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# -*- test-case-name: twisted.internet.test.test_sigchld -*- # Copyright (c) Twisted Matrix Laboratories. # See LICENSE for details. """ This module is used to integrate child process termination into a reactor event loop. This is a challenging feature to provide because most platforms indicate process termination via SIGCHLD and do not provide a way to wait for that signal and arbitrary I/O events at the same time. The naive implementation involves installing a Python SIGCHLD handler; unfortunately this leads to other syscalls being interrupted (whenever SIGCHLD is received) and failing with EINTR (which almost no one is prepared to handle). This interruption can be disabled via siginterrupt(2) (or one of the equivalent mechanisms); however, if the SIGCHLD is delivered by the platform to a non-main thread (not a common occurrence, but difficult to prove impossible), the main thread (waiting on select() or another event notification API) may not wake up leading to an arbitrary delay before the child termination is noticed. The basic solution to all these issues involves enabling SA_RESTART (ie, disabling system call interruption) and registering a C signal handler which writes a byte to a pipe. The other end of the pipe is registered with the event loop, allowing it to wake up shortly after SIGCHLD is received. See L{_SIGCHLDWaker} for the implementation of the event loop side of this solution. The use of a pipe this way is known as the U{self-pipe trick<http://cr.yp.to/docs/selfpipe.html>}. From Python version 2.6, C{signal.siginterrupt} and C{signal.set_wakeup_fd} provide the necessary C signal handler which writes to the pipe to be registered with C{SA_RESTART}. """ from __future__ import annotations import contextlib import errno import os import signal import socket from types import FrameType from typing import Callable, Optional, Sequence from zope.interface import Attribute, Interface, implementer from attrs import define, frozen from typing import Protocol, TypeAlias from twisted.internet.interfaces import IReadDescriptor from twisted.python import failure, log, util from twisted.python.runtime import platformType if platformType == "posix": from . import fdesc, process SignalHandler: TypeAlias = Callable[[int, Optional[FrameType]], None] def installHandler(fd: int) -> int: """ Install a signal handler which will write a byte to C{fd} when I{SIGCHLD} is received. This is implemented by installing a SIGCHLD handler that does nothing, setting the I{SIGCHLD} handler as not allowed to interrupt system calls, and using L{signal.set_wakeup_fd} to do the actual writing. @param fd: The file descriptor to which to write when I{SIGCHLD} is received. @return: The file descriptor previously configured for this use. """ if fd == -1: signal.signal(signal.SIGCHLD, signal.SIG_DFL) else: def noopSignalHandler(*args): pass signal.signal(signal.SIGCHLD, noopSignalHandler) signal.siginterrupt(signal.SIGCHLD, False) return signal.set_wakeup_fd(fd) def isDefaultHandler(): """ Determine whether the I{SIGCHLD} handler is the default or not. """ return signal.getsignal(signal.SIGCHLD) == signal.SIG_DFL class SignalHandling(Protocol): """ The L{SignalHandling} protocol enables customizable signal-handling behaviors for reactors. A value that conforms to L{SignalHandling} has install and uninstall hooks that are called by a reactor at the correct times to have the (typically) process-global effects necessary for dealing with signals. """ def install(self) -> None: """ Install the signal handlers. """ def uninstall(self) -> None: """ Restore signal handlers to their original state. """ @frozen class _WithoutSignalHandling: """ A L{SignalHandling} implementation that does no signal handling. This is the implementation of C{installSignalHandlers=False}. """ def install(self) -> None: """ Do not install any signal handlers. """ def uninstall(self) -> None: """ Do nothing because L{install} installed nothing. """ @frozen class _WithSignalHandling: """ A reactor core helper that can manage signals: it installs signal handlers at start time. """ _sigInt: SignalHandler _sigBreak: SignalHandler _sigTerm: SignalHandler def install(self) -> None: """ Install the signal handlers for the Twisted event loop. """ if signal.getsignal(signal.SIGINT) == signal.default_int_handler: # only handle if there isn't already a handler, e.g. for Pdb. signal.signal(signal.SIGINT, self._sigInt) signal.signal(signal.SIGTERM, self._sigTerm) # Catch Ctrl-Break in windows SIGBREAK = getattr(signal, "SIGBREAK", None) if SIGBREAK is not None: signal.signal(SIGBREAK, self._sigBreak) def uninstall(self) -> None: """ At the moment, do nothing (for historical reasons). """ # This should really do something. # https://github.com/twisted/twisted/issues/11761 @define class _MultiSignalHandling: """ An implementation of L{SignalHandling} which propagates protocol method calls to a number of other implementations. This supports composition of multiple signal handling implementations into a single object so the reactor doesn't have to be concerned with how those implementations are factored. @ivar _signalHandlings: The other C{SignalHandling} implementations to which to propagate calls. @ivar _installed: If L{install} has been called but L{uninstall} has not. This is used to avoid double cleanup which otherwise results (at least during test suite runs) because twisted.internet.reactormixins doesn't keep track of whether a reactor has run or not but always invokes its cleanup logic. """ _signalHandlings: Sequence[SignalHandling] _installed: bool = False def install(self) -> None: for d in self._signalHandlings: d.install() self._installed = True def uninstall(self) -> None: if self._installed: for d in self._signalHandlings: d.uninstall() self._installed = False @define class _ChildSignalHandling: """ Signal handling behavior which supports I{SIGCHLD} for notification about changes to child process state. @ivar _childWaker: L{None} or a reference to the L{_SIGCHLDWaker} which is used to properly notice child process termination. This is L{None} when this handling behavior is not installed and non-C{None} otherwise. This is mostly an unfortunate implementation detail due to L{_SIGCHLDWaker} allocating file descriptors as a side-effect of its initializer. """ _addInternalReader: Callable[[IReadDescriptor], object] _removeInternalReader: Callable[[IReadDescriptor], object] _childWaker: Optional[_SIGCHLDWaker] = None def install(self) -> None: """ Extend the basic signal handling logic to also support handling SIGCHLD to know when to try to reap child processes. """ # This conditional should probably not be necessary. # https://github.com/twisted/twisted/issues/11763 if self._childWaker is None: self._childWaker = _SIGCHLDWaker() self._addInternalReader(self._childWaker) self._childWaker.install() # Also reap all processes right now, in case we missed any # signals before we installed the SIGCHLD waker/handler. # This should only happen if someone used spawnProcess # before calling reactor.run (and the process also exited # already). process.reapAllProcesses() def uninstall(self) -> None: """ If a child waker was created and installed, uninstall it now. Since this disables reactor functionality and is only called when the reactor is stopping, it doesn't provide any directly useful functionality, but the cleanup of reactor-related process-global state that it does helps in unit tests involving multiple reactors and is generally just a nice thing. """ assert self._childWaker is not None # XXX This would probably be an alright place to put all of the # cleanup code for all internal readers (here and in the base class, # anyway). See #3063 for that cleanup task. self._removeInternalReader(self._childWaker) self._childWaker.uninstall() self._childWaker.connectionLost(failure.Failure(Exception("uninstalled"))) # We just spoiled the current _childWaker so throw it away. We can # make a new one later if need be. self._childWaker = None class _IWaker(Interface): """ Interface to wake up the event loop based on the self-pipe trick. The U{I{self-pipe trick}<http://cr.yp.to/docs/selfpipe.html>}, used to wake up the main loop from another thread or a signal handler. This is why we have wakeUp together with doRead This is used by threads or signals to wake up the event loop. """ disconnected = Attribute("") def wakeUp() -> None: """ Called when the event should be wake up. """ def doRead() -> None: """ Read some data from my connection and discard it. """ def connectionLost(reason: failure.Failure) -> None: """ Called when connection was closed and the pipes. """ @implementer(_IWaker) class _SocketWaker(log.Logger): """ The I{self-pipe trick<http://cr.yp.to/docs/selfpipe.html>}, implemented using a pair of sockets rather than pipes (due to the lack of support in select() on Windows for pipes), used to wake up the main loop from another thread. """ disconnected = 0 def __init__(self) -> None: """Initialize.""" # Following select_trigger (from asyncore)'s example; client = socket.socket(socket.AF_INET, socket.SOCK_STREAM) client.setsockopt(socket.IPPROTO_TCP, socket.TCP_NODELAY, 1) with contextlib.closing( socket.socket(socket.AF_INET, socket.SOCK_STREAM) ) as server: server.bind(("127.0.0.1", 0)) server.listen(1) client.connect(server.getsockname()) reader, clientaddr = server.accept() client.setblocking(False) reader.setblocking(False) self.r = reader self.w = client self.fileno = self.r.fileno def wakeUp(self): """Send a byte to my connection.""" try: util.untilConcludes(self.w.send, b"x") except OSError as e: if e.args[0] != errno.WSAEWOULDBLOCK: raise def doRead(self): """ Read some data from my connection. """ try: self.r.recv(8192) except OSError: pass def connectionLost(self, reason): self.r.close() self.w.close() @implementer(IReadDescriptor) class _FDWaker(log.Logger): """ The I{self-pipe trick<http://cr.yp.to/docs/selfpipe.html>}, used to wake up the main loop from another thread or a signal handler. L{_FDWaker} is a base class for waker implementations based on writing to a pipe being monitored by the reactor. @ivar o: The file descriptor for the end of the pipe which can be written to wake up a reactor monitoring this waker. @ivar i: The file descriptor which should be monitored in order to be awoken by this waker. """ disconnected = 0 i: int o: int def __init__(self) -> None: """Initialize.""" self.i, self.o = os.pipe() fdesc.setNonBlocking(self.i) fdesc._setCloseOnExec(self.i) fdesc.setNonBlocking(self.o) fdesc._setCloseOnExec(self.o) self.fileno = lambda: self.i def doRead(self) -> None: """ Read some bytes from the pipe and discard them. """ fdesc.readFromFD(self.fileno(), lambda data: None) def connectionLost(self, reason): """Close both ends of my pipe.""" if not hasattr(self, "o"): return for fd in self.i, self.o: try: os.close(fd) except OSError: pass del self.i, self.o @implementer(_IWaker) class _UnixWaker(_FDWaker): """ This class provides a simple interface to wake up the event loop. This is used by threads or signals to wake up the event loop. """ def wakeUp(self): """Write one byte to the pipe, and flush it.""" # We don't use fdesc.writeToFD since we need to distinguish # between EINTR (try again) and EAGAIN (do nothing). if self.o is not None: try: util.untilConcludes(os.write, self.o, b"x") except OSError as e: # XXX There is no unit test for raising the exception # for other errnos. See #4285. if e.errno != errno.EAGAIN: raise if platformType == "posix": _Waker = _UnixWaker else: # Primarily Windows and Jython. _Waker = _SocketWaker # type: ignore[misc,assignment] class _SIGCHLDWaker(_FDWaker): """ L{_SIGCHLDWaker} can wake up a reactor whenever C{SIGCHLD} is received. """ def install(self) -> None: """ Install the handler necessary to make this waker active. """ installHandler(self.o) def uninstall(self) -> None: """ Remove the handler which makes this waker active. """ installHandler(-1) def doRead(self) -> None: """ Having woken up the reactor in response to receipt of C{SIGCHLD}, reap the process which exited. This is called whenever the reactor notices the waker pipe is writeable, which happens soon after any call to the C{wakeUp} method. """ super().doRead() process.reapAllProcesses()