[p0lrb3ar]

This one took about ten minutes to fail. I'm thinking there should be a cursory check if the PDF is locked or protected in any way, then a shorter timeout period possibly?

I can post more examples if needed:

calibre, version 8.12.0
WARNING: Failed for some files: Failed to embed metadata into some book files. Click "Show details" for details.

The PDF format of Oxford Handbook of Respiratory Medicine 4E:

Traceback (most recent call last):
File "calibre\utils\podofo\__init__.py", line 36, in set_metadata
File "calibre\utils\ipc\simple_worker.py", line 251, in fork_job
File "calibre\utils\ipc\simple_worker.py", line 176, in run_job
File "calibre\utils\ipc\simple_worker.py", line 104, in communicate
calibre.utils.ipc.simple_worker.WorkerError: Worker appears to have hung

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "calibre\customize\ui.py", line 529, in set_file_type_metadata
File "calibre\customize\builtins.py", line 531, in set_metadata
File "calibre\ebooks\metadata\pdf.py", line 193, in set_metadata
File "calibre\utils\podofo\__init__.py", line 40, in set_metadata
Exception: Failed to set PDF metadata in (Oxford Handbook of Respiratory Medicine 4E):