[New-bugs-announce] [issue37830] continue in finally with return in try results with segfault

[Batuhan]

New submission from Batuhan <batuhanosmantaskaya at gmail.com>:

When you try to put a return statement with the iterated value inside of try/finally and if you have continue in finally it will result with segfault.

def simple():
    for number in range(2):
        try:
            return number
        finally:
            continue

simple()
SEGV


My first debugs shows TOS is the next value int(1) when it comes to FOR_ITER, instead of the range instance. 
So when python try to call next() with (*iter->ob_type->tp_iternext)(iter) in FOR_ITER it gets a non iterator, int(1). 

Adding an assert can prove it, 
python: Python/ceval.c:3198: _PyEval_EvalFrameDefault: Assertion `PyIter_Check(iter)' failed.

For seeing how stack changed i enabled lltrace and formatted it little bit;

>>> STACK_SIZE          0                       
LOAD_GLOBAL             0                       
>>> STACK_SIZE          0                       
>>> push                <class 'range'>         
LOAD_CONST              1                       
>>> STACK_SIZE          1                       
>>> push                2                       
CALL_FUNCTION           1                       
>>> STACK_SIZE          2                       
>>> ext_pop             2                       
>>> ext_pop             <class 'range'>         
>>> push                range(0, 2)             
GET_ITER                None                    
>>> STACK_SIZE          1                       
FOR_ITER                24                      
>>> STACK_SIZE          1                       
>>> push                0                       
STORE_FAST              0                       
>>> STACK_SIZE          2                       
>>> pop                 0                       
SETUP_FINALLY           12                      
>>> STACK_SIZE          1                       
LOAD_FAST               0                       
>>> STACK_SIZE          1                       
>>> push                0                       
POP_BLOCK               None                    
>>> STACK_SIZE          2                       
CALL_FINALLY            6                       
>>> STACK_SIZE          2                       
>>> push                20                      
POP_FINALLY             0                       
>>> STACK_SIZE          3                       
>>> pop                 20                      
JUMP_ABSOLUTE           8                       
>>> STACK_SIZE          2                       
FOR_ITER                24                      
>>> STACK_SIZE          2
[SEGV]

And the oddity is STACK_SIZE should be 1 before the FOR_ITER but it is 2, then i said why dont i try the SECOND() as iter, and it worked. It means an instruction is pushing the value of previous iteration.

There are 3 things we can do;
=> raise a RuntimeError if TOS isn't an iterator (IMHO we should do that)
=> check if try/finally created inside of a function and an iterator. then check inside of try if a return happens with the iterated value and if so set preserve_tos value to false. 
=> dont allow continue in finally

I want to fix this, and i prefer the first one. If you have any suggestion, i am open.

----------
messages: 349450
nosy: BTaskaya
priority: normal
severity: normal
status: open
title: continue in finally with return in try results with segfault

_______________________________________
Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue37830>
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