FinishWithSize() sets recheck_maxchar

Author: vstinner

Include/internal/pycore_unicodeobject.h

@@ -132,6 +132,8 @@ _PyUnicodeWriter_CanWrite(_PyUnicodeWriter *writer)
 static inline void
 _PyUnicodeWriter_SetBuffer(_PyUnicodeWriter *writer, PyObject *buffer)
 {
+    assert(writer->pos <= PyUnicode_GET_LENGTH(buffer));
+
     // Py_DECREF() the previous buffer (if any)
     Py_XSETREF(writer->buffer, buffer);
     writer->data = PyUnicode_DATA(buffer);

Objects/unicode_writer.c

@@ -643,7 +643,12 @@ _PyUnicodeWriter_FinishWithSize(_PyUnicodeWriter *writer, Py_ssize_t size)
 {
     assert(0 <= size);
     if (writer->buffer != NULL) {
+        assert(size <= writer->pos);
         assert(size <= PyUnicode_GET_LENGTH(writer->buffer));
+        if (size < writer->pos) {
+            // Truncate the string: we may need to adjust the string kind
+            writer->recheck_maxchar = 1;
+        }
     }
     else {
         assert(size == 0);