gh-156812: Reject a trailing '-' when serializing an xml.dom.minidom comment

Author: tonghuaroot

Lib/test/test_minidom.py

@@ -1721,6 +1721,12 @@ def testSerializeCommentNodeWithDoubleHyphen(self):
         doc.appendChild(doc.createComment("foo--bar"))
         self.assertRaises(ValueError, doc.toxml)
 
+    def testSerializeCommentNodeWithTrailingHyphen(self):
+        # A trailing '-' would fuse with the closing '-->' into '--->'.
+        doc = create_doc_without_doctype()
+        doc.appendChild(doc.createComment("foo-"))
+        self.assertRaises(ValueError, doc.toxml)
+
 
     def testEmptyXMLNSValue(self):
         doc = parseString("<element xmlns=''>\n"

Lib/xml/dom/minidom.py

@@ -1253,8 +1253,10 @@ def __init__(self, data):
         self._data = data
 
     def writexml(self, writer, indent="", addindent="", newl=""):
-        if "--" in self.data:
-            raise ValueError("'--' is not allowed in a comment node")
+        if "--" in self.data or self.data.endswith("-"):
+            raise ValueError(
+                "'--' is not allowed in a comment node, and a comment "
+                "node cannot end with '-'")
         writer.write("%s<!--%s-->%s" % (indent, self.data, newl))
 
 

Misc/NEWS.d/next/Library/2026-09-02-17-35-20.gh-issue-156812.mdCmt1.rst

@@ -0,0 +1,3 @@
+Serializing an :mod:`xml.dom.minidom` comment node whose data ends with ``-``
+now raises :exc:`ValueError` instead of producing non-well-formed XML,
+completing the existing check for ``--``.