gh-155742: Use PyBytesWriter in _Py_strhex_impl() (#157734)

Replace soft deprecated PyBytes_FromStringAndSize() with
PyBytesWriter.

Replace also PyBytes_FromStringAndSize(NULL, 0) with
Py_GetConstant(Py_CONSTANT_EMPTY_BYTES) in other functions.

Replace also PyBytes_FromStringAndSize(NULL, 0) with
PyBytes_FromStringAndSize("", 0) in _Py_GetConstant_Init().

Author: vstinner

Modules/_io/bytesio.c

@@ -486,7 +486,7 @@ peek_bytes_lock_held(bytesio *self, Py_ssize_t size)
        is beyond the size of self->buf. Assert above validates size is always in
        bounds. When self->pos is out of bounds calling code sets size to 0. */
     if (size == 0) {
-        return PyBytes_FromStringAndSize(NULL, 0);
+        return Py_GetConstant(Py_CONSTANT_EMPTY_BYTES);
     }
 
     output = PyBytes_AS_STRING(self->buf) + self->pos;
@@ -1109,7 +1109,7 @@ bytesio_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
     /* tp_alloc initializes all the fields to zero. So we don't have to
        initialize them here. */
 
-    self->buf = PyBytes_FromStringAndSize(NULL, 0);
+    self->buf = Py_GetConstant(Py_CONSTANT_EMPTY_BYTES);
     if (self->buf == NULL) {
         Py_DECREF(self);
         return PyErr_NoMemory();

Modules/_io/winconsoleio.c

@@ -942,7 +942,7 @@ _io__WindowsConsoleIO_readall_impl(winconsoleio *self)
     if (len == 0 && _buflen(self) == 0) {
         /* when the result starts with ^Z we return an empty buffer */
         PyMem_Free(buf);
-        return PyBytes_FromStringAndSize(NULL, 0);
+        return Py_GetConstant(Py_CONSTANT_EMPTY_BYTES);
     }
 
     if (len) {

Modules/_sqlite/blob.c

@@ -447,7 +447,7 @@ subscript_slice(pysqlite_Blob *self, PyObject *item)
     }
 
     if (len == 0) {
-        return PyBytes_FromStringAndSize(NULL, 0);
+        return Py_GetConstant(Py_CONSTANT_EMPTY_BYTES);
     }
 
     if (step == 1) {

Objects/object.c

@@ -3464,7 +3464,7 @@ _Py_GetConstant_Init(void)
     constants[Py_CONSTANT_ZERO] = _PyLong_GetZero();
     constants[Py_CONSTANT_ONE] = _PyLong_GetOne();
     constants[Py_CONSTANT_EMPTY_STR] = PyUnicode_New(0, 0);
-    constants[Py_CONSTANT_EMPTY_BYTES] = PyBytes_FromStringAndSize(NULL, 0);
+    constants[Py_CONSTANT_EMPTY_BYTES] = PyBytes_FromStringAndSize("", 0);
     constants[Py_CONSTANT_EMPTY_TUPLE] = PyTuple_New(0);
 #ifndef NDEBUG
     for (size_t i=0; i < Py_ARRAY_LENGTH(constants); i++) {

Python/pystrhex.c

@@ -168,15 +168,16 @@ _Py_strhex_impl(const char* argbuf, Py_ssize_t arglen,
         abs_bytes_per_sep = 0;
     }
 
-    PyObject *retval;
+    PyObject *retval = NULL;
+    PyBytesWriter *bytes_writer = NULL;
     Py_UCS1 *retbuf;
     if (return_bytes) {
         /* If _PyBytes_FromSize() were public we could avoid malloc+copy. */
-        retval = PyBytes_FromStringAndSize(NULL, resultlen);
-        if (!retval) {
+        bytes_writer = PyBytesWriter_Create(resultlen);
+        if (!bytes_writer) {
             return NULL;
         }
-        retbuf = (Py_UCS1 *)PyBytes_AS_STRING(retval);
+        retbuf = PyBytesWriter_GetData(bytes_writer);
     }
     else {
         retval = PyUnicode_New(resultlen, 127);
@@ -244,13 +245,15 @@ _Py_strhex_impl(const char* argbuf, Py_ssize_t arglen,
         }
     }
 
+    if (return_bytes) {
+        return PyBytesWriter_Finish(bytes_writer);
+    }
+    else {
 #ifdef Py_DEBUG
-    if (!return_bytes) {
         assert(_PyUnicode_CheckConsistency(retval, 1));
-    }
 #endif
-
-    return retval;
+        return retval;
+    }
 }
 
 PyObject * _Py_strhex(const char* argbuf, Py_ssize_t arglen)