[1.6.x] Fixed #21643 -- repeated execution of qs with F() + timedelta

Thanks Tim Graham for review and Tai Lee for the additional test to prove
this was a regression in 1.6.

Backport of 7f2485b4d1 and 8137215973 from master

django/db/models/sql/expressions.py

@@ -108,6 +108,7 @@ class SQLEvaluator(object):
     def evaluate_date_modifier_node(self, node, qn, connection):
         timedelta = node.children.pop()
         sql, params = self.evaluate_node(node, qn, connection)
+        node.children.append(timedelta)
 
         if timedelta.days == 0 and timedelta.seconds == 0 and \
                 timedelta.microseconds == 0:

docs/releases/1.6.3.txt

@@ -14,3 +14,7 @@ several bugs in 1.6.2:
 * Fixed ``AttributeError`` when using
   :meth:`~django.db.models.query.QuerySet.bulk_create` with ``ForeignObject``
   (`#21566 <http://code.djangoproject.com/ticket/21566>`_).
+
+* Fixed crash of ``QuerySet``\s that use ``F() + timedelta()`` when their query
+  was compiled more once
+  (`#21643 <http://code.djangoproject.com/ticket/21643>`_).

tests/expressions_regress/tests.py

@@ -259,6 +259,20 @@ class FTimeDeltaTests(TestCase):
         self.days_long.append(e4.completed-e4.assigned)
         self.expnames = [e.name for e in Experiment.objects.all()]
 
+    def test_multiple_query_compilation(self):
+        # Ticket #21643
+        queryset = Experiment.objects.filter(end__lt=F('start') + datetime.timedelta(hours=1))
+        q1 = str(queryset.query)
+        q2 = str(queryset.query)
+        self.assertEqual(q1, q2)
+
+    def test_query_clone(self):
+        # Ticket #21643
+        qs = Experiment.objects.filter(end__lt=F('start') + datetime.timedelta(hours=1))
+        qs2 = qs.all()
+        list(qs)
+        list(qs2)
+
     def test_delta_add(self):
         for i in range(len(self.deltas)):
             delta = self.deltas[i]