gh-103583: Always pass multibyte codec structs as const (#103588)

Author: erlend-aasland

Modules/cjkcodecs/multibytecodec.c

@@ -67,7 +67,7 @@ typedef struct {
 static char *incnewkwarglist[] = {"errors", NULL};
 static char *streamkwarglist[] = {"stream", "errors", NULL};
 
-static PyObject *multibytecodec_encode(MultibyteCodec *,
+static PyObject *multibytecodec_encode(const MultibyteCodec *,
                 MultibyteCodec_State *, PyObject *, Py_ssize_t *,
                 PyObject *, int);
 
@@ -221,7 +221,7 @@ expand_encodebuffer(MultibyteEncodeBuffer *buf, Py_ssize_t esize)
  */
 
 static int
-multibytecodec_encerror(MultibyteCodec *codec,
+multibytecodec_encerror(const MultibyteCodec *codec,
                         MultibyteCodec_State *state,
                         MultibyteEncodeBuffer *buf,
                         PyObject *errors, Py_ssize_t e)
@@ -375,7 +375,7 @@ multibytecodec_encerror(MultibyteCodec *codec,
 }
 
 static int
-multibytecodec_decerror(MultibyteCodec *codec,
+multibytecodec_decerror(const MultibyteCodec *codec,
                         MultibyteCodec_State *state,
                         MultibyteDecodeBuffer *buf,
                         PyObject *errors, Py_ssize_t e)
@@ -479,7 +479,7 @@ multibytecodec_decerror(MultibyteCodec *codec,
 }
 
 static PyObject *
-multibytecodec_encode(MultibyteCodec *codec,
+multibytecodec_encode(const MultibyteCodec *codec,
                       MultibyteCodec_State *state,
                       PyObject *text, Py_ssize_t *inpos_t,
                       PyObject *errors, int flags)
@@ -1953,7 +1953,7 @@ _multibytecodec___create_codec(PyObject *module, PyObject *arg)
 /*[clinic end generated code: output=cfa3dce8260e809d input=6840b2a6b183fcfa]*/
 {
     MultibyteCodecObject *self;
-    MultibyteCodec *codec;
+    const MultibyteCodec *codec;
 
     if (!PyCapsule_IsValid(arg, PyMultibyteCodec_CAPSULE_NAME)) {
         PyErr_SetString(PyExc_ValueError, "argument type invalid");

Modules/cjkcodecs/multibytecodec.h

@@ -62,14 +62,14 @@ typedef struct {
 
 typedef struct {
     PyObject_HEAD
-    MultibyteCodec *codec;
+    const MultibyteCodec *codec;
 } MultibyteCodecObject;
 
 #define MultibyteCodec_Check(state, op) Py_IS_TYPE((op), state->multibytecodec_type)
 
 #define _MultibyteStatefulCodec_HEAD            \
     PyObject_HEAD                               \
-    MultibyteCodec *codec;                      \
+    const MultibyteCodec *codec;                \
     MultibyteCodec_State state;                 \
     PyObject *errors;
 typedef struct {