[1.6.x] Fixed #21146 - DatabaseCache converts expires to python value

DatabaseCache uses raw cursors to bypass the ORM. This prevents it from
being used by database backends that require special handling of datetime
values.

There is no easy way to test this, so no tests added.

Backport of d5606b5763 from master

django/core/cache/backends/db.py

@@ -11,6 +11,7 @@ except ImportError:
 from django.conf import settings
 from django.core.cache.backends.base import BaseCache, DEFAULT_TIMEOUT
 from django.db import connections, transaction, router, DatabaseError
+from django.db.backends.utils import typecast_timestamp
 from django.utils import timezone, six
 from django.utils.encoding import force_bytes
 
@@ -65,8 +66,13 @@ class DatabaseCache(BaseDatabaseCache):
         if row is None:
             return default
         now = timezone.now()
-
-        if row[2] < now:
+        expires = row[2]
+        if connections[db].features.needs_datetime_string_cast and not isinstance(expires, datetime):
+            # Note: typecasting is needed by some 3rd party database backends.
+            # All core backends work without typecasting, so be careful about
+            # changes here - test suite will NOT pick regressions here.
+            expires = typecast_timestamp(str(expires))
+        if expires < now:
             db = router.db_for_write(self.cache_model_class)
             cursor = connections[db].cursor()
             cursor.execute("DELETE FROM %s "
@@ -112,12 +118,21 @@ class DatabaseCache(BaseDatabaseCache):
         if six.PY3:
             b64encoded = b64encoded.decode('latin1')
         try:
+            # Note: typecasting for datetimes is needed by some 3rd party
+            # database backends. All core backends work without typecasting,
+            # so be careful about changes here - test suite will NOT pick
+            # regressions.
             with transaction.atomic(using=db):
                 cursor.execute("SELECT cache_key, expires FROM %s "
                                "WHERE cache_key = %%s" % table, [key])
                 result = cursor.fetchone()
+                if result:
+                    current_expires = result[1]
+                    if (connections[db].features.needs_datetime_string_cast and not
+                            isinstance(current_expires, datetime)):
+                        current_expires = typecast_timestamp(str(current_expires))
                 exp = connections[db].ops.value_to_db_datetime(exp)
-                if result and (mode == 'set' or (mode == 'add' and result[1] < now)):
+                if result and (mode == 'set' or (mode == 'add' and current_expires < now)):
                     cursor.execute("UPDATE %s SET value = %%s, expires = %%s "
                                    "WHERE cache_key = %%s" % table,
                                    [b64encoded, exp, key])