t.i._signals : 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 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.

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.

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.