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.

django/db/models/sql/query.py

@@ -6,6 +6,8 @@ themselves do not have to (and could be backed by things other than SQL
 databases). The abstraction barrier only works one way: this module has to know
 all about the internals of models in order to get the information it needs.
 """
+from string import ascii_uppercase
+from itertools import count, product
 
 from collections import Mapping, OrderedDict
 import copy
@@ -815,13 +817,37 @@ class Query(object):
         conflict. Even tables that previously had no alias will get an alias
         after this call.
         """
+        def prefix_gen():
+            """
+            Generates a sequence of characters in alphabetical order:
+                -> 'A', 'B', 'C', ...
+
+            When the alphabet is finished, the sequence will continue with the
+            Cartesian product:
+                -> 'AA', 'AB', 'AC', ...
+            """
+            alphabet = ascii_uppercase
+            prefix = chr(ord(self.alias_prefix) + 1)
+            yield prefix
+            for n in count(1):
+                seq = alphabet[alphabet.index(prefix):] if prefix else alphabet
+                for s in product(seq, repeat=n):
+                    yield ''.join(s)
+                prefix = None
+
         if self.alias_prefix != outer_query.alias_prefix:
             # No clashes between self and outer query should be possible.
             return
-        self.alias_prefix = chr(ord(self.alias_prefix) + 1)
-        while self.alias_prefix in self.subq_aliases:
-            self.alias_prefix = chr(ord(self.alias_prefix) + 1)
-            assert self.alias_prefix < 'Z'
+
+        local_recursion_limit = 127  # explicitly avoid infinite loop
+        for pos, prefix in enumerate(prefix_gen()):
+            if prefix not in self.subq_aliases:
+                self.alias_prefix = prefix
+                break
+            if pos > local_recursion_limit:
+                raise RuntimeError(
+                    'Maximum recursion depth exceeded: too many subqueries.'
+                )
         self.subq_aliases = self.subq_aliases.union([self.alias_prefix])
         outer_query.subq_aliases = outer_query.subq_aliases.union(self.subq_aliases)
         change_map = OrderedDict()

docs/releases/1.7.2.txt

@@ -183,3 +183,6 @@ Bugfixes
   convention in the template engine (:ticket:`23831`).
 
 * Prevented extraneous ``DROP DEFAULT`` SQL in migrations (:ticket:`23581`).
+
+* Restored the ability to use more than five levels of subqueries
+  (:ticket:`23758`).

tests/queries/tests.py

@@ -383,6 +383,25 @@ 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 xrange(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.