gh-123803: Support arbitrary code page encodings on Windows

If the cpXXX encoding is not directly implemented in Python, fall back
to use the Windows-specific API codecs.code_page_encode() and
codecs.code_page_decode().

Author: serhiy-storchaka

Lib/encodings/__init__.py

@@ -156,19 +156,45 @@ def search_function(encoding):
 codecs.register(search_function)
 
 if sys.platform == 'win32':
-    # bpo-671666, bpo-46668: If Python does not implement a codec for current
-    # Windows ANSI code page, use the "mbcs" codec instead:
-    # WideCharToMultiByte() and MultiByteToWideChar() functions with CP_ACP.
-    # Python does not support custom code pages.
-    def _alias_mbcs(encoding):
+    def _code_page_search_function(encoding):
+        encoding = encoding.lower()
+        if not encoding.startswith('cp'):
+            return None
         try:
-            import _winapi
-            ansi_code_page = "cp%s" % _winapi.GetACP()
-            if encoding == ansi_code_page:
-                import encodings.mbcs
-                return encodings.mbcs.getregentry()
-        except ImportError:
-            # Imports may fail while we are shutting down
-            pass
-
-    codecs.register(_alias_mbcs)
+            cp = int(encoding[2:])
+        except ValueError:
+            return None
+
+        def encode(input, errors='strict'):
+            return codecs.code_page_encode(cp, input, errors)
+
+        def decode(input, errors='strict'):
+            return codecs.code_page_decode(cp, input, errors, True)
+
+        class IncrementalEncoder(codecs.IncrementalEncoder):
+            def encode(self, input, final=False):
+                return codecs.code_page_encode(cp, input, self.errors)[0]
+
+        class IncrementalDecoder(codecs.BufferedIncrementalDecoder):
+            def _buffer_decode(self, input, errors, final):
+                return codecs.code_page_decode(cp, input, errors, final)
+
+        class StreamWriter(Codec, codecs.StreamWriter):
+            def encode(self, input, errors='strict'):
+                return codecs.code_page_encode(cp, input, errors)
+
+        class StreamReader(Codec, codecs.StreamReader):
+            def decode(self, input, errors, final):
+                return codecs.code_page_decode(cp, input, errors, final)
+
+        return codecs.CodecInfo(
+            name=f'cp{cp}',
+            encode=encode,
+            decode=decode,
+            incrementalencoder=IncrementalEncoder,
+            incrementaldecoder=IncrementalDecoder,
+            streamreader=StreamReader,
+            streamwriter=StreamWriter,
+        )
+
+    codecs.register(_code_page_search_function)