gh-150557: Fix os.path.ismount() for UNC paths with no share on Windows

[amruthamodela06]

Bug

On Windows, os.path.ismount() (ntpath.ismount()) reports a UNC path that names a server but no share as a mount point. A UNC mount point requires both a server and a share (\\server\share); a bare server name is not one.

>>> import os.path
>>> os.path.ismount(r"\\server")        # server, no share
True                                     # should be False
>>> os.path.ismount(r"\\?\UNC\server")  # extended UNC, no share
True                                     # should be False

Genuine share roots are correct and stay unchanged:

>>> os.path.ismount(r"\\server\share")
True
>>> os.path.ismount(r"\\?\UNC\server\share")
True

Regression

This is partly a regression from 3.11, where os.path.ismount(r"\\?\UNC\server") returned False. The splitroot()-based rewrite of ismount() (gh-101000) changed it to True. (That same rewrite correctly fixed \\?\UNC\server\share, which was wrongly False on 3.11.)

Root cause

ismount() treats the UNC/device branch as a mount point whenever the path equals its drive with no trailing component:

drive, root, rest = splitroot(path)
if drive and drive[0] in seps:
    return not rest

But splitroot() packs an incomplete, one-component UNC entirely into drive:

>>> ntpath.splitroot(r"\\server")
('\\server', '', '')
>>> ntpath.splitroot(r"\\?\UNC\server")
('\\?\UNC\server', '', '')

So rest is empty and the check passes despite there being no share.

Fix

Require a UNC drive to name both a server and a share - i.e. a separator with non-empty text on both sides must appear after the leading \\ (or after the \\?\UNC\ prefix for extended UNC):

if drive and drive[0] in seps:
    if rest:
        return False
    # A UNC mount point must name both a server and a share; a bare
    # server (e.g. \\server or \\?\UNC\server) is not one.
    if isinstance(drive, bytes):
        sep, altsep, unc_prefix = b'\\', b'/', b'\\\\?\\UNC\\'
    else:
        sep, altsep, unc_prefix = '\\', '/', '\\\\?\\UNC\\'
    normd = drive.replace(altsep, sep)
    if normd[:len(unc_prefix)].upper() == unc_prefix:
        start = len(unc_prefix)
    else:
        start = 2
    return start < normd.find(sep, start) < len(normd) - 1

Device paths (\\.\dev, \\?\dev), extended drive-letter paths (\\?\c:), volume GUID paths, and drive-letter roots all keep their current behavior.

Scope & safety

ismount() is a leaf function: nothing in ntpath calls it, and join() / normpath() / split() / splitdrive() are all driven by splitroot(), not ismount(). Its only stdlib consumer is pathlib.Path.is_mount(). splitroot() is not modified, so no other path operation changes. This also aligns with the existing docs, which already state that "a drive letter root and a share UNC are always mount points" - a bare server is not a share UNC.

Tests

News entry added via blurb. Regression tests added to test_ntpath.test_ismount:

• \\server -> False
• \\server\ (trailing separator) -> False
• \\?\UNC\server and \\?\UNC\server\ -> False
• bytes variants b"\\server" and b"\\?\UNC\server" -> False
• \\server\share and \\?\UNC\server\share (str and bytes) -> still True

The existing assertions for \\localhost\c$ (with/without trailing slash, str and bytes) continue to pass. Verified locally on Windows after rebuild: test_ntpath passes, and the full test suite passes with no new failures.

Backport

The bug exists on every branch since 3.12 (where the splitroot() rewrite landed), so this looks like a backport candidate for the active maintenance branches - deferring to maintainers on exactly which.

Split off from #139916 per discussion in #150557.

• Issue: ntpath.ismount() returns True for a UNC server with no share (e.g. \\server, \\?\UNC\server) #150557

[python-cla-bot]

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[amruthamodela06]

The failing Windows CI checks appear unrelated to this change — test_ntpath passes on the Win32 builder. The 3 failures are in unrelated subsystems:

test_msvcrt.test_kbhit — console keyboard input
test_profiling … test_generator_not_under_consumer_arithmetic — sampling-profiler stack accuracy (reports "torn stacks", timing-sensitive)
test_cmd_line.test_python_legacy_windows_stdio — exits 0xC0000142
This PR only modifies ntpath.ismount(). Happy to rebase or re-run if that would help.