gh-146196: Fix potential Undefined Behavior in _PyUnicodeWriter_WriteASCIIString

Author: ashm-dev

Misc/NEWS.d/next/Core_and_Builtins/2026-03-20-13-55-14.gh-issue-146196.Zg70Kb.rst

@@ -0,0 +1,3 @@
+Fix potential Undefined Behavior in :c:func:`PyUnicodeWriter_WriteASCII` by
+adding a zero-length check and using memmove() instead of memcpy(). Patch by
+Shamil Abdulaev.

Objects/unicode_writer.c

@@ -465,6 +465,10 @@ _PyUnicodeWriter_WriteASCIIString(_PyUnicodeWriter *writer,
     if (len == -1)
         len = strlen(ascii);
 
+    if (len == 0) {
+        return 0;
+    }
+
     assert(ucs1lib_find_max_char((const Py_UCS1*)ascii, (const Py_UCS1*)ascii + len) < 128);
 
     if (writer->buffer == NULL && !writer->overallocate) {