[Python-Dev] Default SIGINT handling dangerous?

[Jurko Gospodnetić]

   Hi all.

   Is there any way to disable the default asynchronous exception 
raising SIGINT signal handling? Or a plan to add support for this? My 
searches on the net found several bug reports and discussions regarding 
this, but nothing seems final... :-(

   I believe the current Python default SIGINT handling by raising an 
asynchronous KeyboardInterrupt exception is dangerous, and can directly 
cause Python interpreter crashes even if no user code is involved, or 
any sort of other 'undefined behaviour' if it is.

   On Windows at least (using Python 3.3.3 at the moment, but seen with 
earlier versions as well), I occasionally get 'python.exe has stopped 
working' dialogs when Ctrl-C is pressed (most often if pressed during 
Python's initialization - loading site.py or earlier).

   The problem with asynchronous exceptions is, of course, that they can 
occur at _any_ time, even at the start of a finally: block, effectively 
causing cleanup code to be skipped even though the programmer intended 
it to be called even in case of exceptions.

   My scripts replace the default SIGINT/SIGBREAK signal handlers as 
soon as possible, and everything works great after that, but things get 
ugly if Ctrl-C is pressed before the script gets a chance to do this. I 
could even live with an 'exit with an ugly traceback', but having the 
process hang or fail so that Windows reports it as non-responding or 
reports it as 'stopped working' and offers to send failure information 
to Microsoft? That just leaves a bad taste in my mouth. :-(

   Also, Python documentation should not helpfully state or infer in 
several places that user can handle KeyboardInterrupt exceptions to 
implement Ctrl-C handling. Even if you do manage to catch it, you must 
never ever ignore it and must terminate your application since it might 
have already been left in an inconsistent state internally (e.g. some 
important finally clause got skipped, even one located deep in the 
Python standard library internals). Doing anything else can only be 
considered a partially working hack. Another problem is that that 
multiple SIGINT signals might occur in a sequence and any such 
KeyboardInterrupt handling can itself be interrupted in the same way as 
the original code. You can say that this is 'unlikely', or even add 
additional code to make this even more unlikely, but it still smells 
bad. :-(

   Hope this helps.

   Best regards,
     Jurko Gospodnetić


   Below, I've included a few script outputs (tracebacks included) from 
instances where Python interpreter crashed due to pressing Ctrl-C soon 
after an empty Python script has been run.

In the first of these instances I got the Microsoft's error reporting 
dialog. In all the later instances I checked the Python process's debug 
output and every time it included the message: 'Microsoft Visual Studio 
C Runtime Library has detected a fatal error in python.exe.'.

Using:
   - OS: Windows 7 SP1 x64
   - Python 3.3.3 (64-it)
   - default site.py

--------------
Occurrence #1:
--------------
> D:\Workplace>run.py
> Fatal Python error: Py_Initialize: can't initialize sys standard streams
> Traceback (most recent call last):
>   File "<frozen importlib._bootstrap>", line 755, in _open_registry
> FileNotFoundError: [WinError 2] The system cannot find the file specified
>
> During handling of the above exception, another exception occurred:
>
> Traceback (most recent call last):
>   File "C:\Program Files\Python\Python333\lib\encodings\__init__.py", line 98, in search_function
>     level=0)
>   File "<frozen importlib._bootstrap>", line 1565, in _find_and_load
>   File "<frozen importlib._bootstrap>", line 1523, in_find_and_load_unlocked
>   File "<frozen importlib._bootstrap>", line 1477, in _find_module
>   File "<frozen importlib._bootstrap>", line 777, in find_module
>   File "<frozen importlib._bootstrap>", line 768, in _search_registry
>   File "<frozen importlib._bootstrap>", line 755, in _open_registry
> KeyboardInterrupt


--------------
Occurrence #2:
--------------
> D:\Workplace>run.py
> Fatal Python error: Py_Initialize: can't initialize sys standard streams
> Traceback (most recent call last):
>   File "C:\Program Files\Python\Python333\lib\codecs.py", line 165, in __init__
>     def __init__(self, errors='strict'):
> KeyboardInterrupt


--------------
Occurrence #3:
--------------
> D:\Workplace>run.py
> Fatal Python error: Py_Initialize: can't initialize sys standard streams
> Traceback (most recent call last):
>   File "<frozen importlib._bootstrap>", line 1565, in _find_and_load
>   File "<frozen importlib._bootstrap>", line 1523, in _find_and_load_unlocked
>   File "<frozen importlib._bootstrap>", line 1477, in _find_module
>   File "<frozen importlib._bootstrap>", line 1309, in find_module
>   File "<frozen importlib._bootstrap>", line 1288, in _get_loader
>   File "<frozen importlib._bootstrap>", line 1387, in find_loader
>   File "<frozen importlib._bootstrap>", line 110, in _path_isfile
>   File "<frozen importlib._bootstrap>", line 101, in _path_is_mode_type
> KeyboardInterrupt


--------------
Occurrence #4:
--------------
> D:\Workplace>run.py
> Fatal Python error: Py_Initialize: can't initialize sys standard streams
> Traceback (most recent call last):
>   File "C:\Program Files\Python\Python333\lib\io.py", line 52, in <module>
>     import abc
>   File "<frozen importlib._bootstrap>", line 1565, in _find_and_load
>   File "<frozen importlib._bootstrap>", line 1533, in _find_and_load_unlocked
>   File "<frozen importlib._bootstrap>", line 486, in _verbose_message
> KeyboardInterrupt


--------------
Occurrence #5:
--------------
> D:\Workplace>run.py
> Fatal Python error: Py_Initialize: can't initialize sys standard streams
> Traceback (most recent call last):
>   File "C:\Program Files\Python\Python333\lib\io.py", line 52, in <module>
>     import abc
>   File "C:\Program Files\Python\Python333\lib\abc.py", line 6, in <module>
>     from _weakrefset import WeakSet
>   File "<frozen importlib._bootstrap>", line 1559, in _find_and_load
> KeyboardInterrupt


--------------
Occurrence #6:
--------------
> D:\Workplace>run.py
> Fatal Python error: Py_Initialize: can't initialize sys standard streams
> Traceback (most recent call last):
>   File "C:\Program Files\Python\Python333\lib\encodings\__init__.py", line 69, in search_function
>     def search_function(encoding):
> KeyboardInterrupt


--------------
Occurrence #7:
--------------
> D:\Workplace>run.py
> Fatal Python error: Py_Initialize: can't initialize sys standard streams
> Traceback (most recent call last):
>   File "C:\Program Files\Python\Python333\lib\io.py", line 52, in <module>
>     import abc
>   File "C:\Program Files\Python\Python333\lib\abc.py", line 6, in <module>
>     from _weakrefset import WeakSet
>   File "<frozen importlib._bootstrap>", line 1565, in _find_and_load
>   File "<frozen importlib._bootstrap>", line 1532, in _find_and_load_unlocked
>   File "<frozen importlib._bootstrap>", line 584, in _check_name_wrapper
>   File "<frozen importlib._bootstrap>", line 1022, in load_module
>   File "<frozen importlib._bootstrap>", line 1003, in load_module
> KeyboardInterrupt


--------------
Occurrence #8:
--------------
> D:\Workplace>run.py
> Fatal Python error: Py_Initialize: can't initialize sys standard streams
> Traceback (most recent call last):
>   File "C:\Program Files\Python\Python333\lib\encodings\__init__.py", line 98, in search_function
>     level=0)
>   File "<frozen importlib._bootstrap>", line 1565, in _find_and_load
>   File "<frozen importlib._bootstrap>", line 1532, in _find_and_load_unlocked
>   File "<frozen importlib._bootstrap>", line 584, in _check_name_wrapper
>   File "<frozen importlib._bootstrap>", line 1022, in load_module
>   File "<frozen importlib._bootstrap>", line 1003, in load_module
>   File "<frozen importlib._bootstrap>", line 560, in module_for_loader_wrapper
>   File "<frozen importlib._bootstrap>", line 857, in _load_module
>   File "<frozen importlib._bootstrap>", line 439, in cache_from_source
> KeyboardInterrupt


--------------
Occurrence #9:
--------------
> D:\Workplace>run.py
> Fatal Python error: Py_Initialize: can't initialize sys standard streams
> Traceback (most recent call last):
>   File "C:\Program Files\Python\Python333\lib\encodings\__init__.py", line 98, in search_function
>     level=0)
>   File "C:\Program Files\Python\Python333\lib\encodings\cp852.py", line 175, in <module>
>     0x00ff: 0x00a0,     #  NO-BREAK SPACE
> KeyboardInterrupt