remove dead code in PyErr_GivenExceptionMatches()

[scoder]

BPO	31091
Nosy	@pitrou, @scoder, @serhiy-storchaka
PRs	bpo-31091: remove dead code. #2963

^{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 = <Date 2017-07-31.20:30:21.339>
created_at = <Date 2017-07-31.19:24:27.609>
labels = ['interpreter-core', '3.7', 'performance']
title = 'remove dead code in PyErr_GivenExceptionMatches()'
updated_at = <Date 2017-07-31.20:30:21.338>
user = 'https://github.com/scoder'

bugs.python.org fields:

activity = <Date 2017-07-31.20:30:21.338>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2017-07-31.20:30:21.339>
closer = 'serhiy.storchaka'
components = ['Interpreter Core']
creation = <Date 2017-07-31.19:24:27.609>
creator = 'scoder'
dependencies = []
files = []
hgrepos = []
issue_num = 31091
keywords = []
message_count = 4.0
messages = ['299579', '299580', '299584', '299585']
nosy_count = 3.0
nosy_names = ['pitrou', 'scoder', 'serhiy.storchaka']
pr_nums = ['2963']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'performance'
url = 'https://bugs.python.org/issue31091'
versions = ['Python 3.7']

[scoder]

PyObject *exception, *value, *tb;
        PyErr_Fetch(&exception, &value, &tb);
        /* PyObject_IsSubclass() can recurse and therefore is
           not safe (see test_bad_getattr in test.pickletester). */
        res = PyType_IsSubtype((PyTypeObject *)err, (PyTypeObject *)exc);
        /* This function must not fail, so print the error here */
        if (res == -1) {
            PyErr_WriteUnraisable(err);
            res = 0;
        }
        PyErr_Restore(exception, value, tb);

According to the comment, there was previously a call to PyObject_IsSubclass() involved which could fail, but since it was replaced with a call to PyType_IsSubtype(), it can no longer fail.
See pull request.

[scoder]

Looks like the switch from PyObject_IsSubclass() to PyType_IsSubtype() was made during the original Py3 development cycle. It should thus be safe to assume that the semantics are "as designed". :)

What about applying the patch also to 3.6 and older?

[serhiy-storchaka]

New changeset e4c06bc by Serhiy Storchaka (scoder) in branch 'master':
bpo-31091: Remove dead code in PyErr_GivenExceptionMatches(). (bpo-2963)
e4c06bc

[serhiy-storchaka]

This is just a cleaning up. There is no bug in the old code. Usually such kind of changes are not backported.