From f386454d1302b66d0eb331ed0ae9e4811e2f3a15 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Guillermo=20Bonveh=C3=AD?= <gbonvehi@gmail.com>
Date: Sat, 20 Jun 2020 04:33:24 -0300
Subject: [PATCH] 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.
---
 django/core/cache/backends/db.py |  9 ++++++---
 tests/cache/tests.py             | 14 ++++++++++++++
 2 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/django/core/cache/backends/db.py b/django/core/cache/backends/db.py
index 33c79c5a14..acbe702255 100644
--- a/django/core/cache/backends/db.py
+++ b/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)
diff --git a/tests/cache/tests.py b/tests/cache/tests.py
index d404475e16..dc4a221060 100644
--- a/tests/cache/tests.py
+++ b/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