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#!/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.
# https://www.gnu.org/software/make/manual/html_node/POSIX-Jobserver.html#POSIX-Jobserver
from __future__ import print_function
import os, sys, fcntl, errno

# Default parallelism is "1" unless overridden on the command-line.
default="1"
if len(sys.argv) > 1:
	default=sys.argv[1]

# Set non-blocking for a given file descriptor.
def nonblock(fd):
	flags = fcntl.fcntl(fd, fcntl.F_GETFL)
	fcntl.fcntl(fd, fcntl.F_SETFL, flags | os.O_NONBLOCK)
	return fd

# Extract and prepare jobserver file descriptors from envirnoment.
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)]
	reader = nonblock(reader)
except (KeyError, IndexError, ValueError, IOError):
	# Any missing environment strings or bad fds should result in just
	# using the default specified parallelism.
	print(default)
	sys.exit(0)

# Read out as many jobserver slots as possible.
jobs = b""
while True:
	try:
		slot = os.read(reader, 1)
		jobs += slot
	except (OSError, IOError) as e:
		if e.errno == errno.EWOULDBLOCK:
			# Stop when reach the end of the jobserver queue.
			break
		raise e
# Return all the reserved slots.
os.write(writer, jobs)

# If the jobserver was (impossibly) full or communication failed, use default.
if len(jobs) < 1:
	print(default)
	sys.exit(0)

# Report available slots (with a bump for our caller's reserveration).
print(len(jobs) + 1)