gh-129813, PEP 782: Use PyBytesWriter in socket and mmap (#138831)

Replace PyBytes_FromStringAndSize(NULL, size) and _PyBytes_Resize()
with the new public PyBytesWriter API.

Author: vstinner

Modules/mmapmodule.c

@@ -448,7 +448,8 @@ _safe_PyBytes_ReverseFind(Py_ssize_t *out, mmap_object *self,
 }
 
 PyObject *
-_safe_PyBytes_FromStringAndSize(char *start, size_t num_bytes) {
+_safe_PyBytes_FromStringAndSize(char *start, size_t num_bytes)
+{
     if (num_bytes == 1) {
         char dest;
         if (safe_byte_copy(&dest, start) < 0) {
@@ -459,14 +460,15 @@ _safe_PyBytes_FromStringAndSize(char *start, size_t num_bytes) {
         }
     }
     else {
-        PyObject *result = PyBytes_FromStringAndSize(NULL, num_bytes);
-        if (result == NULL) {
+        PyBytesWriter *writer = PyBytesWriter_Create(num_bytes);
+        if (writer == NULL) {
             return NULL;
         }
-        if (safe_memcpy(PyBytes_AS_STRING(result), start, num_bytes) < 0) {
-            Py_CLEAR(result);
+        if (safe_memcpy(PyBytesWriter_GetData(writer), start, num_bytes) < 0) {
+            PyBytesWriter_Discard(writer);
+            return NULL;
         }
-        return result;
+        return PyBytesWriter_Finish(writer);
     }
 }
 

Modules/socketmodule.c

@@ -3467,7 +3467,6 @@ sock_getsockopt(PyObject *self, PyObject *args)
     int level;
     int optname;
     int res;
-    PyObject *buf;
     socklen_t buflen = 0;
     int flag = 0;
     socklen_t flagsize;
@@ -3512,17 +3511,17 @@ sock_getsockopt(PyObject *self, PyObject *args)
                         "getsockopt buflen out of range");
         return NULL;
     }
-    buf = PyBytes_FromStringAndSize((char *)NULL, buflen);
-    if (buf == NULL)
+    PyBytesWriter *writer = PyBytesWriter_Create(buflen);
+    if (writer == NULL) {
         return NULL;
+    }
     res = getsockopt(get_sock_fd(s), level, optname,
-                     (void *)PyBytes_AS_STRING(buf), &buflen);
+                     PyBytesWriter_GetData(writer), &buflen);
     if (res < 0) {
-        Py_DECREF(buf);
+        PyBytesWriter_Discard(writer);
         return s->errorhandler();
     }
-    _PyBytes_Resize(&buf, buflen);
-    return buf;
+    return PyBytesWriter_FinishWithSize(writer, buflen);
 }
 
 PyDoc_STRVAR(getsockopt_doc,