perf: use pre-allocated constant for null sentinel in collection serialization

Replace per-call int32_pack(-1) with a module-level _INT32_NULL constant
in serialize methods of ListType, SetType, MapType, TupleType, and
UserType. This avoids a struct.pack call on every null element during
collection serialization.

Affected sites:
  - _SimpleParameterizedType.serialize_safe (ListType/SetType)
  - MapType.serialize_safe (null key + null value)
  - TupleType.serialize_safe
  - UserType.serialize_safe

Author: mykaul

cassandra/cqltypes.py

@@ -52,6 +52,7 @@
 from cassandra import util
 
 _little_endian_flag = 1  # we always serialize LE
+_INT32_NULL = int32_pack(-1)  # pre-allocated null sentinel for collection serialization
 import ipaddress
 
 apache_cassandra_type_prefix = 'org.apache.cassandra.db.marshal.'
@@ -841,7 +842,7 @@ def serialize_safe(cls, items, protocol_version):
         inner_proto = max(3, protocol_version)
         for item in items:
             if item is None:
-                buf.write(int32_pack(-1))
+                buf.write(_INT32_NULL)
             else:
                 itembytes = subtype.to_binary(item, inner_proto)
                 buf.write(int32_pack(len(itembytes)))
@@ -912,13 +913,13 @@ def serialize_safe(cls, themap, protocol_version):
                 buf.write(int32_pack(len(keybytes)))
                 buf.write(keybytes)
             else:
-                buf.write(int32_pack(-1))
+                buf.write(_INT32_NULL)
             if val is not None:
                 valbytes = value_type.to_binary(val, inner_proto)
                 buf.write(int32_pack(len(valbytes)))
                 buf.write(valbytes)
             else:
-                buf.write(int32_pack(-1))
+                buf.write(_INT32_NULL)
         return buf.getvalue()
 
 
@@ -964,7 +965,7 @@ def serialize_safe(cls, val, protocol_version):
                 buf.write(int32_pack(len(packed_item)))
                 buf.write(packed_item)
             else:
-                buf.write(int32_pack(-1))
+                buf.write(_INT32_NULL)
         return buf.getvalue()
 
     @classmethod
@@ -1041,7 +1042,7 @@ def serialize_safe(cls, val, protocol_version):
                 buf.write(int32_pack(len(packed_item)))
                 buf.write(packed_item)
             else:
-                buf.write(int32_pack(-1))
+                buf.write(_INT32_NULL)
         return buf.getvalue()
 
     @classmethod

tests/unit/test_types.py

@@ -26,7 +26,7 @@
     EmptyValue, LongType, SetType, UTF8Type,
     cql_typename, int8_pack, int64_pack, int64_unpack, lookup_casstype,
     lookup_casstype_simple, parse_casstype_args,
-    int32_pack, Int32Type, ListType, MapType, VectorType,
+    int32_pack, Int32Type, ListType, MapType, TupleType, UserType, VectorType,
     FloatType
 )
 from cassandra.encoder import cql_quote
@@ -1117,3 +1117,54 @@ def test_token_order(self):
         tokens_equal = [Token(1), Token(1)]
         check_sequence_consistency(tokens)
         check_sequence_consistency(tokens_equal, equal=True)
+
+
+class CollectionNullSentinelTests(unittest.TestCase):
+    """
+    Tests that collection types correctly round-trip None/null elements
+    using the pre-allocated _INT32_NULL sentinel.
+    """
+
+    def test_list_with_none_roundtrip(self):
+        proto = 4
+        parameterized = ListType.apply_parameters([Int32Type])
+        original = [1, None, 3]
+        serialized = parameterized.serialize(original, proto)
+        deserialized = parameterized.deserialize(serialized, proto)
+        self.assertEqual(deserialized, original)
+
+    def test_set_with_none_roundtrip(self):
+        proto = 4
+        parameterized = SetType.apply_parameters([Int32Type])
+        original = [1, None, 3]
+        serialized = parameterized.serialize(original, proto)
+        deserialized = parameterized.deserialize(serialized, proto)
+        self.assertEqual(set(deserialized), set(original))
+
+    def test_map_with_none_values_roundtrip(self):
+        proto = 4
+        parameterized = MapType.apply_parameters([Int32Type, Int32Type])
+        original = {1: None, 2: 10}
+        serialized = parameterized.serialize(original, proto)
+        deserialized = parameterized.deserialize(serialized, proto)
+        self.assertEqual(dict(deserialized.items()), original)
+
+    def test_tuple_with_none_roundtrip(self):
+        proto = 4
+        parameterized = TupleType.apply_parameters([Int32Type, UTF8Type, Int32Type])
+        original = (1, None, 3)
+        serialized = parameterized.serialize(original, proto)
+        deserialized = parameterized.deserialize(serialized, proto)
+        self.assertEqual(deserialized, original)
+
+    def test_usertype_with_none_roundtrip(self):
+        proto = 4
+        udt_class = UserType.make_udt_class(
+            'test_ks', 'test_udt',
+            ('field_a', 'field_b', 'field_c'),
+            (Int32Type, UTF8Type, Int32Type)
+        )
+        original = (1, None, 3)
+        serialized = udt_class.serialize(original, proto)
+        deserialized = udt_class.deserialize(serialized, proto)
+        self.assertEqual(deserialized, original)