[1.7.x] Fixed #23758 -- Allowed more than 5 levels of subqueries

Refactored bump_prefix() to avoid infinite loop and allow more than
than 5 subquires by extending the alphabet to use multi-letters.

Backport of 41fc1c0b5e from master

tests/queries/tests.py

@@ -15,6 +15,7 @@ from django.db.models.sql.datastructures import EmptyResultSet
 from django.test import TestCase, skipUnlessDBFeature
 from django.test.utils import str_prefix, CaptureQueriesContext
 from django.utils import six
+from django.utils.six.moves import range
 
 from .models import (
     Annotation, Article, Author, Celebrity, Child, Cover, Detail, DumbCategory,
@@ -380,6 +381,25 @@ class Queries1Tests(BaseQuerysetTest):
             ['<Item: four>']
         )
 
+    def test_avoid_infinite_loop_on_too_many_subqueries(self):
+        x = Tag.objects.filter(pk=1)
+        local_recursion_limit = 127
+        msg = 'Maximum recursion depth exceeded: too many subqueries.'
+        with self.assertRaisesMessage(RuntimeError, msg):
+            for i in six.moves.range(local_recursion_limit * 2):
+                x = Tag.objects.filter(pk__in=x)
+
+    def test_reasonable_number_of_subq_aliases(self):
+        x = Tag.objects.filter(pk=1)
+        for _ in range(20):
+            x = Tag.objects.filter(pk__in=x)
+        self.assertEqual(
+            x.query.subq_aliases, {
+                'T', 'U', 'V', 'W', 'X', 'Y', 'Z', 'AA', 'AB', 'AC', 'AD',
+                'AE', 'AF', 'AG', 'AH', 'AI', 'AJ', 'AK', 'AL', 'AM', 'AN',
+            }
+        )
+
     def test_heterogeneous_qs_combination(self):
         # Combining querysets built on different models should behave in a well-defined
         # fashion. We raise an error.