slice-override.py

#!/usr/bin/env python3

import os, sys, glob, re, pathlib as pl

src = pl.Path('/etc/systemd/system')
dst = pl.Path('/run/systemd/system')

for p in map(pl.Path, glob.glob(str(src / '*.slice'))):
	unit = p.read_text()
	for line in unit.splitlines():
		if re.search(r'\s*IO\w+Max=', line): break
	else: continue

	p_run, p_run_tmp = dst / p.name, dst / (p.name + '.tmp')
	try:
		with p_run_tmp.open('w') as tmp:
			for line in unit.strip().split('\n'):
				m = re.search(r'\s*(IO\w+Max)\s*=\s*(/dev/\S+)\s+(\S.*)$', line)
				if m:
					key, dev, limit = m.groups()
					try: dev = pl.Path(dev).resolve(strict=True).stat().st_rdev
					except OSError: pass
					else: line = f'{key}={os.major(dev)}:{os.minor(dev)} {limit.strip()}'
				tmp.write(f'{line}\n')
	except: p_run_tmp.unlink(missing_ok=True)
	else: p_run_tmp.rename(p_run)