Semaphore hissy fit at the end of subset run

[mp15]

Issue Description

Process completes successfully but multithreading throws a hissy fit about not being able to unlink its semaphores.

Logs

$ pod5 subset -t 50 PAO27011_pass_7b4991d0_ec3250cb.pod5 --missing-ok --summary sequencing_summary_PAO27011_7b4991d0_ec3250cb.txt --columns channel --output /tmp/tmp.Al3fg28Kg1
Subsetting: 99%|#########9| 2603/2623 [32:00<00:14, 1.36Files/s]
Traceback (most recent call last):
File "/software/python-3.10.1/lib/python3.10/multiprocessing/util.py", line 300, in _run_finalizers
finalizer()
File "/software/python-3.10.1/lib/python3.10/multiprocessing/util.py", line 224, in call
res = self._callback(*self._args, **self._kwargs)
File "/software/python-3.10.1/lib/python3.10/multiprocessing/synchronize.py", line 87, in _cleanup
sem_unlink(name)
FileNotFoundError: [Errno 2] No such file or directory

Specifications

• Pod5 Version: 0.3.6
• Python Version: Python 3.10.1
• Platform: Ubuntu Bionic 18.04

[HalfPhoton]

Hi @mp15,
Would you be able to add POD5_DEBUG=1 next time to hopefully capture in more detail what's going wrong here?

The number of "threads" (which are actually processes) -t 50 is quite high and the number of outputs (splitting by channel) is also quite high. This could be causing issues.
Please consider lowering this value - the subsetting process is predominantly IO bound and there's diminishing returns with increasing threads.