bpo-29240: Fix locale encodings in UTF-8 Mode (#5170)

Modify locale.localeconv(), time.tzname, os.strerror() and other
functions to ignore the UTF-8 Mode: always use the current locale
encoding.

Changes:

* Add _Py_DecodeLocaleEx() and _Py_EncodeLocaleEx(). On decoding or
  encoding error, they return the position of the error and an error
  message which are used to raise Unicode errors in
  PyUnicode_DecodeLocale() and PyUnicode_EncodeLocale().
* Replace _Py_DecodeCurrentLocale() with _Py_DecodeLocaleEx().
* PyUnicode_DecodeLocale() now uses _Py_DecodeLocaleEx() for all
  cases, especially for the strict error handler.
* Add _Py_DecodeUTF8Ex(): return more information on decoding error
  and supports the strict error handler.
* Rename _Py_EncodeUTF8_surrogateescape() to _Py_EncodeUTF8Ex().
* Replace _Py_EncodeCurrentLocale() with _Py_EncodeLocaleEx().
* Ignore the UTF-8 mode to encode/decode localeconv(), strerror()
  and time zone name.
* Remove PyUnicode_DecodeLocale(), PyUnicode_DecodeLocaleAndSize()
  and PyUnicode_EncodeLocale() now ignore the UTF-8 mode: always use
  the "current" locale.
* Remove _PyUnicode_DecodeCurrentLocale(),
  _PyUnicode_DecodeCurrentLocaleAndSize() and
  _PyUnicode_EncodeCurrentLocale().

Doc/c-api/sys.rst

@@ -106,6 +106,16 @@ Operating System Utilities
    surrogate character, escape the bytes using the surrogateescape error
    handler instead of decoding them.
 
+   Encoding, highest priority to lowest priority:
+
+   * ``UTF-8`` on macOS and Android;
+   * ``UTF-8`` if the Python UTF-8 mode is enabled;
+   * ``ASCII`` if the ``LC_CTYPE`` locale is ``"C"``,
+     ``nl_langinfo(CODESET)`` returns the ``ASCII`` encoding (or an alias),
+     and :c:func:`mbstowcs` and :c:func:`wcstombs` functions uses the
+     ``ISO-8859-1`` encoding.
+   * the current locale encoding.
+
    Return a pointer to a newly allocated wide character string, use
    :c:func:`PyMem_RawFree` to free the memory. If size is not ``NULL``, write
    the number of wide characters excluding the null character into ``*size``
@@ -137,6 +147,18 @@ Operating System Utilities
    :ref:`surrogateescape error handler <surrogateescape>`: surrogate characters
    in the range U+DC80..U+DCFF are converted to bytes 0x80..0xFF.
 
+   Encoding, highest priority to lowest priority:
+
+   * ``UTF-8`` on macOS and Android;
+   * ``UTF-8`` if the Python UTF-8 mode is enabled;
+   * ``ASCII`` if the ``LC_CTYPE`` locale is ``"C"``,
+     ``nl_langinfo(CODESET)`` returns the ``ASCII`` encoding (or an alias),
+     and :c:func:`mbstowcs` and :c:func:`wcstombs` functions uses the
+     ``ISO-8859-1`` encoding.
+   * the current locale encoding.
+
+   The function uses the UTF-8 encoding in the Python UTF-8 mode.
+
    Return a pointer to a newly allocated byte string, use :c:func:`PyMem_Free`
    to free the memory. Return ``NULL`` on encoding error or memory allocation
    error

Doc/c-api/unicode.rst

@@ -770,12 +770,20 @@ system.
    :c:data:`Py_FileSystemDefaultEncoding` (the locale encoding read at
    Python startup).
 
+   This function ignores the Python UTF-8 mode.
+
    .. seealso::
 
       The :c:func:`Py_DecodeLocale` function.
 
    .. versionadded:: 3.3
 
+   .. versionchanged:: 3.7
+      The function now also uses the current locale encoding for the
+      ``surrogateescape`` error handler. Previously, :c:func:`Py_DecodeLocale`
+      was used for the ``surrogateescape``, and the current locale encoding was
+      used for ``strict``.
+
 
 .. c:function:: PyObject* PyUnicode_DecodeLocale(const char *str, const char *errors)
 
@@ -797,12 +805,20 @@ system.
    :c:data:`Py_FileSystemDefaultEncoding` (the locale encoding read at
    Python startup).
 
+   This function ignores the Python UTF-8 mode.
+
    .. seealso::
 
       The :c:func:`Py_EncodeLocale` function.
 
    .. versionadded:: 3.3
 
+   .. versionchanged:: 3.7
+      The function now also uses the current locale encoding for the
+      ``surrogateescape`` error handler. Previously, :c:func:`Py_EncodeLocale`
+      was used for the ``surrogateescape``, and the current locale encoding was
+      used for ``strict``.
+
 
 File System Encoding
 """"""""""""""""""""