Windows CI failure:test_itertools corrupts Unicode temp directory name undermbcs encoding, breaks subsequent tests

Summary

On Windows (official Azure CI), the CPython 3.15 test suite produces cascading failures originating fromtest_itertools. The test creates a temporary directory whose name contains a non-ASCII character. Because the Windows default filesystem encoding ismbcs, the Unicode byte sequence is misinterpreted and the directory name becomes corrupted on disk. The directory is never cleaned up, which causes all subsequent tests that create temporary directories to fail withFileNotFoundError.

CPython Version

Python 3.15.0a7 (main branch)

Operating System / Platform

Windows (Azure Pipelines Microsoft-hosted runner)

Filesystem Encoding

>>>importsys>>>sys.getfilesystemencoding()'mbcs'

CI Pipeline Context

Build step:

PCbuild\build.bat -e $(buildOpt)

Test step:

PCbuild\rt.bat -q -uall -u-cpu -rwW --slowest --timeout=1200 -j0 \  --junit-xml="$(Build.BinariesDirectory)\test-results.xml" \  --tempdir="$(Build.BinariesDirectory)\test"

Azure DevOps build log:
https://dev.azure.com/Python/cpython/_build/results?buildId=168864&view=logs&j=c8a71634-e5ec-54a0-3958-760f4148b765&t=ddcdae4e-111a-5c2a-2289-6b784c553924

Steps to Reproduce

1. Check out the CPythonmain branch.
2. Build on Windows:

   PCbuild\build.bat -e <buildOpt>

3. Run the test suite:

   PCbuild\rt.bat -q -uall -u-cpu -rwW --slowest --timeout=1200 ^    -j0 --tempdir="%TEMP%"

4. Observe a directory with a corrupted name (e.g.packageæ/) left behind in the temp directory.
5. Observe that subsequent tests fail withFileNotFoundError.

Expected Behavior

• All tests pass cleanly on Windows during CI builds.
• No test leaves behind artifacts that interfere with other tests.
• Test isolation is preserved — temporary directories are created and removed correctly.
• Unicode filenames are handled consistently regardless of platform filesystem encoding.

Actual Behavior

• test_itertools creates a directory whose name contains a non-ASCII character (e.g.æ).
• Under the Windows default codepage (mbcs), the Unicode name is misinterpreted:æ (U+00E6) is stored asæ.
• The corrupted directory isnot removed aftertest_itertools completes.
• Subsequent tests (e.g.test_class,test_pdb) fail when attempting to create temporary directories because the corrupted entry causesos_helper.temp_dir to raiseFileNotFoundError.
• The runner reports a warning fortest_itertools and marks unrelated tests as failed, significantly increasing triage cost.

Relevant Log Excerpt

From Azure build 168864:

Warning -- files was modified by test_itertoolsAfter: ... 'packageæ/'test_class failed (env changed)FileNotFoundError: [WinError 3] unable to create temporary directory:'D:\a\1\s\build\test_python_9140æ'

Root Cause (Analysis)

Two independent problems combine to produce this failure:

1. Encoding mismatch intest_itertools

The test constructs or receives a directory name containing a non-ASCII character. On Windows,sys.getfilesystemencoding() returns'mbcs' rather than'utf-8'. The UTF-8 byte sequence foræ (0xC3 0xA6) is reinterpreted under the ANSI code page as two separate characters, producing the visible corruptionæ. This is a violation of cross-platform Unicode path handling requirements.

2. Missing cleanup intest_itertools

The test creates the directory but does not register it for cleanup (e.g. viaaddCleanup,self.addCleanup, or a context manager such astempfile.TemporaryDirectory). The corrupted directory persists in the shared--tempdir location for the duration of the entire test run, causing every subsequent call toos_helper.temp_dir to fail withWinError 3.

Impact

Files Involved

• Lib/test/test_itertools.py — primary location of the encoding issue and missing cleanup
• Lib/test/support/os_helper.py — secondary impact point (temp_dir context manager)

Linked Resources

• Azure DevOps build 168864:https://dev.azure.com/Python/cpython/_build/results?buildId=168864
• PEP 529 — Change Windows filesystem encoding to UTF-8:https://peps.python.org/pep-0529/
• sys.getfilesystemencoding() docs:https://docs.python.org/3/library/sys.html#sys.getfilesystemencoding
• os.fsencode() docs:https://docs.python.org/3/library/os.html#os.fsencode

Linked PRs

• gh-146202: Create tmp_dir in regrtest worker #146347
• [3.14] gh-146202: Create tmp_dir in regrtest worker (GH-146347) #146349
• [3.13] gh-146202: Create tmp_dir in regrtest worker (GH-146347) #146350