It is possible to create a loop by setting the __context__ attribute of
the raised exception, either explicitly, or implicitly, using "raise ...
from ...". Creating a loop can leads to the hanging in infinite loop or
crash due to stack overflow. Several years ago we did have issues
related to ExitStack and contextmanager() [1] [2], and recently we have
found other issue related to asynccontextmanager() [3]. There may be
other issues in the stdlib or third-party libraries.
To solve all such issues we can prevent creating the loop in the
__context__ setter. Unfortunately there is no way to return an error in
PyException_SetContext(), so it is always success. So we should break a
loop at some point. There was proposed several options. If we have a chain
A -> B -> C -> D -> E -> NULL
after raising C in the context of A ("raise C from A") or setting
C.__context__ = A we will get a chain:
0. C -> A -> B -> C -> A -> B -> ...
A loop. This is the current behavior.
1. C -> D -> E -> NULL
The operation is no-op. We lose information about exceptions preceding
C: A and B.
2. C -> NULL
Lose all context.
3. C -> A -> B -> D -> E -> NULL
C is moved at the front of the chain. No exception is lost, but their
order is changed.
4. C -> A -> B -> NULL
Remove C from the chain and rewrite its context. We lose information
about the old context of C: D and E.
5. C -> A -> B -> C' -> D -> E -> NULL
Create a copy of C (how?) and replace C with its copy in the chain. But
what to do with other references to C?
There may be other options.
We need to solve this issue, otherwise we will continue to stick with
other bugs related to the exceptions loop.
[1] https://bugs.python.org/issue25779
[2] https://bugs.python.org/issue25786
[3] https://bugs.python.org/issue40696
[4] https://bugs.python.org/issue25782
_______________________________________________
Python-Dev mailing list -- python-dev@python.org
To unsubscribe send an email to python-dev-le...@python.org
https://mail.python.org/mailman3/lists/python-dev.python.org/
Message archived at
https://mail.python.org/archives/list/python-dev@python.org/message/MHH2BOG2BIK7MLGKZP7Q5S6MQDCJPF3S/
Code of Conduct: http://python.org/psf/codeofconduct/
