[issue25782] CPython hangs on error __context__ set to the error itself

[Yury Selivanov]

Yury Selivanov added the comment:

> Should we do the same for __cause__? Is it possible to create __context__ or __cause__ loop without assigning these attributes directly?

Yes, let's mirror the __context__ behaviour for __cause__.  New patch attached.


Serhiy, Guido,

The new patch raises a TypeError in __cause__ and __context__ setters when a cycle was introduced, so in pure Python the following won't work:


   # will throw TypeError("cycle in exception context chain")
   ex.__context__ = ex chain")

   # will throw TypeError("cycle in exception cause chain")
   ex.__cause__ = ex


However, since PyException_SetContext and PyException_SetCause are public APIs, and their return type is 'void', I can't raise an error when a C code introduces a cycle, in that case, the exc->cause/exc->context will be set to NULL.

Thoughts?

I think that this approach is the only sane one here.  We can certainly fix the infinite loop in PyErr_SetObject, but it will only be a matter of time until we discover a similar loop somewhere else.

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Added file: http://bugs.python.org/file41214/Issue25782_2.patch

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