[ python-Bugs-780714 ] infinite __str__ recursion in thread causes seg fault

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Bugs item #780714, was opened at 2003-07-31 08:37
Message generated for change (Settings changed) made by gbrandl
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Category: Python Interpreter Core
Group: Python 2.3
>Status: Open
Resolution: Fixed
Priority: 5
Submitted By: Stefan Gregory (stefan_gregory)
Assigned to: Nobody/Anonymous (nobody)
Summary: infinite __str__ recursion in thread causes seg fault

Initial Comment:
The following code reliably produces a segmentation
fault under Solaris8 in Python2.3, Python2.2,
Python2.1.3 and Python1.5. It produces a Bus Error when
run on Python2.2 under OSX. Clearly it should produce a
Python RuntimeError.

import thread, time

class Frog:

    def __str__(self):
        return str(self)

f = Frog()
thread.start_new_thread(str,(f,))
time.sleep(1000)

This problem manifests in scenarios such as when you
have 2 publishing objects (such as HTMLgen objects) A
and B and you put A inside B and B inside A and each
objects __str__ method calls its childrens __str__
methods and voila! (I hope this might be the reason my
Zope server has been intermitently crashing for the
past year or so though I have not found any recursive
structures yet.)

With Python2.3 gdb reports:

vgetargskeywords (args=0x1bdaf0, keywords=0x0,
format=0xd2579 "0:str", kwlist=0xf7b4c,
p_va=0xfed0C02c) at Python/getargs.c:1167

Though with Python2.1 it dies at line 330 in getargs.c.



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Comment By: Josiah Carlson (josiahcarlson)
Date: 2006-08-16 07:42

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>>> import sys
>>> sys.setrecursionlimit(10000)
>>> class foo:
...     def __str__(self):
...             return str(self)
...
>>> import threading
>>> threading.Thread(target=foo().__str__).start()

Kills 2.3, 2.4, and 2.5 on Windows, and 2.3 and 2.4 on linux
(I can't test 2.5 on linux).  Running in the main thread on
Windows doesn't seem to be a big deal:

>>> import sys
>>> sys.setrecursionlimit(10000)
>>> class foo:
...     def __str__(self):
...             return str(self)
...
>>> try:
...     str(foo())
... except Exception, why:
...     print why
...
Stack overflow
>>>

Note that the above crashes 2.3 and 2.4 on Linux.

This is still a bug, at least in maintenance on 2.4. 
Suggested reopen.


----------------------------------------------------------------------

Comment By: SourceForge Robot (sf-robot)
Date: 2006-08-14 02:20

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This Tracker item was closed automatically by the system. It was
previously set to a Pending status, and the original submitter
did not respond within 14 days (the time period specified by
the administrator of this Tracker).

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Comment By: Georg Brandl (gbrandl)
Date: 2006-07-30 11:16

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Under 2.5, I get a runtime error now, so this might be fixed.

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Comment By: Brett Cannon (bcannon)
Date: 2003-08-05 19:22

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Forgot to mention that Modules/threadmodule.c has 
thread_PyThread_start_new_thread which uses Python/
thread_*.h's PyThread_start_new_thread for executing threads;  
the '*' is replaced with the threading library used on your 
platform.  OS X should use thread_pthread.h (I think).

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Comment By: Brett Cannon (bcannon)
Date: 2003-08-05 19:19

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If you run ``str(Frog())`` instead of a bunch of threads you do get 
a RuntimeError.  Looks like this bus error has something to do 
with thread.start_new_thread and not 'str'.  It might be that since 
it runs using pthreads it does not have the built-in recursion limit 
check that the Python interpreter has.  Anyone with more 
experience with the 'thread' module have any ideas?

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