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Re: Hang in "double/triple" select()

From: Jan Djärv
Subject: Re: Hang in "double/triple" select()
Date: Sat, 23 Sep 2006 15:52:53 +0200
User-agent: Thunderbird (Macintosh/20060719)

Kim F. Storm skrev:
I happened to be using a "slightly old" version of CVS Emacs
GNU Emacs (i686-pc-linux-gnu, X toolkit, Xaw3d scroll bars)
of 2006-01-19 on kfs-l.imdomain.dk

and went away from the screen for an hours so, and when I returned,
Emacs was stuck in a non-responsive mode with a "blank" emacs frame.

I attached to it with GDB, and this is what I got:

#0  0x403d8122 in select () from /lib/libc.so.6
#1  0x402bbdc4 in _XlcPublicMethods () from /usr/X11R6/lib/libX11.so.6
#2  0x40210ee8 in _XFlush () from /usr/X11R6/lib/libX11.so.6
#3  0x402124fd in _XReply () from /usr/X11R6/lib/libX11.so.6
#4  0x401ff722 in XInternAtom () from /usr/X11R6/lib/libX11.so.6
#5  0x4066bf31 in _XimExtension ()
   from /usr/X11R6/lib/X11/locale/lib/common/ximcp.so.2
#6  0x4022e515 in XFilterEvent () from /usr/X11R6/lib/libX11.so.6
#7  0x080e59f5 in x_filter_event (dpyinfo=0x8621100, event=0xbfffd8a4)
    at xterm.c:5586
#8  0x080e7ad1 in XTread_socket (sd=0, expected=1, hold_quit=0xbfffe974)
    at xterm.c:7016
#9  0x0811a03b in read_avail_input (expected=1) at keyboard.c:6712
#10 0x0811a232 in handle_async_input () at keyboard.c:6858
#11 0x0811a275 in input_available_signal (signo=29) at keyboard.c:6900
#12 <signal handler called>
#13 0x403d8122 in select () from /lib/libc.so.6
#14 0x00000010 in ?? ()
#15 0x081164a6 in kbd_buffer_get_event (kbp=0xbffff0a0,
    used_mouse_menu=0xbffff39c) at keyboard.c:3935

It looks like select is not signalsafe and/or reentrant. If that is the case, there is always a risk that Emacs will hang. The best solution would be to move to SYNC_INPUT. The second solution would be to have a flag that tells the signal handler if we are in select. If so, it just writes a byte to a pipe we have in the select read set and exits. This makes select exit, and we can handle X input.

Has this only happened once? Can you provoke it? I would think marking some text in Emacs so it grabs the PRIMARY selection, and then yank that text in an xterm for example. If Emacs is idle, there is a fair chanse that it is in select() in this case. Unless there are timers and such running.

        Jan D.

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