really minimal jobserver

js.py

import sys
import subprocess
import queue
import threading
import functools
import multiprocessing

exit = []

def worker(q):
    print("Worker spawned", file=sys.stderr)
    while not exit:
        try:
            item = q.get(timeout=0.1)
        except queue.Empty:
            pass
        else:
            run(item)

def run(cmd):
    print("Running", cmd, file=sys.stderr)

    try:
        retcode = subprocess.call(cmd, shell=True)
        if retcode < 0:
            print("Child was terminated by signal", -retcode, file=sys.stderr)
            exit.append(1)
        elif retcode > 0:
            print("Child returned with error", retcode, file=sys.stderr)
            exit.append(1)
    except OSError as e:
        print("Execution failed:", e, file=sys.stderr)
        exit.append(1)

try:
    njobs = int(sys.argv[1])
except:
    njobs = multiprocessing.cpu_count()

print("njobs:", njobs, file=sys.stderr)

if njobs == 0:
    threads = [] # threads will be created as needed
elif njobs == 1:
    pass # no setup needed
else:
    work_items = queue.Queue()
    threads = [threading.Thread(target=functools.partial(worker, work_items))
               for _ in range(njobs)]
    for t in threads:
        t.start()

for cmd in sys.stdin:
    if njobs == 0:
        t = threading.Thread(target=functools.partial(run, cmd))
        thread.append(t)
        t.start()
    elif njobs == 1:
        run(cmd)
    else:
        work_items.put(cmd)

    if exit:
        break

exit.append(0)

if njobs != 1:
    for t in threads:
        t.join()

if exit:
    sys.exit(exit[0])