bpo-27946: Fix possible crash in ElementTree.Element (GH-29915)

Getting an attribute via attrib.get() simultaneously with replacing
the attrib dict can lead to access to deallocated dict.
(cherry picked from commit d15cdb2f32f572ce56d7120135da24b9fdce4c99)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/Lib/test/test_xml_etree_c.py b/Lib/test/test_xml_etree_c.py
index e68613b..bec8208 100644
--- a/Lib/test/test_xml_etree_c.py
+++ b/Lib/test/test_xml_etree_c.py
@@ -169,6 +169,18 @@ class MiscTests(unittest.TestCase):
         del parser
         support.gc_collect()
 
+    def test_dict_disappearing_during_get_item(self):
+        # test fix for seg fault reported in issue 27946
+        class X:
+            def __hash__(self):
+                e.attrib = {} # this frees e->extra->attrib
+                [{i: i} for i in range(1000)] # exhaust the dict keys cache
+                return 13
+
+        e = cET.Element("elem", {1: 2})
+        r = e.get(X())
+        self.assertIsNone(r)
+
 
 @unittest.skipUnless(cET, 'requires _elementtree')
 class TestAliasWorking(unittest.TestCase):