gh-156939: Do no modify writer->size directly in UCS1 encoder

unicode_encode_ucs1() now calls PyBytesWriter_Grow() to update the
PyBytesWriter size.

Author: vstinner

Objects/unicodeobject.c

@@ -796,6 +796,8 @@ backslashreplace(PyBytesWriter *writer, char *str,
         }
         size += incr;
     }
+    /* subtract preallocated bytes */
+    size -= (collend - collstart);
 
     str = PyBytesWriter_GrowAndUpdatePointer(writer, size, str);
     if (str == NULL) {
@@ -871,6 +873,8 @@ xmlcharrefreplace(PyBytesWriter *writer, char *str,
         }
         size += incr;
     }
+    /* subtract preallocated bytes */
+    size -= (collend - collstart);
 
     str = PyBytesWriter_GrowAndUpdatePointer(writer, size, str);
     if (str == NULL) {
@@ -7262,8 +7266,6 @@ unicode_encode_ucs1(PyObject *unicode,
                 break;
 
             case _Py_ERROR_BACKSLASHREPLACE:
-                /* subtract preallocated bytes */
-                writer->size -= (collend - collstart);
                 str = backslashreplace(writer, str,
                                        unicode, collstart, collend);
                 if (str == NULL)
@@ -7272,8 +7274,6 @@ unicode_encode_ucs1(PyObject *unicode,
                 break;
 
             case _Py_ERROR_XMLCHARREFREPLACE:
-                /* subtract preallocated bytes */
-                writer->size -= (collend - collstart);
                 str = xmlcharrefreplace(writer, str,
                                         unicode, collstart, collend);
                 if (str == NULL)
@@ -7314,10 +7314,13 @@ unicode_encode_ucs1(PyObject *unicode,
                     }
                 }
                 else {
-                    /* subtract preallocated bytes */
-                    writer->size -= newpos - collstart;
                     /* Only overallocate the buffer if it's not the last write */
                     writer->overallocate = (newpos < size);
+
+                    /* subtract preallocated bytes */
+                    if (PyBytesWriter_Grow(writer, -(newpos - collstart)) < 0) {
+                        goto onError;
+                    }
                 }
 
                 const char *rep_str;