forked from goodboy/tractor
Hackery to override the stdlib's forkserver
The stdlib insists on creating multiple forkservers and semaphore trackers for each sub-sub-process launched. This isn't ideal since it costs each `tractor` sub-actor an additional 2 more processes then necessary and is confusing when viewed as a process tree (eg. via `pstree`). The majority of the change is simply avoiding the call to `forkserver.ensure_running()` and `semaphore_tracker.ensure_running()` in `ForkServer.connect_new_process()` and instead treating the user like an adult and expecting those calls to be made *once* in the parent most process (i.e. what `multiprocessing` calls the `MainProcess`). Really a proper patch should be made against cpython which allows for similar manual management of the server along with a mechanism to communicate forkserver and semaphore tracker fd info to sub-processes such that further calls to `Process.start()` work as expected. Relates to #6forkserver_singleton
Tyler Goodlet
parent
d6d7fea708
commit
f46d5b2b62
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@ -0,0 +1,275 @@
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"""
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This is near-copy of the 3.8 stdlib's ``multiprocessing.forkserver.py``
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with some hackery to prevent any more then a single forkserver and
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semaphore tracker per ``MainProcess``.
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"""
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import os
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import socket
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import signal
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import struct
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import sys
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import errno
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import selectors
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import warnings
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from multiprocessing import (
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forkserver, semaphore_tracker, spawn, process, util
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)
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from multiprocessing.forkserver import (
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ForkServer, MAXFDS_TO_SEND, write_unsigned,
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# _serve_one,
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)
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from multiprocessing.context import reduction
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# taken from 3.8
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SIGNED_STRUCT = struct.Struct('q') # large enough for pid_t
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class AdultForkServer(ForkServer):
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def connect_to_new_process(self, fds):
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'''Request forkserver to create a child process.
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Returns a pair of fds (status_r, data_w). The calling process can read
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the child process's pid and (eventually) its returncode from status_r.
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The calling process should write to data_w the pickled preparation and
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process data.
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'''
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# self.ensure_running() # treat our users like adults expecting
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# them to spawn the server on their own
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if len(fds) + 4 >= MAXFDS_TO_SEND:
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raise ValueError('too many fds')
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with socket.socket(socket.AF_UNIX) as client:
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client.connect(self._forkserver_address)
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parent_r, child_w = os.pipe()
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child_r, parent_w = os.pipe()
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allfds = [child_r, child_w, self._forkserver_alive_fd,
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semaphore_tracker.getfd()]
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allfds += fds
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# This is the only part changed
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try:
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while True:
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try:
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reduction.sendfds(client, allfds)
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break
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except OSError as err:
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if err.args[0] == 9:
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print(f"Bad FD {err}")
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client = socket.socket(socket.AF_UNIX)
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client.connect(self._forkserver_address)
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continue
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raise
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# This is the only part changed
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return parent_r, parent_w
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except Exception:
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os.close(parent_r)
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os.close(parent_w)
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raise
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finally:
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os.close(child_r)
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os.close(child_w)
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def main(listener_fd, alive_r, preload, main_path=None, sys_path=None):
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'''Run forkserver.'''
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if preload:
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if '__main__' in preload and main_path is not None:
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process.current_process()._inheriting = True
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try:
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spawn.import_main_path(main_path)
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finally:
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del process.current_process()._inheriting
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for modname in preload:
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try:
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__import__(modname)
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except ImportError:
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pass
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util._close_stdin()
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sig_r, sig_w = os.pipe()
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os.set_blocking(sig_r, False)
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os.set_blocking(sig_w, False)
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def sigchld_handler(*_unused):
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# Dummy signal handler, doesn't do anything
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pass
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handlers = {
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# unblocking SIGCHLD allows the wakeup fd to notify our event loop
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signal.SIGCHLD: sigchld_handler,
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# protect the process from ^C
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signal.SIGINT: signal.SIG_IGN,
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}
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old_handlers = {sig: signal.signal(sig, val)
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for (sig, val) in handlers.items()}
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# calling os.write() in the Python signal handler is racy
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signal.set_wakeup_fd(sig_w)
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# map child pids to client fds
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pid_to_fd = {}
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with socket.socket(socket.AF_UNIX, fileno=listener_fd) as listener, \
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selectors.DefaultSelector() as selector:
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_forkserver._forkserver_address = listener.getsockname()
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selector.register(listener, selectors.EVENT_READ)
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selector.register(alive_r, selectors.EVENT_READ)
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selector.register(sig_r, selectors.EVENT_READ)
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while True:
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try:
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while True:
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rfds = [key.fileobj for (key, events) in selector.select()]
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if rfds:
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break
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if alive_r in rfds:
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# EOF because no more client processes left
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assert os.read(alive_r, 1) == b'', "Not at EOF?"
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raise SystemExit
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if sig_r in rfds:
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# Got SIGCHLD
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os.read(sig_r, 65536) # exhaust
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while True:
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# Scan for child processes
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try:
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pid, sts = os.waitpid(-1, os.WNOHANG)
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except ChildProcessError:
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break
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if pid == 0:
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break
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child_w = pid_to_fd.pop(pid, None)
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if child_w is not None:
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if os.WIFSIGNALED(sts):
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returncode = -os.WTERMSIG(sts)
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else:
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if not os.WIFEXITED(sts):
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raise AssertionError(
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"Child {0:n} status is {1:n}".format(
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pid, sts))
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returncode = os.WEXITSTATUS(sts)
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# Send exit code to client process
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try:
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# write_signed(child_w, returncode)
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write_unsigned(child_w, returncode)
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except BrokenPipeError:
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# client vanished
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pass
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os.close(child_w)
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else:
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# This shouldn't happen really
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warnings.warn('forkserver: waitpid returned '
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'unexpected pid %d' % pid)
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if listener in rfds:
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# Incoming fork request
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with listener.accept()[0] as s:
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# Thing changed - be tolerant of socket disconnects
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try:
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# Receive fds from client
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fds = reduction.recvfds(s, MAXFDS_TO_SEND + 1)
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except EOFError:
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# broken socket due to reconnection on client-side
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continue
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# Thing changed - be tolerant of socket disconnects
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if len(fds) > MAXFDS_TO_SEND:
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raise RuntimeError(
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"Too many ({0:n}) fds to send".format(
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len(fds)))
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child_r, child_w, *fds = fds
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s.close()
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pid = os.fork()
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if pid == 0:
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# Child
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code = 1
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try:
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listener.close()
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selector.close()
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unused_fds = [alive_r, child_w, sig_r, sig_w]
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unused_fds.extend(pid_to_fd.values())
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code = _serve_one(child_r, fds,
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unused_fds,
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old_handlers)
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except Exception:
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sys.excepthook(*sys.exc_info())
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sys.stderr.flush()
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finally:
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os._exit(code)
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else:
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# Send pid to client process
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try:
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# write_signed(child_w, pid)
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write_unsigned(child_w, pid)
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except BrokenPipeError:
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# client vanished
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pass
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pid_to_fd[pid] = child_w
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os.close(child_r)
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for fd in fds:
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os.close(fd)
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except OSError as e:
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if e.errno != errno.ECONNABORTED:
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raise
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def _serve_one(child_r, fds, unused_fds, handlers):
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# close unnecessary stuff and reset signal handlers
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signal.set_wakeup_fd(-1)
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for sig, val in handlers.items():
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signal.signal(sig, val)
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for fd in unused_fds:
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os.close(fd)
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(_forkserver._forkserver_alive_fd,
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semaphore_tracker._semaphore_tracker._fd,
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*_forkserver._inherited_fds) = fds
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# Run process object received over pipe
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code = spawn._main(child_r)
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return code
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def write_signed(fd, n):
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msg = SIGNED_STRUCT.pack(n)
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while msg:
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nbytes = os.write(fd, msg)
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if nbytes == 0:
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raise RuntimeError('should not get here')
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msg = msg[nbytes:]
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class AdultSemaphoreTracker(semaphore_tracker.SemaphoreTracker):
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"""Stop GD ensuring everything is running...
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"""
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def getfd(self):
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# self.ensure_running()
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return self._fd
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# override the stdlib's stuff
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_semaphore_tracker = AdultSemaphoreTracker()
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semaphore_tracker._semaphore_tracker = _semaphore_tracker
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semaphore_tracker.ensure_running = _semaphore_tracker.ensure_running
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semaphore_tracker.register = _semaphore_tracker.register
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semaphore_tracker.unregister = _semaphore_tracker.unregister
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semaphore_tracker.getfd = _semaphore_tracker.getfd
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_forkserver = AdultForkServer()
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forkserver._forkserver = _forkserver
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forkserver.main = main
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forkserver._serve_one = _serve_one
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forkserver.ensure_running = _forkserver.ensure_running
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forkserver.get_inherited_fds = _forkserver.get_inherited_fds
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forkserver.connect_to_new_process = _forkserver.connect_to_new_process
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forkserver.set_forkserver_preload = _forkserver.set_forkserver_preload
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