[Windows] Can't import extension modules resolved via relative paths in sys.path

[smunday]

BPO	43105
Nosy	@brettcannon, @gpshead, @pfmoore, @orsenthil, @tjguk, @zware, @eryksun, @zooba, @neonene
PRs	bpo-43105: Importlib now resolves relative paths when creating module spec objects from file locations #25121 [3.9] bpo-43105: Importlib now resolves relative paths when creating module spec objects from file locations (GH-25121) #25237 [3.8] bpo-43105: Importlib now resolves relative paths when creating module spec objects from file locations (GH-25121) #25318

^{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 2021-04-12.15:36:46.754>
created_at = <Date 2021-02-02.17:03:00.187>
labels = ['interpreter-core', '3.8', 'type-bug', 'library', 'OS-windows']
title = "[Windows] Can't import extension modules resolved via relative paths in sys.path"
updated_at = <Date 2021-06-14.06:28:27.775>
user = 'https://bugs.python.org/smunday'

bugs.python.org fields:

activity = <Date 2021-06-14.06:28:27.775>
actor = 'neonene'
assignee = 'steve.dower'
closed = True
closed_date = <Date 2021-04-12.15:36:46.754>
closer = 'steve.dower'
components = ['Interpreter Core', 'Library (Lib)', 'Windows']
creation = <Date 2021-02-02.17:03:00.187>
creator = 'smunday'
dependencies = []
files = []
hgrepos = []
issue_num = 43105
keywords = ['patch']
message_count = 13.0
messages = ['386155', '386898', '387056', '389930', '390388', '390421', '390423', '390445', '390476', '390672', '393129', '395032', '395775']
nosy_count = 10.0
nosy_names = ['brett.cannon', 'gregory.p.smith', 'paul.moore', 'orsenthil', 'tim.golden', 'zach.ware', 'eryksun', 'steve.dower', 'neonene', 'smunday']
pr_nums = ['25121', '25237', '25318']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue43105'
versions = ['Python 3.8']

[smunday]

If I attempt to import an extension module via something like this:

from pkg import extmodule

and it happens that "pkg" is found in a folder that is given in sys.path as a relative path, then the import fails, with

ImportError: DLL load failed while importing extmodule: The parameter is incorrect.

This only happens with 3.8 and later.

AFAICS the reason is that if the module resolution is done via a relative path, it results in dynload_win.c:_PyImport_FindSharedFuncptrWindows attempting to feed LoadLibraryExW a relative path for the .pyd file. But LoadLibraryExW treats relative paths completely differently from absolute ones, in that it searches for the given path relative to the library search path entries rather than simply loading the DLL at that path. But as of 3.8 the current directory is removed from the search path, so the .pyd is never found. Since Python knows the specific file it wants LoadLibraryExW to load, having just resolved it via the import mechanism, I guess it ought to ensure it only ever calls LoadLibraryExW with an absolute path.

(I'm assuming use of relative paths in sys.path is officially supported, since nothing I've found in the docs says otherwise.)