module documentation

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 _SIGCHLDWaker for the implementation of the event loop side of this solution. The use of a pipe this way is known as the self-pipe trick.

From Python version 2.6, signal.siginterrupt and signal.set_wakeup_fd provide the necessary C signal handler which writes to the pipe to be registered with SA_RESTART.

Class SignalHandling The SignalHandling protocol enables customizable signal-handling behaviors for reactors.
Function installHandler Install a signal handler which will write a byte to fd when SIGCHLD is received.
Function isDefaultHandler Determine whether the SIGCHLD handler is the default or not.
Type Alias SignalHandler Undocumented
Interface _IWaker Interface to wake up the event loop based on the self-pipe trick.
Class _ChildSignalHandling Signal handling behavior which supports SIGCHLD for notification about changes to child process state.
Class _FDWaker The self-pipe trick<>, used to wake up the main loop from another thread or a signal handler.
Class _MultiSignalHandling An implementation of SignalHandling which propagates protocol method calls to a number of other implementations.
Class _SIGCHLDWaker _SIGCHLDWaker can wake up a reactor whenever SIGCHLD is received.
Class _SocketWaker The self-pipe trick<>, 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.
Class _UnixWaker This class provides a simple interface to wake up the event loop.
Class _WithoutSignalHandling A SignalHandling implementation that does no signal handling.
Class _WithSignalHandling A reactor core helper that can manage signals: it installs signal handlers at start time.
def installHandler(fd): (source)

Install a signal handler which will write a byte to fd when SIGCHLD is received.

This is implemented by installing a SIGCHLD handler that does nothing, setting the SIGCHLD handler as not allowed to interrupt system calls, and using signal.set_wakeup_fd to do the actual writing.

fd:intThe file descriptor to which to write when SIGCHLD is received.
intThe file descriptor previously configured for this use.
def isDefaultHandler(): (source)

Determine whether the SIGCHLD handler is the default or not.

SignalHandler: TypeAlias = (source)


Callable[[int, Optional[FrameType]], None]