bpo-42236: Use UTF-8 encoding if nl_langinfo(CODESET) fails (GH-23086)

If the nl_langinfo(CODESET) function returns an empty string, Python
now uses UTF-8 as the filesystem encoding.

In May 2010 (commit b744ba1d14), I
modified Python to log a warning and use UTF-8 as the filesystem
encoding (instead of None) if nl_langinfo(CODESET) returns an empty
string.

In August 2020 (commit 94908bbc15), I
modified Python startup to fail with a fatal error and a specific
error message if nl_langinfo(CODESET) returns an empty string. The
intent was to prevent guessing the encoding and also investigate user
configuration where this case happens.

In 10 years (2010 to 2020), I saw zero user report about the error
message related to nl_langinfo(CODESET) returning an empty string.

Today, UTF-8 became the defacto standard and it's safe to make the
assumption that the user expects UTF-8. For example,
nl_langinfo(CODESET) can return an empty string on macOS if the
LC_CTYPE locale is not supported, and UTF-8 is the default encoding
on macOS.

While this change is likely to not affect anyone in practice, it
should make UTF-8 lover happy ;-)

Rewrite also the documentation explaining how Python selects the
filesystem encoding and error handler.

Doc/c-api/init_config.rst

@@ -253,10 +253,16 @@ PyPreConfig
 
       See :c:member:`PyConfig.isolated`.
 
-   .. c:member:: int legacy_windows_fs_encoding (Windows only)
+   .. c:member:: int legacy_windows_fs_encoding
 
-      If non-zero, disable UTF-8 Mode, set the Python filesystem encoding to
-      ``mbcs``, set the filesystem error handler to ``replace``.
+      If non-zero:
+
+      * Set :c:member:`PyPreConfig.utf8_mode` to ``0``,
+      * Set :c:member:`PyConfig.filesystem_encoding` to ``"mbcs"``,
+      * Set :c:member:`PyConfig.filesystem_errors` to ``"replace"``.
+
+      Initialized the from :envvar:`PYTHONLEGACYWINDOWSFSENCODING` environment
+      variable value.
 
       Only available on Windows. ``#ifdef MS_WINDOWS`` macro can be used for
       Windows specific code.
@@ -499,11 +505,47 @@ PyConfig
 
    .. c:member:: wchar_t* filesystem_encoding
 
-      Filesystem encoding, :func:`sys.getfilesystemencoding`.
+      Filesystem encoding: :func:`sys.getfilesystemencoding`.
+
+      On macOS, Android and VxWorks: use ``"utf-8"`` by default.
+
+      On Windows: use ``"utf-8"`` by default, or ``"mbcs"`` if
+      :c:member:`~PyPreConfig.legacy_windows_fs_encoding` of
+      :c:type:`PyPreConfig` is non-zero.
+
+      Default encoding on other platforms:
+
+      * ``"utf-8"`` if :c:member:`PyPreConfig.utf8_mode` is non-zero.
+      * ``"ascii"`` if Python detects that ``nl_langinfo(CODESET)`` announces
+        the ASCII encoding (or Roman8 encoding on HP-UX), whereas the
+        ``mbstowcs()`` function decodes from a different encoding (usually
+        Latin1).
+      * ``"utf-8"`` if ``nl_langinfo(CODESET)`` returns an empty string.
+      * Otherwise, use the LC_CTYPE locale encoding:
+        ``nl_langinfo(CODESET)`` result.
+
+      At Python statup, the encoding name is normalized to the Python codec
+      name. For example, ``"ANSI_X3.4-1968"`` is replaced with ``"ascii"``.
+
+      See also the :c:member:`~PyConfig.filesystem_errors` member.
 
    .. c:member:: wchar_t* filesystem_errors
 
-      Filesystem encoding errors, :func:`sys.getfilesystemencodeerrors`.
+      Filesystem error handler: :func:`sys.getfilesystemencodeerrors`.
+
+      On Windows: use ``"surrogatepass"`` by default, or ``"replace"``  if
+      :c:member:`~PyPreConfig.legacy_windows_fs_encoding` of
+      :c:type:`PyPreConfig` is non-zero.
+
+      On other platforms: use ``"surrogateescape"`` by default.
+
+      Supported error handlers:
+
+      * ``"strict"``
+      * ``"surrogateescape"``
+      * ``"surrogatepass"`` (only supported with the UTF-8 encoding)
+
+      See also the :c:member:`~PyConfig.filesystem_encoding` member.
 
    .. c:member:: unsigned long hash_seed
    .. c:member:: int use_hash_seed

Doc/library/sys.rst

@@ -616,29 +616,20 @@ always available.
 .. function:: getfilesystemencoding()
 
    Return the name of the encoding used to convert between Unicode
-   filenames and bytes filenames. For best compatibility, str should be
-   used for filenames in all cases, although representing filenames as bytes
-   is also supported. Functions accepting or returning filenames should support
-   either str or bytes and internally convert to the system's preferred
-   representation.
+   filenames and bytes filenames.
+
+   For best compatibility, str should be used for filenames in all cases,
+   although representing filenames as bytes is also supported. Functions
+   accepting or returning filenames should support either str or bytes and
+   internally convert to the system's preferred representation.
 
    This encoding is always ASCII-compatible.
 
    :func:`os.fsencode` and :func:`os.fsdecode` should be used to ensure that
    the correct encoding and errors mode are used.
 
-   * In the UTF-8 mode, the encoding is ``utf-8`` on any platform.
-
-   * On macOS, the encoding is ``'utf-8'``.
-
-   * On Unix, the encoding is the locale encoding.
-
-   * On Windows, the encoding may be ``'utf-8'`` or ``'mbcs'``, depending
-     on user configuration.
-
-   * On Android, the encoding is ``'utf-8'``.
-
-   * On VxWorks, the encoding is ``'utf-8'``.
+   The filesystem encoding is initialized from
+   :c:member:`PyConfig.filesystem_encoding`.
 
    .. versionchanged:: 3.2
       :func:`getfilesystemencoding` result cannot be ``None`` anymore.
@@ -660,6 +651,9 @@ always available.
    :func:`os.fsencode` and :func:`os.fsdecode` should be used to ensure that
    the correct encoding and errors mode are used.
 
+   The filesystem error handler is initialized from
+   :c:member:`PyConfig.filesystem_errors`.
+
    .. versionadded:: 3.6
 
 .. function:: getrefcount(object)
@@ -1457,6 +1451,9 @@ always available.
    This is equivalent to defining the :envvar:`PYTHONLEGACYWINDOWSFSENCODING`
    environment variable before launching Python.
 
+   See also :func:`sys.getfilesystemencoding` and
+   :func:`sys.getfilesystemencodeerrors`.
+
    .. availability:: Windows.
 
    .. versionadded:: 3.6

Include/cpython/initconfig.h

@@ -156,36 +156,13 @@ typedef struct {
     /* Python filesystem encoding and error handler:
        sys.getfilesystemencoding() and sys.getfilesystemencodeerrors().
 
-       Default encoding and error handler:
-
-       * if Py_SetStandardStreamEncoding() has been called: they have the
-         highest priority;
-       * PYTHONIOENCODING environment variable;
-       * The UTF-8 Mode uses UTF-8/surrogateescape;
-       * If Python forces the usage of the ASCII encoding (ex: C locale
-         or POSIX locale on FreeBSD or HP-UX), use ASCII/surrogateescape;
-       * locale encoding: ANSI code page on Windows, UTF-8 on Android and
-         VxWorks, LC_CTYPE locale encoding on other platforms;
-       * On Windows, "surrogateescape" error handler;
-       * "surrogateescape" error handler if the LC_CTYPE locale is "C" or "POSIX";
-       * "surrogateescape" error handler if the LC_CTYPE locale has been coerced
-         (PEP 538);
-       * "strict" error handler.
-
-       Supported error handlers: "strict", "surrogateescape" and
-       "surrogatepass". The surrogatepass error handler is only supported
-       if Py_DecodeLocale() and Py_EncodeLocale() use directly the UTF-8 codec;
-       it's only used on Windows.
-
-       initfsencoding() updates the encoding to the Python codec name.
-       For example, "ANSI_X3.4-1968" is replaced with "ascii".
-
-       On Windows, sys._enablelegacywindowsfsencoding() sets the
-       encoding/errors to mbcs/replace at runtime.
-
-
-       See Py_FileSystemDefaultEncoding and Py_FileSystemDefaultEncodeErrors.
-       */
+       The Doc/c-api/init_config.rst documentation explains how Python selects
+       the filesystem encoding and error handler.
+
+      _PyUnicode_InitEncodings() updates the encoding name to the Python codec
+      name. For example, "ANSI_X3.4-1968" is replaced with "ascii". It also
+      sets Py_FileSystemDefaultEncoding to filesystem_encoding and
+      sets Py_FileSystemDefaultEncodeErrors to filesystem_errors. */
     wchar_t *filesystem_encoding;
     wchar_t *filesystem_errors;
 

Include/internal/pycore_fileutils.h

@@ -50,7 +50,7 @@ PyAPI_FUNC(int) _Py_GetLocaleconvNumeric(
 
 PyAPI_FUNC(void) _Py_closerange(int first, int last);
 
-PyAPI_FUNC(wchar_t*) _Py_GetLocaleEncoding(const char **errmsg);
+PyAPI_FUNC(wchar_t*) _Py_GetLocaleEncoding(void);
 PyAPI_FUNC(PyObject*) _Py_GetLocaleEncodingObject(void);
 
 #ifdef __cplusplus

Include/pyport.h

@@ -841,12 +841,16 @@ extern _invalid_parameter_handler _Py_silent_invalid_parameter_handler;
 #endif
 
 #if defined(__ANDROID__) || defined(__VXWORKS__)
-   /* Ignore the locale encoding: force UTF-8 */
+   // Use UTF-8 as the locale encoding, ignore the LC_CTYPE locale.
+   // See _Py_GetLocaleEncoding(), PyUnicode_DecodeLocale()
+   // and PyUnicode_EncodeLocale().
 #  define _Py_FORCE_UTF8_LOCALE
 #endif
 
 #if defined(_Py_FORCE_UTF8_LOCALE) || defined(__APPLE__)
-   /* Use UTF-8 as filesystem encoding */
+   // Use UTF-8 as the filesystem encoding.
+   // See PyUnicode_DecodeFSDefaultAndSize(), PyUnicode_EncodeFSDefault(),
+   // Py_DecodeLocale() and Py_EncodeLocale().
 #  define _Py_FORCE_UTF8_FS_ENCODING
 #endif
 

Misc/NEWS.d/next/Core and Builtins/2020-11-01-21-21-38.bpo-42236.MPx-NK.rst

@@ -0,0 +1,2 @@
+If the ``nl_langinfo(CODESET)`` function returns an empty string, Python now
+uses UTF-8 as the filesystem encoding. Patch by Victor Stinner.

Python/fileutils.c

@@ -826,20 +826,15 @@ _Py_EncodeLocaleEx(const wchar_t *text, char **str,
 // - Return "UTF-8" if _Py_FORCE_UTF8_LOCALE macro is defined (ex: on Android)
 // - Return "UTF-8" if the UTF-8 Mode is enabled
 // - On Windows, return the ANSI code page (ex: "cp1250")
-// - Return "UTF-8" if nl_langinfo(CODESET) returns an empty string
-//   and if the _Py_FORCE_UTF8_FS_ENCODING macro is defined (ex: on macOS).
+// - Return "UTF-8" if nl_langinfo(CODESET) returns an empty string.
 // - Otherwise, return nl_langinfo(CODESET).
 //
-// Return NULL and set errmsg to an error message
-// if nl_langinfo(CODESET) fails.
-//
-// Return NULL and set errmsg to NULL on memory allocation failure.
+// Return NULL on memory allocation failure.
 //
 // See also config_get_locale_encoding()
 wchar_t*
-_Py_GetLocaleEncoding(const char **errmsg)
+_Py_GetLocaleEncoding(void)
 {
-    *errmsg = NULL;
 #ifdef _Py_FORCE_UTF8_LOCALE
     // On Android langinfo.h and CODESET are missing,
     // and UTF-8 is always used in mbstowcs() and wcstombs().
@@ -859,21 +854,14 @@ _Py_GetLocaleEncoding(const char **errmsg)
 #else
     const char *encoding = nl_langinfo(CODESET);
     if (!encoding || encoding[0] == '\0') {
-#ifdef _Py_FORCE_UTF8_FS_ENCODING
-        // nl_langinfo() can return an empty string when the LC_CTYPE locale is
-        // not supported. Default to UTF-8 in that case, because UTF-8 is the
-        // default charset on macOS.
+        // Use UTF-8 if nl_langinfo() returns an empty string. It can happen on
+        // macOS if the LC_CTYPE locale is not supported.
         return _PyMem_RawWcsdup(L"UTF-8");
-#else
-        *errmsg = "failed to get the locale encoding: "
-                  "nl_langinfo(CODESET) returns an empty string";
-        return NULL;
-#endif
     }
 
     wchar_t *wstr;
     int res = decode_current_locale(encoding, &wstr, NULL,
-                                    errmsg, _Py_ERROR_SURROGATEESCAPE);
+                                    NULL, _Py_ERROR_SURROGATEESCAPE);
     if (res < 0) {
         return NULL;
     }
@@ -887,15 +875,9 @@ _Py_GetLocaleEncoding(const char **errmsg)
 PyObject *
 _Py_GetLocaleEncodingObject(void)
 {
-    const char *errmsg;
-    wchar_t *encoding = _Py_GetLocaleEncoding(&errmsg);
+    wchar_t *encoding = _Py_GetLocaleEncoding();
     if (encoding == NULL) {
-        if (errmsg != NULL) {
-            PyErr_SetString(PyExc_ValueError, errmsg);
-        }
-        else {
-            PyErr_NoMemory();
-        }
+        PyErr_NoMemory();
         return NULL;
     }
 

Python/initconfig.c

@@ -1318,7 +1318,7 @@ config_read_env_vars(PyConfig *config)
 
 #ifdef MS_WINDOWS
     _Py_get_env_flag(use_env, &config->legacy_windows_stdio,
-                 "PYTHONLEGACYWINDOWSSTDIO");
+                     "PYTHONLEGACYWINDOWSSTDIO");
 #endif
 
     if (config_get_env(config, "PYTHONDUMPREFS")) {
@@ -1498,15 +1498,9 @@ static PyStatus
 config_get_locale_encoding(PyConfig *config, const PyPreConfig *preconfig,
                            wchar_t **locale_encoding)
 {
-    const char *errmsg;
-    wchar_t *encoding = _Py_GetLocaleEncoding(&errmsg);
+    wchar_t *encoding = _Py_GetLocaleEncoding();
     if (encoding == NULL) {
-        if (errmsg != NULL) {
-            return _PyStatus_ERR(errmsg);
-        }
-        else {
-            return _PyStatus_NO_MEMORY();
-        }
+        return _PyStatus_NO_MEMORY();
     }
     PyStatus status = PyConfig_SetString(config, locale_encoding, encoding);
     PyMem_RawFree(encoding);