Treat bytes objects as immutable in C code

[vstinner]

Currently, in the CPython C code, multiple functions treat bytes objects as mutable. For example, they use char* to access their data, instead of const char*.

In the worst case, it can lead to bad bugs like issue gh-155702 in sqlite3 which modifies an immutable bytes object.

Some functions can be rewritten using PyBytesWriter C API to avoid that.

Linked PRs

• gh-155742: Use PyBytesWriter in io _textiowrapper_writeflush() #155743
• gh-155742: Use PyBytesWriter in json _match_number_unicode() #155744
• gh-155742: Use PyBytesWriter in _zstd.train_dict() #155745
• gh-155742: Use PyBytesWriter in CJK codecs #155746
• gh-155742: Use PyBytesWriter in Python/assemble.c #155747
• gh-155742: Use PyBytesWriter in marshal #155748
• gh-155742: Use PyBytesWriter in xml.etree #155749
• gh-155742: Use PyBytesWriter in codecs #155750
• gh-155742: Get PyBytes_AS_STRING() as const char* #155751
• gh-155742: Test PyBytesWriter_FinishWithSize() with negative size #155783
• [3.15] gh-155742: Test PyBytesWriter_FinishWithSize() with negative size (GH-155783) #155791
• gh-155742: Get singleton in PyBytesWriter_FinishWithSize() #155795
• gh-155742: Check singletons consistencty in the garbage collector #155801

[vstinner]

I modified Python locally with following patch to replace char* with const char* in functions PyBytes_AS_STRING(), PyBytes_AsString() and PyBytes_AsStringAndSize(). So I can detect when a bytes object is treated as mutable.

diff --git a/Include/bytesobject.h b/Include/bytesobject.h
index c5a24195be6..4abd89469ad 100644
--- a/Include/bytesobject.h
+++ b/Include/bytesobject.h
@@ -36,7 +36,7 @@ PyAPI_FUNC(PyObject *) PyBytes_FromFormatV(const char*, va_list)
 PyAPI_FUNC(PyObject *) PyBytes_FromFormat(const char*, ...)
                                 Py_GCC_ATTRIBUTE((format(printf, 1, 2)));
 PyAPI_FUNC(Py_ssize_t) PyBytes_Size(PyObject *);
-PyAPI_FUNC(char *) PyBytes_AsString(PyObject *);
+PyAPI_FUNC(const char *) PyBytes_AsString(PyObject *);
 PyAPI_FUNC(PyObject *) PyBytes_Repr(PyObject *, int);
 PyAPI_FUNC(void) PyBytes_Concat(PyObject **, PyObject *);
 PyAPI_FUNC(void) PyBytes_ConcatAndDel(PyObject **, PyObject *);
@@ -50,7 +50,7 @@ PyAPI_FUNC(PyObject *) PyBytes_DecodeEscape(const char *, Py_ssize_t,
    cause an exception).  */
 PyAPI_FUNC(int) PyBytes_AsStringAndSize(
     PyObject *obj,      /* bytes object */
-    char **s,           /* pointer to buffer variable */
+    const char **s,           /* pointer to buffer variable */
     Py_ssize_t *len     /* pointer to length variable or NULL */
     );

diff --git a/Include/cpython/bytesobject.h b/Include/cpython/bytesobject.h
index 85bc2b827df..d7bdebca31e 100644
--- a/Include/cpython/bytesobject.h
+++ b/Include/cpython/bytesobject.h
@@ -20,7 +20,7 @@ PyAPI_FUNC(int) _PyBytes_Resize(PyObject **, Py_ssize_t);
 #define _PyBytes_CAST(op) \
     (assert(PyBytes_Check(op)), _Py_CAST(PyBytesObject*, op))

-static inline char* PyBytes_AS_STRING(PyObject *op)
+static inline const char* PyBytes_AS_STRING(PyObject *op)
 {
     return _PyBytes_CAST(op)->ob_sval;
 }
diff --git a/Objects/bytesobject.c b/Objects/bytesobject.c
index ef35dad82e8..6bd53958ac1 100644
--- a/Objects/bytesobject.c
+++ b/Objects/bytesobject.c
@@ -1338,7 +1338,7 @@ PyBytes_Size(PyObject *op)
     return Py_SIZE(op);
 }

-char *
+const char *
 PyBytes_AsString(PyObject *op)
 {
     if (!PyBytes_Check(op)) {
@@ -1351,7 +1351,7 @@ PyBytes_AsString(PyObject *op)

 int
 PyBytes_AsStringAndSize(PyObject *obj,
-                         char **s,
+                         const char **s,
                          Py_ssize_t *len)
 {
     if (s == NULL) {