Exceptions can be suppressed by improper lock release

[pennedav]

I had a case where the crawler just seemed to get stuck. Turns out it was because of a dml_write_exception being thrown from within crawl() in classes/robot/crawler.php. In my case it was because there was a page having a link that was longer than 1033 characters (the size of the url field). (This alone needs to be submitted as a separate issue.) What was confounding to me was that I had to dig to find the actual dml_write_exception because all I was seeing in my cron logs (PHP error logs were silent) was:

Adhoc task failed: tool_crawler\task\adhoc_crawl_task,Coding error detected, it must be fixed by a programmer: A lock was created but not released at: /path/to/moodle/admin/tool/crawler/classes/robot/crawler.php on line 526

 Code should look like:

 $factory = \core\lock\lock_config::get_lock_factory('type');
 $lock = $factory->get_lock(Resource id #28399);
 $lock->release();  // Locks must ALWAYS be released like this.

Wrapping the call to crawl() on line 541 in a try-catch-finally resolves this and you then actually see the error report in Moodle's cron output:

Execute adhoc task: tool_crawler\task\adhoc_crawl_task
... started 13:49:07. Current memory use 44.2MB.
... used 51 dbqueries
... used 0.75267791748047 seconds
Adhoc task failed: tool_crawler\task\adhoc_crawl_task,Error writing to database