scripts/jobserver-exec: move the code to a class

#!/usr/bin/env python3

# SPDX-License-Identifier: GPL-2.0+

#

# pylint: disable=C0103,C0209

#

# 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

"""

Interacts with the POSIX jobserver during the Kernel build time.

A "normal" jobserver task, like the one initiated by a make subrocess would do:

    - open read/write file descriptors to communicate with the job server;

    - ask for one slot by calling:

        claim = os.read(reader, 1)

    - when the job finshes, call:

        os.write(writer, b"+")  # os.write(writer, claim)

Here, the goal is different: This script aims to get the remaining number

of slots available, using all of them to run a command which handle tasks in

parallel. To to that, it has a loop that ends only after there are no

slots left. It then increments the number by one, in order to allow a

call equivalent to make -j$((claim+1)), e.g. having a parent make creating

$claim child to do the actual work.

The end goal here is to keep the total number of build tasks under the

limit established by the initial make -j$n_proc call.

"""

import errno

import os

import subprocess

import sys

class JobserverExec:

    """

    Claim all slots from make using POSIX Jobserver.

    The main methods here are:

    - open(): reserves all slots;

    - close(): method returns all used slots back to make;

    - run(): executes a command setting PARALLELISM=<available slots jobs + 1>

    """

    def __init__(self):

        """Initialize internal vars"""

        self.claim = 0

        self.jobs = b""

        self.reader = None

        self.writer = None

        self.is_open = False

    def open(self):

        """Reserve all available slots to be claimed later on"""

        if self.is_open:

            return

# Extract and prepare jobserver file descriptors from environment.

claim = 0

jobs = b""

        try:

            # Fetch the make environment options.

	flags = os.environ['MAKEFLAGS']

            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.

            # --jobserver-auth= option, the last one is relevant.

            fds = opts[-1].split("=", 1)[1]

	# Starting with GNU Make 4.4, named pipes are used for reader and writer.

            # Starting with GNU Make 4.4, named pipes are used for reader

            # and writer.

            # Example argument: --jobserver-auth=fifo:/tmp/GMfifo8134

	_, _, path = fds.partition('fifo:')

            _, _, path = fds.partition("fifo:")

            if path:

		reader = os.open(path, os.O_RDONLY | os.O_NONBLOCK)

		writer = os.open(path, os.O_WRONLY)

                self.reader = os.open(path, os.O_RDONLY | os.O_NONBLOCK)

                self.writer = os.open(path, os.O_WRONLY)

            else:

		reader, writer = [int(x) for x in fds.split(",", 1)]

                self.reader, self.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),

                self.reader = os.open("/proc/self/fd/%d" % (self.reader),

                                      os.O_RDONLY | os.O_NONBLOCK)

	# Read out as many jobserver slots as possible.

            # Read out as many jobserver slots as possible

            while True:

                try:

			slot = os.read(reader, 8)

			jobs += slot

                    slot = os.read(self.reader, 8)

                    self.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)

                    if self.jobs:

                        os.write(self.writer, self.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:

            self.claim = len(self.jobs) + 1

        except (KeyError, IndexError, ValueError, OSError, IOError):

            # Any missing environment strings or bad fds should result in just

            # not being parallel.

	pass

            self.claim = None

        self.is_open = True

# 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)

    def close(self):

        """Return all reserved slots to Jobserver"""

rc = subprocess.call(sys.argv[1:])

        if not self.is_open:

            return

        # Return all the reserved slots.

if len(jobs):

	os.write(writer, jobs)

        if len(self.jobs):

            os.write(self.writer, self.jobs)

        self.is_open = False

    def __enter__(self):

        self.open()

        return self

    def __exit__(self, exc_type, exc_value, exc_traceback):

        self.close()

    def run(self, cmd):

        """

        Run a command setting PARALLELISM env variable to the number of

        available job slots (claim) + 1, e.g. it will reserve claim slots

        to do the actual build work, plus one to monitor its children.

        """

        self.open()             # Ensure that self.claim is set

        # 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 self.claim:

            os.environ["PARALLELISM"] = str(self.claim)

        return subprocess.call(cmd)

def main():

    """Main program"""

    with JobserverExec() as jobserver:

        jobserver.run(sys.argv[1:])

sys.exit(rc)

if __name__ == "__main__":

    main()