Cannot import an extension module if its path is not encodable in UTF-8

Bug report

Importing an extension module fails with UnicodeEncodeError if its path contains characters unencodable in UTF-8, e.g. surrogate escapes of a file name undecodable in the filesystem encoding.

$ .../nd\udcffwt/python -c "import _json"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
UnicodeEncodeError: 'utf-8' codec can't encode character '\udcff' in position 45: surrogates not allowed

As a result, CPython cannot be run in a directory whose name contains undecodable bytes, and cannot even be built there -- make fails in checksharedmods. On 3.13 and 3.14 every import of a shared extension module from such a path fails.

The cause is the key of the extensions cache: hashtable_key_from_2_strings() in Python/import.c encodes the module file name with _PyUnicode_AsUTF8NoNUL(). It was added in gh-105699.

The raw content of the strings (UCS1, UCS2 or UCS4) can be used for the key instead of the UTF-8 encoding.

Linked PRs

• gh-155249

Start in Python/import.c at hashtable_key_from_2_strings(), where the issue identifies UTF-8 encoding of the extension module filename as the cause. Review linked PR gh-155249 and the checksharedmods failure described in the report. Done should include successful extension-module imports and builds from paths containing undecodable filename bytes.