linux-kernel - Re: [PATCH v8 01/24] scripts/jobserver-exec: move the code to a class

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Message-ID: <87segj7l5r.fsf@trenco.lwn.net>
Date: Thu, 18 Sep 2025 10:58:08 -0600
From: Jonathan Corbet <corbet@....net>
To: Mauro Carvalho Chehab <mchehab+huawei@...nel.org>, Linux Doc Mailing
 List <linux-doc@...r.kernel.org>
Cc: Mauro Carvalho Chehab <mchehab+huawei@...nel.org>, Akira Yokosawa
 <akiyks@...il.com>, Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 01/24] scripts/jobserver-exec: move the code to a class

Mauro Carvalho Chehab <mchehab+huawei@...nel.org> writes:

> +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
> +
> +        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.
> +            # If the MAKEFLAGS variable contains multiple instances of the
> +            # --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.
> +            # Example argument: --jobserver-auth=fifo:/tmp/GMfifo8134
> +            _, _, path = fds.partition("fifo:")
> +
> +            if path:
> +                self.reader = os.open(path, os.O_RDONLY | os.O_NONBLOCK)
> +                self.writer = os.open(path, os.O_WRONLY)
> +            else:
> +                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.
> +                self.reader = os.open("/proc/self/fd/%d" % (self.reader),
> +                                      os.O_RDONLY | os.O_NONBLOCK)
> +
> +            # Read out as many jobserver slots as possible
> +            while True:
> +                try:
> +                    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 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.
> +            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.
> +            self.claim = None

Sigh ... this kind of massive try/except block is one of the best ways
I've found in Python to hide bugs - and Python has a few of those.  I
realize this is existing code, this is not the place to fix it, but I
would like to clean that up at some point.

jon