rule->compile($env);
            if (is_array($rules)) {
                array_splice(
                    $this->rules,
                    $i,
                    1,
                    $rules
                );
                $temp_count = count($rules) - 1;
                $i += $temp_count;
                $rules_len += $temp_count;
            }
            else {
                array_splice(
                    $this->rules,
                    $i,
                    1,
                    array($rules)
                );
            }
            $this->resetCache();
            $deepLevel--;
            $logger->debug(str_repeat("\t", $deepLevel) . 'END RULE: ' . $rule->path->value);
        }
    }
    $logger->debug(str_repeat("\t", $deepLevel) . 'evalImports END');
    $deepLevel--;
}

Then execute the problem request (styles-m.css in my case):

wget -t 1 http://<your domain>/pub/static/frontend/Magento/luma/en_US/css/styles-m.css

And you will see in var/log/debug.log what the Less rule breaks Oyejorge LESS processor.