[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

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 OrderedDict
 import copy
@@ -837,13 +839,37 @@ 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()