Determining the failure of C API call is ambiguous

[serhiy-storchaka]

BPO	26776
Nosy	@vstinner, @serhiy-storchaka
Files	check_error_occurred.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2016-04-16.08:02:38.488>
labels = ['interpreter-core']
title = 'Determining the failure of C API call is ambiguous'
updated_at = <Date 2016-04-16.09:27:24.259>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2016-04-16.09:27:24.259>
actor = 'vstinner'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2016-04-16.08:02:38.488>
creator = 'serhiy.storchaka'
dependencies = []
files = ['42478']
hgrepos = []
issue_num = 26776
keywords = ['patch']
message_count = 2.0
messages = ['263540', '263544']
nosy_count = 2.0
nosy_names = ['vstinner', 'serhiy.storchaka']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue26776'
versions = []

[serhiy-storchaka]

C API functions returns a special value unambiguously signaling about a raised exception (NULL or -1). But in some cases this is ambiguous, because the special value is a legitimate value (e.g. -1 for PyLong_AsLong() or NULL for PyDict_GetItem()). Needed to use PyErr_Occurred() to distinguish between successful and failed call.

The problem is that if PyLong_AsLong() is called when the exception is set, successful call returned -1 is interpreted as failed. Since it is happen in very rare case, this bug is usually unnoticed.

Attached experimental patch makes some functions like PyLong_AsLong() always failing if called with an exception set. Some tests are failed with it applied: test_compile test_datetime test_io test_os test_symtable test_syntax test_xml_etree_c.

[vstinner]

Ah yes. Some months (years?) ago, i started to add assertions to fail if
some functions are called with an exception set.

Checking if an exception is set at runtime adds a low overhead. Maybe it's
better to fail with a fatal error (like an assertion error) in debug mode
(and do nothing in release mode).

But I'm not sure that developers of C extensions are all able to get a
Python compiled in debug mode :-/ That's also why I added a check a
runtime: raise a SystemError if a function with an exception set. I also
added PYTHONMALLOC=debug to make some debug checks easily available on
release builds.

The bug also reminds me my PEP-490 to chain exceptions. It would be
yet another option...

So well, I don't know what is the best option.