[issue29096] Signal Handlers reliably cause UnboundLocalErrors

[Joe Jevnik]

Joe Jevnik added the comment:

The issue appears to be in ceval.c:unicode_concatenate (or the py2 equivalent)

The code sets the local variable on the lhs to NULL before doing a potentially 
inplace append to the string. This means that if a signal is raised during the 
concat, we never hit the STORE_FAST instruction following the INPLACE_ADD so 
the local or cell still holds NULL, triggering an error.

I am not really sure what can be done to prevent the failure while still 
allowing the optimization.

--
components: +Interpreter Core
nosy: +ll
type: crash -> 

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[issue29096] Signal Handlers reliably cause UnboundLocalErrors

[Ted Meyer]

Ted Meyer added the comment:

Sorry everyone, it appears I made a mistake pasting the code in here.
There was a type where I was trying to print local_val instead of local_var. 
the code should be this:

import signal

def sig_hdlr(signum, frame):
raise ValueError()

def faulty():
local_var = ""
signal.signal(signal.SIGALRM, sig_hdlr)
signal.alarm(1)
try:
while True:
local_var += "!"
except ValueError:
print (local_var)

faulty()


and the crash should be this:
Traceback (most recent call last):
  File "pybug.py", line 12, in faulty
local_var += "!"
  File "pybug.py", line 4, in sig_hdlr
raise ValueError()
ValueError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "pybug.py", line 16, in 
faulty()
  File "pybug.py", line 14, in faulty
print (local_var)
UnboundLocalError: local variable 'local_var' referenced before assignment

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[issue29096] Signal Handlers reliably cause UnboundLocalErrors

[Ted Meyer]

Changes by Ted Meyer :


--
type:  -> crash

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[issue29096] Signal Handlers reliably cause UnboundLocalErrors

[Ted Meyer]

New submission from Ted Meyer:

Using this simple snippit of code:
import signal



import signal

def sig_hdlr(signum, frame):
raise ValueError()

def faulty():
local_var = ""
signal.signal(signal.SIGALRM, sig_hdlr)
signal.alarm(1)
try:
while True:
local_var += "!"
except ValueError:
print (local_val)

faulty()



I can reliably get a crash:
tmathmeyer@tmathmeyer-linuxstation:~$ python --version
Python 2.7.6
tmathmeyer@tmathmeyer-linuxstation:~$ python pybug.py 
Traceback (most recent call last):
  File "pybug.py", line 16, in 
faulty()
  File "pybug.py", line 14, in faulty
print local_val
NameError: global name 'local_val' is not defined

tmathmeyer@tmathmeyer-linuxstation:~$ python3.4 pybug.py 
Traceback (most recent call last):
  File "pybug.py", line 12, in faulty
local_var += "!"
  File "pybug.py", line 4, in sig_hdlr
raise ValueError()
ValueError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "pybug.py", line 16, in 
faulty()
  File "pybug.py", line 14, in faulty
print (local_val)
NameError: name 'local_val' is not defined


I can repro this on 2.7.6 and 3.4, but do not have other versions to test on.

--
messages: 284196
nosy: Ted Meyer
priority: normal
severity: normal
status: open
title: Signal Handlers reliably cause UnboundLocalErrors
versions: Python 2.7, Python 3.3, Python 3.4

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