bpo-34523: Py_FileSystemDefaultEncoding NULL by default (GH-9003)

* Py_FileSystemDefaultEncoding and Py_FileSystemDefaultEncodeErrors
  default value is now NULL: initfsencoding() set them
  during Python initialization.
* Document how Python chooses the filesystem encoding and error
  handler.
* Add an assertion to _PyCoreConfig_Read().

Include/coreconfig.h

@@ -66,11 +66,34 @@ typedef struct {
     int coerce_c_locale;    /* PYTHONCOERCECLOCALE, -1 means unknown */
     int coerce_c_locale_warn; /* PYTHONCOERCECLOCALE=warn */
 
-    /* Python filesystem encoding and error handler: see
+    /* Python filesystem encoding and error handler:
        sys.getfilesystemencoding() and sys.getfilesystemencodeerrors().
 
-       Updated later by initfsencoding(). On Windows, can be updated by
-       sys._enablelegacywindowsfsencoding() at runtime.
+       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;
+       * locale encoding: ANSI code page on Windows, UTF-8 on Android,
+         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.
        */

Python/coreconfig.c

@@ -19,25 +19,12 @@
 
 /* Global configuration variables */
 
-/* The default encoding used by the platform file system APIs
-   Can remain NULL for all platforms that don't have such a concept
-
-   Don't forget to modify PyUnicode_DecodeFSDefault() if you touch any of the
-   values for Py_FileSystemDefaultEncoding!
-*/
-#if defined(__APPLE__)
-const char *Py_FileSystemDefaultEncoding = "utf-8";
-int Py_HasFileSystemDefaultEncoding = 1;
-#elif defined(MS_WINDOWS)
-/* may be changed by initfsencoding(), but should never be free()d */
-const char *Py_FileSystemDefaultEncoding = "utf-8";
-int Py_HasFileSystemDefaultEncoding = 1;
-#else
-const char *Py_FileSystemDefaultEncoding = NULL; /* set by initfsencoding() */
+/* The filesystem encoding is chosen by config_init_fs_encoding(),
+   see also initfsencoding(). */
+const char *Py_FileSystemDefaultEncoding = NULL;
 int Py_HasFileSystemDefaultEncoding = 0;
-#endif
-const char *Py_FileSystemDefaultEncodeErrors = "surrogateescape";
-static int _Py_HasFileSystemDefaultEncodeErrors = 1;
+const char *Py_FileSystemDefaultEncodeErrors = NULL;
+static int _Py_HasFileSystemDefaultEncodeErrors = 0;
 
 /* UTF-8 mode (PEP 540): if equals to 1, use the UTF-8 encoding, and change
    stdin and stdout error handler to "surrogateescape". It is equal to
@@ -1362,6 +1349,7 @@ _PyCoreConfig_Read(_PyCoreConfig *config)
     assert(config->filesystem_errors != NULL);
     assert(config->stdio_encoding != NULL);
     assert(config->stdio_errors != NULL);
+    assert(config->_check_hash_pycs_mode != NULL);
 
     return _Py_INIT_OK();
 }