gh-157917: Do not leak the \\?\ extended-length prefix into sys.prefix on Windows venvs

Lib/test/test_venv.py

@@ -302,6 +302,28 @@ def test_prefixes(self):
             self.assertEqual(pathlib.Path(out.strip().decode()),
                              pathlib.Path(expected), prefix)
 
+    # gh-157917: the "\\?\" extended-length prefix must not leak into the prefix
+    @requireVenvCreate
+    @unittest.skipUnless(os.name == 'nt', 'only relevant on Windows')
+    def test_prefixes_extended_length_path(self):
+        """
+        Test that invoking a venv through a "\\?\" extended-length path does
+        not leave that prefix behind in the prefixes.  Windows does not
+        resolve relative components (such as ".." or "/") in such paths, so
+        they break tools that join them onto sysconfig-derived paths.
+        """
+        rmtree(self.env_dir)
+        self.run_with_capture(venv.create, self.env_dir)
+        envpy = os.path.join('\\\\?\\' + self.env_dir, self.bindir, self.exe)
+        for prefix in ('prefix', 'exec_prefix'):
+            with self.subTest(prefix):
+                out, err = check_output(
+                    [envpy, '-c', 'import sys; print(sys.%s)' % prefix],
+                    encoding='utf-8')
+                self.assertFalse(out.strip().startswith('\\\\?\\'), out)
+                self.assertEqual(pathlib.Path(out.strip()),
+                                 pathlib.Path(self.env_dir), prefix)
+
     @requireVenvCreate
     def test_sysconfig(self):
         """

Misc/NEWS.d/next/Windows/2026-09-30-00-00-00.gh-issue-157917.48oa0L.rst

@@ -0,0 +1,9 @@
+Fix :attr:`sys.prefix`, :attr:`sys.exec_prefix` and :attr:`sys.executable`
+retaining the ``\\?\`` extended-length path prefix in a virtual environment
+when the interpreter is invoked through such a path on Windows.  The venv
+launcher reports its own path through ``GetModuleFileNameW``, which preserves
+the prefix, and that value was used verbatim.  Windows does not resolve
+relative path components in extended-length paths, so the leaked prefix made
+tools that join ``..`` components onto :mod:`sysconfig` paths fail with
+``OSError: [Errno 22]``.  The prefix is now stripped again, except for paths
+that are still too long to be used without it.

Modules/getpath.py

@@ -211,6 +211,28 @@ def search_up(prefix, *landmarks, test=isfile):
         prefix = dirname(prefix)
 
 
+# The traditional Windows path length limit, not counting the terminating NUL.
+MAX_PATH = 260
+
+
+def strip_extended_length_prefix(path):
+    """
+    Remove the leading "\\\\?\\" of a Windows extended-length path, and
+    rewrite the "\\\\?\\UNC\\" form of a UNC path back to a plain "\\\\" one.
+    Windows does not resolve relative components (such as ".." or "/") in an
+    extended-length path, so keeping the prefix in a path that we later join
+    onto is liable to break consumers.  A path that is still too long to be
+    used unprefixed keeps its prefix, as it has no other usable form.
+    """
+    if path.startswith('\\\\?\\UNC\\'):
+        stripped = '\\\\' + path[8:]
+    elif path.startswith('\\\\?\\'):
+        stripped = path[4:]
+    else:
+        return path
+    return stripped if len(stripped) < MAX_PATH else path
+
+
 # ******************************************************************************
 # READ VARIABLES FROM config
 # ******************************************************************************
@@ -319,6 +341,13 @@ def search_up(prefix, *landmarks, test=isfile):
         real_executable = base_executable
         #real_executable_dir = dirname(real_executable)
     executable = ENV_PYTHONEXECUTABLE or ENV___PYVENV_LAUNCHER__
+    if os_name == 'nt':
+        # QUIRK: On Windows the venv launcher reports its own path through
+        # GetModuleFileNameW, which hands back a "\\?\" extended-length path
+        # whenever that is how the interpreter was invoked.  Drop that prefix
+        # again, so the venv prefix and sys.executable stay usable by tools
+        # that join relative components onto them (gh-157917).
+        executable = strip_extended_length_prefix(executable)
     executable_dir = dirname(executable)