1#!/usr/bin/env python3
2# SPDX-License-Identifier: GPL-2.0+
3#
4# This determines how many parallel tasks "make" is expecting, as it is
5# not exposed via an special variables, reserves them all, runs a subprocess
6# with PARALLELISM environment variable set, and releases the jobs back again.
7#
8# https://www.gnu.org/software/make/manual/html_node/POSIX-Jobserver.html#POSIX-Jobserver
9from __future__ import print_function
10import os, sys, errno
11import subprocess
12
13# Extract and prepare jobserver file descriptors from environment.
14claim = 0
15jobs = b""
16try:
17	# Fetch the make environment options.
18	flags = os.environ['MAKEFLAGS']
19
20	# Look for "--jobserver=R,W"
21	# Note that GNU Make has used --jobserver-fds and --jobserver-auth
22	# so this handles all of them.
23	opts = [x for x in flags.split(" ") if x.startswith("--jobserver")]
24
25	# Parse out R,W file descriptor numbers and set them nonblocking.
26	# If the MAKEFLAGS variable contains multiple instances of the
27	# --jobserver-auth= option, the last one is relevant.
28	fds = opts[-1].split("=", 1)[1]
29
30	# Starting with GNU Make 4.4, named pipes are used for reader and writer.
31	# Example argument: --jobserver-auth=fifo:/tmp/GMfifo8134
32	_, _, path = fds.partition('fifo:')
33
34	if path:
35		reader = os.open(path, os.O_RDONLY | os.O_NONBLOCK)
36		writer = os.open(path, os.O_WRONLY)
37	else:
38		reader, writer = [int(x) for x in fds.split(",", 1)]
39		# Open a private copy of reader to avoid setting nonblocking
40		# on an unexpecting process with the same reader fd.
41		reader = os.open("/proc/self/fd/%d" % (reader),
42				 os.O_RDONLY | os.O_NONBLOCK)
43
44	# Read out as many jobserver slots as possible.
45	while True:
46		try:
47			slot = os.read(reader, 8)
48			jobs += slot
49		except (OSError, IOError) as e:
50			if e.errno == errno.EWOULDBLOCK:
51				# Stop at the end of the jobserver queue.
52				break
53			# If something went wrong, give back the jobs.
54			if len(jobs):
55				os.write(writer, jobs)
56			raise e
57	# Add a bump for our caller's reserveration, since we're just going
58	# to sit here blocked on our child.
59	claim = len(jobs) + 1
60except (KeyError, IndexError, ValueError, OSError, IOError) as e:
61	# Any missing environment strings or bad fds should result in just
62	# not being parallel.
63	pass
64
65# We can only claim parallelism if there was a jobserver (i.e. a top-level
66# "-jN" argument) and there were no other failures. Otherwise leave out the
67# environment variable and let the child figure out what is best.
68if claim > 0:
69	os.environ['PARALLELISM'] = '%d' % (claim)
70
71rc = subprocess.call(sys.argv[1:])
72
73# Return all the reserved slots.
74if len(jobs):
75	os.write(writer, jobs)
76
77sys.exit(rc)
78