Background of this is - "Prepare submissions for annotation (\assignfeedback_editpdf\task\convert_submissions)" scheduled task looks for assignment submissions queued in assignfeedback_editpdf_queue table to convert them to PDF files. Normally this process works fine.

However, if the file included in the student assignment submission contains extreme contents, e.g. an Excel file contains over 1 million lines, the conversion would exhaust server resource and eventually fail. See attached file for an example. Depending on the server configuration, this may not bring down the server, but it guarantees to stop the queue processing. Over time, the queue will become very long.

This may not always be a problem, because as I understand, when a teacher tries to grade an assignment submission using PDF annotation, if the submitted file hasn't be converted, it will be converted on the fly, so in most cases (which are submitted waiting in queue, delayed by the failed conversion), the difference may only be a few more seconds' waiting time. But for the submission with the buggy file, conversion on the fly will fail as well, making it impossible for the teacher to grade the assignment submission.

Some prevention mechanism would be preferred to:

1. Allow conversion queue to be processed.
2. Allow teachers to grade the assignment submission.

We have seen this issue in a few different Moodle instances that we manage, in different Moodle versions, e.g. 3.1, 3.3, 3.4, 3.5. Other versions