[Python-Dev] Signals+Threads (PyGTK waking up 10x/sec).

glyph at divmod.com glyph at divmod.com
Sat Dec 8 22:56:33 CET 2007

On 05:20 pm, guido at python.org wrote:
>The best solution I can think of is to add a new API that takes a
>signal and a file descriptor and registers a C-level handler for that
>signal which writes a byte to the file descriptor. You can then create
>a pipe, connect the signal handler to the write end, and add the read
>end to your list of file descriptors passed to select() or poll(). The
>handler must be written in C in order to avoid the race condition
>referred to by Glyph (signals arriving after the signal check in the
>VM main loop but before the select()/poll() system call is entered
>will not be noticed until the select()/poll() call completes).

This paragraph jogged my memory.  I remember this exact solution being 
discussed now, a year ago when I was last talking about these issues.

There's another benefit to implementing a write-a-byte C signal handler. 
Without this feature, it wouldn't make sense to have passed the 
SA_RESTART flag to sigaction, because and GUIs written in Python could 
have spent an indefinite amount of time waiting to deliver their signal 
to Python code.  So, if you had to handle SIGCHLD in Python, for 
example, calls like file().write() would suddenly start raising a new 
exception (EINTR).  With it, you could avoid a whole class of subtle 
error-handling code in Twisted programs.

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