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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]
# 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)]
# 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)
except (KeyError, IndexError, ValueError, IOError, OSError) as e:
print(e, file=sys.stderr)
# 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)
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