os.path.realpath crashes in Windows Store package

[zooba]

BPO	38020
Nosy	@pfmoore, @tjguk, @ambv, @zware, @zooba, @miss-islington
PRs	bpo-38020: Fixes potential crash in os.readlink() on Windows #15663 [3.8] bpo-38020: Fixes crash in os.readlink() on Windows (GH-15663) #15665

^{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 = 'https://github.com/zooba'
closed_at = <Date 2019-09-03.20:16:14.712>
created_at = <Date 2019-09-03.18:28:55.395>
labels = ['release-blocker', '3.8', 'OS-windows', 'type-crash', '3.9']
title = 'os.path.realpath crashes in Windows Store package'
updated_at = <Date 2019-09-03.20:16:14.711>
user = 'https://github.com/zooba'

bugs.python.org fields:

activity = <Date 2019-09-03.20:16:14.711>
actor = 'steve.dower'
assignee = 'steve.dower'
closed = True
closed_date = <Date 2019-09-03.20:16:14.712>
closer = 'steve.dower'
components = ['Windows']
creation = <Date 2019-09-03.18:28:55.395>
creator = 'steve.dower'
dependencies = []
files = []
hgrepos = []
issue_num = 38020
keywords = ['patch', '3.8regression']
message_count = 4.0
messages = ['351094', '351095', '351100', '351102']
nosy_count = 6.0
nosy_names = ['paul.moore', 'tim.golden', 'lukasz.langa', 'zach.ware', 'steve.dower', 'miss-islington']
pr_nums = ['15663', '15665']
priority = 'release blocker'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'crash'
url = 'https://bugs.python.org/issue38020'
versions = ['Python 3.8', 'Python 3.9']

[zooba]

When installed via the Store, "python3.8 -c 'import sysconfig'" crashes hard.

So far, I've discovered it's inside an os.path.realpath() call, and appears to be a refcounting issue (double DECREF I think), but haven't gotten any further yet.

I'm looking into it.

[zooba]

I forgot to initialize the result object in readlink() on Windows, and so when we hit the failure path it returns garbage. Attempting to DECREF that garbage fails.

The call to readlink() fails in the Store app because sys.executable is not a readable link. It should fail in most cases anyway, but apparently outside of the app container the stack is NULL here.

[zooba]

New changeset 993ac92 by Steve Dower in branch 'master':
bpo-38020: Fixes crash in os.readlink() on Windows (GH-15663)
993ac92

[miss-islington]

New changeset 54dac6c by Miss Islington (bot) in branch '3.8':
bpo-38020: Fixes crash in os.readlink() on Windows (GH-15663)
54dac6c