Part of twisted.internet View Source
This module provides a uniform interface to the several mechanisms which are possibly available for dealing with signals.
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 twisted.internet.posixbase._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.
The actual solution implemented in this module depends on the version of
Python. From version 2.6, signal.siginterrupt
and
signal.set_wakeup_fd
allow the necessary C signal handler
which writes to the pipe to be registered with SA_RESTART
.
Prior to 2.6, the twisted.internet._sigchld
extension module
provides similar functionality.
Class | _Handler | _Handler is
a signal handler which writes a byte to a file descriptor whenever it is
invoked.
|
Function | _installHandlerUsingSignal | Install a signal handler which will write a byte to fd when
SIGCHLD is received.
|
Function | _installHandlerUsingSetWakeup | 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. |
Function | _cannotInstallHandler | Fail to install a signal handler for SIGCHLD. |
Function | _cannotDetermineDefault | Undocumented |
Install a signal handler which will write a byte to fd
when
SIGCHLD is received.
_Handler
with fd
and installing it as the signal handler.
Parameters | fd | The file descriptor to which to write when SIGCHLD is
received.
(type: int
) |
Install a signal handler which will write a byte to fd
when
SIGCHLD is received.
_Handler
wrapped around None
, setting the SIGCHLD handler as not
allowed to interrupt system calls, and using
signal.set_wakeup_fd
to do the actual writing.
Parameters | fd | The file descriptor to which to write when SIGCHLD is
received.
(type: int
) |
Fail to install a signal handler for SIGCHLD.
This implementation is used when the supporting code for the other implementations is unavailable (on Python versions 2.5 and older where neither thetwisted.internet._sigchld
extension nor the
standard signal
module is available).
Parameters | fd | Ignored; only for compatibility with the other implementations of this interface. |
Raises | RuntimeError | Always raised to indicate no SIGCHLD handler can be installed. |