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

Mauro Carvalho Chehab
2025-09-18T17:33:26Z

On Thu, Sep 18, 2025 at 10:58:08AM -0600, Jonathan Corbet wrote:
> Mauro Carvalho Chehab <mchehab+huawei@kernel.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.

Agreed: this class deserves some cleanup.

While working here, I considered doing more changes, but I refrained
myself. As you pointed, this is not the right series to do large
changes. Also, this is used not only for documentation build but also
inside scripts/Makefile.vmlinux_o. A major change there could affect
vmlinux generation.

Btw, besides Python cleanups, IMHO jobserver class could benefit
on having a maximum value for claim (which could even be 1). I was
also tempted to add a jobserver-aware subprocess.call here.

That's said, doing a risk analysis, if it fails to properly read
jobserver pipes, the except logic will set claim to None, which:

- for documentation: will use "-jauto";
- for vmlinux generation: will probably use a single job when
building vmlinux. If target dependencies are correct, this
should not cause build failures.

On my tests, I didn't get any such exception for doc build. I
wrote a small testbench to check if jobserver was handling claim
the right way. Before/after the changes, the behavior remained
the same, and caim was always initialized when running via make.

--
Thanks,
Mauro