gh-128302: Fix bugs in xml.dom.xmlbuilder (GH-128284)

* Allow DOMParser.parse() to correctly handle DOMInputSource instances
  that only have a systemId attribute set.
* Fix DOMEntityResolver.resolveEntity(), which was broken by the
  Python 3.0 transition.
* Add Lib/test/test_xml_dom_xmlbuilder.py with few tests.

1 files changed, 7 insertions, 5 deletions

diff --git a/Lib/xml/dom/xmlbuilder.py b/Lib/xml/dom/xmlbuilder.py
index 8a20026..a885262 100644
--- a/Lib/xml/dom/xmlbuilder.py
+++ b/Lib/xml/dom/xmlbuilder.py
@@ -189,7 +189,7 @@ class DOMBuilder:
         options.filter = self.filter
         options.errorHandler = self.errorHandler
         fp = input.byteStream
-        if fp is None and options.systemId:
+        if fp is None and input.systemId:
             import urllib.request
             fp = urllib.request.urlopen(input.systemId)
         return self._parse_bytestream(fp, options)
@@ -247,10 +247,12 @@ class DOMEntityResolver(object):
 
     def _guess_media_encoding(self, source):
         info = source.byteStream.info()
-        if "Content-Type" in info:
-            for param in info.getplist():
-                if param.startswith("charset="):
-                    return param.split("=", 1)[1].lower()
+        # import email.message
+        # assert isinstance(info, email.message.Message)
+        charset = info.get_param('charset')
+        if charset is not None:
+            return charset.lower()
+        return None
 
 
 class DOMInputSource(object):