[issue19835] Add a MemoryError singleton to fix an unlimited loop when the memory is exhausted

[STINNER Victor]

New submission from STINNER Victor:

Under very low memory condition, PyErr_NoMemory() or PyErr_NormalizeException() enters an unlimited loop when the free list of MemoryError becomes empty.

I propose to add a MemoryError read-only singleton to fix this corner case. Attributes cannot be modified, new attributes cannot be added. MemoryError attributes values:

* __cause__ = None
* __context__ = None
* __suppress_context__ = False
* __traceback__ = None
* args = ()

PyException_SetTraceback(), PyException_SetCause() and PyException_SetContext() do nothing when called on the MemoryError singleton.

A MemoryError can be raised on an integer overflow when computing the size of a memory block. In this case, you may still have available free memory and so you may be able to build a traceback object, chain exceptions, etc.

If you consider that attributes must be modifiable in this case, we can keep the MemoryError unchanged and keep the free list, but add a new special "ReadOnlyMemoryError" type which has exactly one instance, preallocated at startup (the singleton).

Read-only attributes are required to not keep references to objects when the MemoryError (singleton) is ne more used ("destroyed"): see for example test_memory_error_cleanup() of test_exceptions.

Python has already a PyExc_RecursionErrorInst singleton, which is used on recursion error. It is just a preallocated RuntimeError error, attributes are modifiable. Does this exception keep a traceback when it is no more used ("destroyed")?

Maybe using read-only attributes is overkill? Replacing the free list with a singleton is enough?

--

See also issue #5437: before Python had a MemoryError singleton, but it was replaced by a free list to not keep references.

changeset:   65690:c6d86439aa91
branch:      3.1
parent:      65672:e4425d68dadf
user:        Antoine Pitrou <solipsis at pitrou.net>
date:        Thu Oct 28 23:06:57 2010 +0000
files:       Include/pyerrors.h Lib/test/test_exceptions.py Misc/NEWS Objects/exceptions.c Python/errors.c
description:
Merged revisions 85896 via svnmerge from
svn+ssh://pythondev@svn.python.org/python/branches/py3k

........
  r85896 | antoine.pitrou | 2010-10-29 00:56:58 +0200 (ven., 29 oct. 2010) | 4 lines

  Issue #5437: A preallocated MemoryError instance should not hold traceback
  data (including local variables caught in the stack trace) alive infinitely.
........

--

Another option is maybe to clear frames of the traceback. Issue #17934 added a clear() method to frame objects for that.

----------
files: memerror_singleton.patch
keywords: patch
messages: 204742
nosy: haypo
priority: normal
severity: normal
status: open
title: Add a MemoryError singleton to fix an unlimited loop when the memory is exhausted
versions: Python 3.4
Added file: http://bugs.python.org/file32892/memerror_singleton.patch

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