Add type checking for the capsule

adopt_external_document() should type check its argument

src/lxml/includes/etree_defs.h

@@ -267,6 +267,12 @@ static void* lxml_unpack_xmldoc_capsule(PyObject* capsule, int* is_owned) {
     xmlDoc *c_doc;
     void *context;
     *is_owned = 0;
+    if (unlikely_condition(!PyCapsule_IsValid(capsule, (const char*)"libxml2:xmlDoc"))) {
+        PyErr_SetString(
+                PyExc_TypeError,
+                "Not a valid capsule. The capsule argument must be a capsule object with name libxml2:xmlDoc");
+        return NULL;
+    }
     c_doc = (xmlDoc*) PyCapsule_GetPointer(capsule, (const char*)"libxml2:xmlDoc");
     if (unlikely_condition(!c_doc)) return NULL;
 

src/lxml/tests/test_external_document.py

@@ -0,0 +1,26 @@
+# -*- coding: utf-8 -*-
+"""
+Test cases related to direct loading of external libxml2 documents
+"""
+
+import unittest
+import sys
+
+from common_imports import etree, HelperTestCase
+
+
+@unittest.skipIf(sys.version_info[:2] < (2, 7),
+                 'Not supported for python < 2.7')
+class ExternalDocumentTestCase(HelperTestCase):
+    def test_external_document_type_checking(self):
+        self.assertRaises(TypeError, etree.adopt_external_document, None)
+
+
+def test_suite():
+    suite = unittest.TestSuite()
+    suite.addTests([unittest.makeSuite(ExternalDocumentTestCase)])
+    return suite
+
+
+if __name__ == '__main__':
+    print('to test use test.py %s' % __file__)