#!/usr/bin/env python
  # SPDX-License-Identifier: GPL-2.0+
  #
  # This determines how many parallel tasks "make" is expecting, as it is
  # not exposed via an special variables, reserves them all, runs a subprocess
  # with PARALLELISM environment variable set, and releases the jobs back again.
  #
  # https://www.gnu.org/software/make/manual/html_node/POSIX-Jobserver.html#POSIX-Jobserver
  from __future__ import print_function
  import os, sys, errno
  import subprocess
  
  # Extract and prepare jobserver file descriptors from envirnoment.
  claim = 0
  jobs = b""
  try:
  	# Fetch the make environment options.
  	flags = os.environ['MAKEFLAGS']
  
  	# Look for "--jobserver=R,W"
  	# Note that GNU Make has used --jobserver-fds and --jobserver-auth
  	# so this handles all of them.
  	opts = [x for x in flags.split(" ") if x.startswith("--jobserver")]
  
  	# Parse out R,W file descriptor numbers and set them nonblocking.
  	fds = opts[0].split("=", 1)[1]
  	reader, writer = [int(x) for x in fds.split(",", 1)]
  	# Open a private copy of reader to avoid setting nonblocking
  	# on an unexpecting process with the same reader fd.
  	reader = os.open("/proc/self/fd/%d" % (reader),
  			 os.O_RDONLY | os.O_NONBLOCK)
  
  	# Read out as many jobserver slots as possible.
  	while True:
  		try:
  			slot = os.read(reader, 8)
  			jobs += slot
  		except (OSError, IOError) as e:
  			if e.errno == errno.EWOULDBLOCK:
  				# Stop at the end of the jobserver queue.
  				break
  			# If something went wrong, give back the jobs.
  			if len(jobs):
  				os.write(writer, jobs)
  			raise e
  	# Add a bump for our caller's reserveration, since we're just going
  	# to sit here blocked on our child.
  	claim = len(jobs) + 1
  except (KeyError, IndexError, ValueError, OSError, IOError) as e:
  	# Any missing environment strings or bad fds should result in just
  	# not being parallel.
  	pass
  
  # We can only claim parallelism if there was a jobserver (i.e. a top-level
  # "-jN" argument) and there were no other failures. Otherwise leave out the
  # environment variable and let the child figure out what is best.
  if claim > 0:
  	os.environ['PARALLELISM'] = '%d' % (claim)
  
  rc = subprocess.call(sys.argv[1:])
  
  # Return all the reserved slots.
  if len(jobs):
  	os.write(writer, jobs)
  
  sys.exit(rc)