bpo-31455: Fix an assertion failure in ElementTree.XMLParser(). (#3545)

* Avoid calling "PyObject_GetAttrString()" (and potentially executing user code) with a live exception set.

* Ignore only AttributeError on attribute lookups in ElementTree.XMLParser() and propagate all other exceptions.

Lib/test/test_xml_etree.py

@@ -2498,6 +2498,31 @@ class TreeBuilderTest(unittest.TestCase):
             ('html', '-//W3C//DTD XHTML 1.0 Transitional//EN',
              'http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd'))
 
+    def test_builder_lookup_errors(self):
+        class RaisingBuilder:
+            def __init__(self, raise_in=None, what=ValueError):
+                self.raise_in = raise_in
+                self.what = what
+
+            def __getattr__(self, name):
+                if name == self.raise_in:
+                    raise self.what(self.raise_in)
+                def handle(*args):
+                    pass
+                return handle
+
+        ET.XMLParser(target=RaisingBuilder())
+        # cET also checks for 'close' and 'doctype', PyET does it only at need
+        for event in ('start', 'data', 'end', 'comment', 'pi'):
+            with self.assertRaisesRegex(ValueError, event):
+                ET.XMLParser(target=RaisingBuilder(event))
+
+        ET.XMLParser(target=RaisingBuilder(what=AttributeError))
+        for event in ('start', 'data', 'end', 'comment', 'pi'):
+            parser = ET.XMLParser(target=RaisingBuilder(event, what=AttributeError))
+            parser.feed(self.sample1)
+            self.assertIsNone(parser.close())
+
 
 class XMLParserTest(unittest.TestCase):
     sample1 = b'<file><line>22</line></file>'

Misc/NEWS.d/next/Library/2017-09-13-19-55-35.bpo-31544.beTh6t.rst

+The C accelerator module of ElementTree ignored exceptions raised when
+looking up TreeBuilder target methods in XMLParser().

Modules/_elementtree.c

@@ -3260,6 +3260,18 @@ xmlparser_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
     return (PyObject *)self;
 }
 
+static int
+ignore_attribute_error(PyObject *value)
+{
+    if (value == NULL) {
+        if (!PyErr_ExceptionMatches(PyExc_AttributeError)) {
+            return -1;
+        }
+        PyErr_Clear();
+    }
+    return 0;
+}
+
 /*[clinic input]
 _elementtree.XMLParser.__init__
 
@@ -3314,14 +3326,33 @@ _elementtree_XMLParser___init___impl(XMLParserObject *self, PyObject *html,
     self->target = target;
 
     self->handle_start = PyObject_GetAttrString(target, "start");
+    if (ignore_attribute_error(self->handle_start)) {
+        return -1;
+    }
     self->handle_data = PyObject_GetAttrString(target, "data");
+    if (ignore_attribute_error(self->handle_data)) {
+        return -1;
+    }
     self->handle_end = PyObject_GetAttrString(target, "end");
+    if (ignore_attribute_error(self->handle_end)) {
+        return -1;
+    }
     self->handle_comment = PyObject_GetAttrString(target, "comment");
+    if (ignore_attribute_error(self->handle_comment)) {
+        return -1;
+    }
     self->handle_pi = PyObject_GetAttrString(target, "pi");
+    if (ignore_attribute_error(self->handle_pi)) {
+        return -1;
+    }
     self->handle_close = PyObject_GetAttrString(target, "close");
+    if (ignore_attribute_error(self->handle_close)) {
+        return -1;
+    }
     self->handle_doctype = PyObject_GetAttrString(target, "doctype");
-
-    PyErr_Clear();
+    if (ignore_attribute_error(self->handle_doctype)) {
+        return -1;
+    }
 
     /* configure parser */
     EXPAT(SetUserData)(self->parser, self);