Issue #23321: Fixed a crash in str.decode() when error handler returned

replacment string longer than mailformed input data.

2 files changed, 9 insertions, 2 deletions

diff --git a/Misc/NEWS b/Misc/NEWS
index 163aff4..3c08363 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -10,6 +10,9 @@ Release date: TBA
 Core and Builtins
 -----------------
 
+- Issue #23321: Fixed a crash in str.decode() when error handler returned
+  replacment string longer than mailformed input data.
+
 - Issue #22286: The "backslashreplace" error handlers now works with
   decoding and translating.
 
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index ee0c044..03f795c 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -4155,9 +4155,13 @@ unicode_decode_call_errorhandler_writer(
     if (PyUnicode_READY(repunicode) < 0)
         goto onError;
     replen = PyUnicode_GET_LENGTH(repunicode);
-    writer->min_length += replen;
-    if (replen > 1)
+    if (replen > 1) {
+        writer->min_length += replen - 1;
         writer->overallocate = 1;
+        if (_PyUnicodeWriter_Prepare(writer, writer->min_length,
+                            PyUnicode_MAX_CHAR_VALUE(repunicode)) == -1)
+            goto onError;
+    }
     if (_PyUnicodeWriter_WriteStr(writer, repunicode) == -1)
         goto onError;