[Tcl-bugs] [ tcl-Bugs-2939073 ] IPC via sockets and traces causes segfault

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Bugs item #2939073, was opened at 2010-01-25 05:54
Message generated for change (Comment added) made by ferrieux
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Category: 46. Traces
Group: current: 8.5.8
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Stephen Huntley (blacksqr)
Assigned to: Don Porter (dgp)
Summary: IPC via sockets and traces causes segfault

Initial Comment:
I wrote a simple application to gather Tk events and pass them from one interpreter to another via tequila (http://wiki.tcl.tk/1243), which uses traces and socket communication to keep array values in sync between Tcl interpreters.

My application displays a canvas, to which a <Motion> event is bound.  Thus as the cursor is moved over the canvas a steady stream of Tk events is generated; the scripts associated with these events are captured and written to an array, which tequila echoes to an interpreter in a separate process.  

When I start this application and move the cursor continually over the canvas, sometime between five and sixty seconds later one of the interpreters segfaults and crashes.

Platform is Ubuntu Hardy, with ActiveTcl 8.5.5 installed.  I duplicated the bug using Pat Thoyt's tclkits versions 8.5.8 (http://www.patthoyts.tk/tclkit/linux-ix86/8.5.8/tclkit%2d8.5.8.gz) and 8.6b1.1 (http://www.patthoyts.tk/tclkit/linux-ix86/8.6%2dbeta/tclkit%2d8.6b1.1.gz).

For ease of duplication I created a starkit containing all relevant code called echoevent.kit (attached).  To duplicate:

1. in a terminal console, execute: tclkit echoevent.kit server
This will start a tequila server.

2. in a separate terminal console, execute: tclkit echoevent.kit master
This will start a simple Tk "master" application that displays a canvas.  Moving the cursor across this "master" canvas will cause the command associated with the <Motion> event to echo repeatedly to the terminal console.

3. in a third terminal console, execute: tclkit echoevent.kit slave
This will cause an identical "slave" Tk application to appear, except event processing for the "slave" canvas is disabled.  Moving the cursor across the "master" canvas will cause the event command to echo to both the "master" console and the "slave" console.  The tequila server is passing event information from the master interpreter to the slave interpreter.

4. move the cursor continually over the master canvas to generate a constant stream of <Motion> events to be echoed to the consoles.  Either the master or the slave process will crash with a segmentation fault within 60 seconds.
            
report cross-posted at: http://bugs.activestate.com/show_bug.cgi?id=85927

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>Comment By: Alexandre Ferrieux (ferrieux)
Date: 2010-01-30 13:53

Message:
Further analysis of the core shows the command

   array unset collab .c,queue,42,*

being executed at the time. So it seems to hint at an interaction between
patterned unsets and traces. I'd appreciate somebody knowing the app at
hand to trim that stuff down to a manageable size... TBC 

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Comment By: Alexandre Ferrieux (ferrieux)
Date: 2010-01-26 01:53

Message:
After Steve kindly brought all the pieces beside my bed, I could get the
attached gdb stack trace under Linux (Fedora 12, 32bits). Nothing about
Metakit there.

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Comment By: Steve Landers (stevel)
Date: 2010-01-26 00:50

Message:
I've reproduced the problem on OSX 10.6.2 using wish 8.5.7.  If I can
figure how, I'll attach the traceback file

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Comment By: Alexandre Ferrieux (ferrieux)
Date: 2010-01-25 09:36

Message:
Since this happens on unix, can you enable core dumps, and provide a gdb
stack trace of the crash ? I'd like to dichotomize between Metakit and the
vanilla Tcl core.

As a side note, a .kit is not the easiest transport for some maintainers
(me at least), because it requires additional effort to crack the hull
open, and reproduce with the core-under-development. I hate efforts ;-)

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