bpo-30647: Check nl_langinfo(CODESET) in locale coercion (GH-2374)

- On some versions of FreeBSD, setting the "UTF-8" locale
  succeeds, but a subsequent "nl_langinfo(CODESET)" fails
- adding a check for this in the coercion logic means that
  coercion will happen on systems where this check succeeds,
  and will be skipped otherwise
- that way CPython should automatically adapt to changes in
  platform behaviour, rather than needing a new release to
  enable coercion at build time
- this also allows UTF-8 to be re-enabled as a coercion
  target, restoring the locale coercion behaviour on Mac OS X

Lib/test/test_c_locale_coercion.py

 # Tests the attempted automatic coercion of the C locale to a UTF-8 locale
 
 import unittest
+import locale
 import os
 import sys
 import sysconfig
@@ -32,24 +33,34 @@ else:
 
 # In order to get the warning messages to match up as expected, the candidate
 # order here must much the target locale order in Python/pylifecycle.c
-_C_UTF8_LOCALES = ("C.UTF-8", "C.utf8") #, "UTF-8")
-
-# XXX (ncoghlan): Using UTF-8 as a target locale is currently disabled due to
-#                 problems encountered on *BSD systems with those test cases
-# For additional details see:
-#     nl_langinfo CODESET error: https://bugs.python.org/issue30647
-#     locale handling differences: https://bugs.python.org/issue30672
+_C_UTF8_LOCALES = ("C.UTF-8", "C.utf8", "UTF-8")
 
 # There's no reliable cross-platform way of checking locale alias
 # lists, so the only way of knowing which of these locales will work
 # is to try them with locale.setlocale(). We do that in a subprocess
 # to avoid altering the locale of the test runner.
+#
+# If the relevant locale module attributes exist, and we're not on a platform
+# where we expect it to always succeed, we also check that
+# `locale.nl_langinfo(locale.CODESET)` works, as if it fails, the interpreter
+# will skip locale coercion for that particular target locale
+_check_nl_langinfo_CODESET = bool(
+    sys.platform not in ("darwin", "linux") and
+    hasattr(locale, "nl_langinfo") and
+    hasattr(locale, "CODESET")
+)
+
 def _set_locale_in_subprocess(locale_name):
     cmd_fmt = "import locale; print(locale.setlocale(locale.LC_CTYPE, '{}'))"
+    if _check_nl_langinfo_CODESET:
+        # If there's no valid CODESET, we expect coercion to be skipped
+        cmd_fmt += "; import sys; sys.exit(not locale.nl_langinfo(locale.CODESET))"
     cmd = cmd_fmt.format(locale_name)
     result, py_cmd = run_python_until_end("-c", cmd, __isolated=True)
     return result.rc == 0
 
+
+
 _fields = "fsencoding stdin_info stdout_info stderr_info lang lc_ctype lc_all"
 _EncodingDetails = namedtuple("EncodingDetails", _fields)
 

Python/pylifecycle.c

@@ -399,17 +399,10 @@ typedef struct _CandidateLocale {
 static _LocaleCoercionTarget _TARGET_LOCALES[] = {
     {"C.UTF-8"},
     {"C.utf8"},
-    /* {"UTF-8"}, */
+    {"UTF-8"},
     {NULL}
 };
 
-/* XXX (ncoghlan): Using UTF-8 as a target locale is currently disabled due to
- *                 problems encountered on *BSD systems with those test cases
- * For additional details see:
- *     nl_langinfo CODESET error: https://bugs.python.org/issue30647
- *     locale handling differences: https://bugs.python.org/issue30672
- */
-
 static char *
 get_default_standard_stream_error_handler(void)
 {
@@ -490,6 +483,16 @@ _Py_CoerceLegacyLocale(void)
                 const char *new_locale = setlocale(LC_CTYPE,
                                                    target->locale_name);
                 if (new_locale != NULL) {
+#if !defined(__APPLE__) && defined(HAVE_LANGINFO_H) && defined(CODESET)
+                    /* Also ensure that nl_langinfo works in this locale */
+                    char *codeset = nl_langinfo(CODESET);
+                    if (!codeset || *codeset == '\0') {
+                        /* CODESET is not set or empty, so skip coercion */
+                        new_locale = NULL;
+                        setlocale(LC_CTYPE, "");
+                        continue;
+                    }
+#endif
                     /* Successfully configured locale, so make it the default */
                     _coerce_default_locale_settings(target);
                     return;