[3.1.x] Fixed #31728 -- Fixed cache culling when no key is found for deletion.

DatabaseCache._cull implementation could fail if no key was found to
perform a deletion in the table. This prevented the new cache key/value
from being correctly added.

Backport of f386454d13 from master

django/core/cache/backends/db.py

@@ -267,9 +267,12 @@ class DatabaseCache(BaseDatabaseCache):
                 cursor.execute(
                     connection.ops.cache_key_culling_sql() % table,
                     [cull_num])
-                cursor.execute("DELETE FROM %s "
-                               "WHERE cache_key < %%s" % table,
-                               [cursor.fetchone()[0]])
+                last_cache_key = cursor.fetchone()
+                if last_cache_key:
+                    cursor.execute(
+                        'DELETE FROM %s WHERE cache_key < %%s' % table,
+                        [last_cache_key[0]],
+                    )
 
     def clear(self):
         db = router.db_for_write(self.cache_model_class)

tests/cache/tests.py

@@ -621,6 +621,20 @@ class BaseCacheTests:
     def test_zero_cull(self):
         self._perform_cull_test('zero_cull', 50, 19)
 
+    def test_cull_delete_when_store_empty(self):
+        try:
+            cull_cache = caches['cull']
+        except InvalidCacheBackendError:
+            self.skipTest("Culling isn't implemented.")
+        old_max_entries = cull_cache._max_entries
+        # Force _cull to delete on first cached record.
+        cull_cache._max_entries = -1
+        try:
+            cull_cache.set('force_cull_delete', 'value', 1000)
+            self.assertIs(cull_cache.has_key('force_cull_delete'), True)
+        finally:
+            cull_cache._max_entries = old_max_entries
+
     def _perform_invalid_key_test(self, key, expected_warning):
         """
         All the builtin backends should warn (except memcached that should