[1.5.x] Fixed #19895 -- Made second iteration over invalid queryset raise an exception too

When iteration over a queryset raised an exception, the result cache
remained initialized with an empty list, so subsequent iterations returned
an empty list instead of raising an exception

Backport of 2cd0edaa477b327024e4007c8eaf46646dcd0f21 from master.

django/db/models/query.py

@@ -102,7 +102,7 @@ class QuerySet(object):
             len(self)
 
         if self._result_cache is None:
-            self._iter = self.iterator()
+            self._iter = self._safe_iterator(self.iterator())
             self._result_cache = []
         if self._iter:
             return self._result_iter()
@@ -337,6 +337,18 @@ class QuerySet(object):
 
             yield obj
 
+    def _safe_iterator(self, iterator):
+        # ensure result cache is cleared when iterating over a queryset
+        # raises an exception
+        try:
+            for item in iterator:
+                yield item
+        except StopIteration:
+            raise
+        except Exception:
+            self._result_cache = None
+            raise
+
     def aggregate(self, *args, **kwargs):
         """
         Returns a dictionary containing the calculations (aggregation)

tests/modeltests/basic/tests.py

@@ -2,7 +2,7 @@ from __future__ import absolute_import, unicode_literals
 
 from datetime import datetime
 
-from django.core.exceptions import ObjectDoesNotExist, MultipleObjectsReturned
+from django.core.exceptions import ObjectDoesNotExist, MultipleObjectsReturned, FieldError
 from django.db.models.fields import Field, FieldDoesNotExist
 from django.test import TestCase, skipIfDBFeature, skipUnlessDBFeature
 from django.utils import six
@@ -639,3 +639,8 @@ class ModelTest(TestCase):
         Article.objects.bulk_create([Article(headline=lazy, pub_date=datetime.now())])
         article = Article.objects.get()
         self.assertEqual(article.headline, notlazy)
+
+    def test_invalid_qs_list(self):
+        qs = Article.objects.order_by('invalid_column')
+        self.assertRaises(FieldError, list, qs)
+        self.assertRaises(FieldError, list, qs)