gh-90548: Allow Alpine/MUSL to pass test_c_locale_coercion. (GH-134454)

Like cygwin, MUSL defaults to utf-8 if no variables are set.  I have no
idea if the existing tests pass on cygwin, but I made the modifications
such that I shouldn't break it if is.  The additional checks needed for
MUSL are guarded by DEFAULT_LOCALE_IS_C being False.  Based on this
flag, we expect utf-8 for the encodings and no coercion message, as
long as LC_ALL is not set to C.  (That looks like a bit of an issue with
the test structure, but I'm not going to attempt to "fix" that.)
DEFAULT_ENCODING is intentionally not given a default since it is only
used when DEFAULT_LOCALE_IS_C is False, and if you use the flag you'll
need to set it.

After reading through issue 30672, looking at the source, and running a
test on Android, I *think* the current situation is that coercion will
be done if the local is set to POSIX regardless of platform.  However,
if the platform doesn't make POSIX equivalent to C, the encodings when
coercion is disabled will not be the same as for C (it is utf-8 on
android, for example).  This means the tests would fail if POSIX were
added unconditionally to the EXPECTED_C_LOCALE_EQUIVALENTS as envisioned
in the issue.  This *could* be fixed with another flag, but I'm not sure
it is worth the effort.  I'm not even sure Python is behaving optimally
in this case (assuming my analysis is correct).  So I just altered the
comment and add POSIX if and only if the platform is linux.

Author: bitdancer

Lib/test/test_c_locale_coercion.py

@@ -12,7 +12,9 @@
 from test.support.script_helper import run_python_until_end
 
 
-# Set the list of ways we expect to be able to ask for the "C" locale
+# Set the list of ways we expect to be able to ask for the "C" locale.
+# 'invalid.ascii' is an invalid LOCALE name and so should get turned in to the
+# default locale, which is traditionally C.
 EXPECTED_C_LOCALE_EQUIVALENTS = ["C", "invalid.ascii"]
 
 # Set our expectation for the default encoding used in the C locale
@@ -21,6 +23,7 @@
 EXPECTED_C_LOCALE_FS_ENCODING = "ascii"
 
 # Set our expectation for the default locale used when none is specified
+DEFAULT_LOCALE_IS_C = True
 EXPECT_COERCION_IN_DEFAULT_LOCALE = True
 
 TARGET_LOCALES = ["C.UTF-8", "C.utf8", "UTF-8"]
@@ -30,12 +33,12 @@
     # Android defaults to using UTF-8 for all system interfaces
     EXPECTED_C_LOCALE_STREAM_ENCODING = "utf-8"
     EXPECTED_C_LOCALE_FS_ENCODING = "utf-8"
-elif sys.platform.startswith("linux"):
-    # Linux distros typically alias the POSIX locale directly to the C
-    # locale.
-    # TODO: Once https://bugs.python.org/issue30672 is addressed, we'll be
-    #       able to check this case unconditionally
-    EXPECTED_C_LOCALE_EQUIVALENTS.append("POSIX")
+elif support.linked_to_musl():
+    # MUSL defaults to utf-8 unless the C locale is set explicitly.
+    EXPECTED_C_LOCALE_EQUIVALENTS = ["C"]
+    DEFAULT_LOCALE_IS_C = False
+    DEFAULT_ENCODING = 'utf-8'
+    EXPECT_COERCION_IN_DEFAULT_LOCALE = False
 elif sys.platform.startswith("aix"):
     # AIX uses iso8859-1 in the C locale, other *nix platforms use ASCII
     EXPECTED_C_LOCALE_STREAM_ENCODING = "iso8859-1"
@@ -52,6 +55,11 @@
     # VxWorks defaults to using UTF-8 for all system interfaces
     EXPECTED_C_LOCALE_STREAM_ENCODING = "utf-8"
     EXPECTED_C_LOCALE_FS_ENCODING = "utf-8"
+if sys.platform.startswith("linux"):
+    # Linux recognizes POSIX as a synonym for C.  Python will always coerce
+    # if the locale is set to POSIX, but not all platforms will use the
+    # C locale encodings if POSIX is set, so we'll only test it on linux.
+    EXPECTED_C_LOCALE_EQUIVALENTS.append("POSIX")
 
 # Note that the above expectations are still wrong in some cases, such as:
 # * Windows when PYTHONLEGACYWINDOWSFSENCODING is set
@@ -362,9 +370,14 @@ def _check_c_locale_coercion(self,
             base_var_dict["PYTHONCOERCECLOCALE"] = coerce_c_locale
 
         # Check behaviour for the default locale
+        _fs_encoding = fs_encoding
+        _stream_encoding = stream_encoding
+        if not DEFAULT_LOCALE_IS_C and 'LC_ALL' not in extra_vars:
+            _fs_encoding = _stream_encoding = DEFAULT_ENCODING
         with self.subTest(default_locale=True,
                           PYTHONCOERCECLOCALE=coerce_c_locale):
-            if EXPECT_COERCION_IN_DEFAULT_LOCALE:
+            if (EXPECT_COERCION_IN_DEFAULT_LOCALE
+                    or (not DEFAULT_LOCALE_IS_C and 'LC_ALL' in extra_vars)):
                 _expected_warnings = expected_warnings
                 _coercion_expected = coercion_expected
             else:
@@ -378,8 +391,8 @@ def _check_c_locale_coercion(self,
                     _expected_warnings == [CLI_COERCION_WARNING]):
                 _expected_warnings = None
             self._check_child_encoding_details(base_var_dict,
-                                               fs_encoding,
-                                               stream_encoding,
+                                               _fs_encoding,
+                                               _stream_encoding,
                                                None,
                                                _expected_warnings,
                                                _coercion_expected)