Address CoPilot review: bound caches, add clear API, fix benchmark imports

- Add 256-entry size cap to both _deserializer_cache and
  _make_deserializers_cache to prevent unbounded growth from
  non-interned parameterized types in unprepared queries.
- Add clear_deserializer_caches() public API so that runtime Des*
  class overrides (e.g. DesBytesType = DesBytesTypeByteArray for
  cqlsh) can flush stale cached instances.
- Add get_deserializer_cache_sizes() diagnostic helper.
- Document override/cache interaction in code comments.
- Fix benchmark copyright (DataStax -> ScyllaDB), add
  pytest.importorskip guards for pytest-benchmark and Cython.
- Add 11 unit tests for cache hit/miss, clear, eviction bounds,
  and size reporting.

Author: mykaul

benchmarks/test_deserializer_cache_benchmark.py

@@ -1,4 +1,4 @@
-# Copyright DataStax, Inc.
+# Copyright ScyllaDB, Inc.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -16,10 +16,16 @@
 Benchmarks for find_deserializer / make_deserializers with and without caching.
 
 Run with: pytest benchmarks/test_deserializer_cache_benchmark.py -v
+
+Requires the ``pytest-benchmark`` plugin and Cython extensions to be built.
+Skipped automatically when either dependency is unavailable.
 """
 
 import pytest
 
+pytest.importorskip("pytest_benchmark")
+pytest.importorskip("cassandra.deserializers")
+
 from cassandra import cqltypes
 from cassandra.deserializers import (
     find_deserializer,

cassandra/deserializers.pyx

@@ -440,6 +440,14 @@ cdef class GenericDeserializer(Deserializer):
 #--------------------------------------------------------------------------
 # Helper utilities
 
+# Maximum number of entries in each deserializer cache.  In practice the
+# caches are bounded by the number of distinct column-type signatures in
+# the schema (typically dozens to low hundreds), but parameterized types
+# created via apply_parameters() for unprepared queries are *not*
+# interned, so repeated simple queries could accumulate entries.  The cap
+# prevents unbounded growth in such edge cases.
+cdef int _CACHE_MAX_SIZE = 256
+
 # Cache make_deserializers results keyed on the tuple of cqltype objects.
 # Using the cqltype objects themselves (rather than id()) as keys ensures
 # the dict holds strong references, preventing GC and id() reuse issues
@@ -454,6 +462,8 @@ def make_deserializers(cqltypes):
     except KeyError:
         pass
     result = obj_array([find_deserializer(ct) for ct in cqltypes])
+    if len(_make_deserializers_cache) >= _CACHE_MAX_SIZE:
+        _make_deserializers_cache.clear()
     _make_deserializers_cache[key] = result
     return result
 
@@ -464,6 +474,11 @@ cdef dict classes = globals()
 # repeated class lookups and object creation on every result set.
 # Using the object as key (rather than id()) holds a strong reference,
 # preventing GC and id() reuse issues with parameterized types.
+#
+# Note: if a Des* class is overridden at runtime (e.g. DesBytesType =
+# DesBytesTypeByteArray for cqlsh), callers must invoke
+# clear_deserializer_caches() to flush stale entries so that subsequent
+# find_deserializer() calls pick up the new class.
 cdef dict _deserializer_cache = {}
 
 cpdef Deserializer find_deserializer(cqltype):
@@ -501,10 +516,29 @@ cpdef Deserializer find_deserializer(cqltype):
         cls = GenericDeserializer
 
     cdef Deserializer result = cls(cqltype)
+    if len(_deserializer_cache) >= _CACHE_MAX_SIZE:
+        _deserializer_cache.clear()
     _deserializer_cache[cqltype] = result
     return result
 
 
+def clear_deserializer_caches():
+    """Clear the find_deserializer and make_deserializers caches.
+
+    Call this after overriding a Des* class at runtime (e.g.
+    ``deserializers.DesBytesType = deserializers.DesBytesTypeByteArray``)
+    so that subsequent lookups pick up the new class instead of returning
+    stale cached instances.
+    """
+    _deserializer_cache.clear()
+    _make_deserializers_cache.clear()
+
+
+def get_deserializer_cache_sizes():
+    """Return ``(find_cache_size, make_cache_size)`` for diagnostic use."""
+    return len(_deserializer_cache), len(_make_deserializers_cache)
+
+
 def obj_array(list objs):
     """Create a (Cython) array of objects given a list of objects"""
     cdef object[:] arr

tests/unit/cython/test_deserializer_cache.py

@@ -0,0 +1,190 @@
+# Copyright ScyllaDB, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""
+Unit tests for the deserializer caches in deserializers.pyx.
+
+Validates cache hit/miss behaviour, bounded eviction, the
+clear_deserializer_caches() API (needed after runtime Des* overrides),
+and the get_deserializer_cache_sizes() diagnostic helper.
+"""
+
+import unittest
+
+from tests.unit.cython.utils import cythontest
+
+try:
+    from cassandra.deserializers import (
+        clear_deserializer_caches,
+        find_deserializer,
+        get_deserializer_cache_sizes,
+        make_deserializers,
+    )
+
+    _HAS_DESERIALIZERS = True
+except ImportError:
+    _HAS_DESERIALIZERS = False
+
+from cassandra import cqltypes
+
+
+# ---------------------------------------------------------------------------
+# Tests
+# ---------------------------------------------------------------------------
+
+
+class DeserializerCacheTest(unittest.TestCase):
+    """Tests for find_deserializer / make_deserializers caching."""
+
+    def setUp(self):
+        if _HAS_DESERIALIZERS:
+            clear_deserializer_caches()
+
+    def tearDown(self):
+        if _HAS_DESERIALIZERS:
+            clear_deserializer_caches()
+
+    # -- find_deserializer cache -------------------------------------------
+
+    @cythontest
+    def test_find_cache_hit_same_object(self):
+        """Repeated calls for the same cqltype return the same instance."""
+        d1 = find_deserializer(cqltypes.Int32Type)
+        d2 = find_deserializer(cqltypes.Int32Type)
+        self.assertIs(d1, d2)
+
+    @cythontest
+    def test_find_cache_miss_different_types(self):
+        """Different cqltypes produce different deserializer instances."""
+        d_int = find_deserializer(cqltypes.Int32Type)
+        d_utf = find_deserializer(cqltypes.UTF8Type)
+        self.assertIsNot(d_int, d_utf)
+
+    @cythontest
+    def test_find_returns_correct_deserializer_class(self):
+        """The returned deserializer class name matches the cqltype."""
+        d = find_deserializer(cqltypes.Int32Type)
+        self.assertEqual(type(d).__name__, "DesInt32Type")
+
+    # -- make_deserializers cache ------------------------------------------
+
+    @cythontest
+    def test_make_cache_hit_same_object(self):
+        """Repeated calls with the same type list return the same array."""
+        types = [cqltypes.Int32Type, cqltypes.UTF8Type]
+        r1 = make_deserializers(types)
+        r2 = make_deserializers(types)
+        self.assertIs(r1, r2)
+
+    @cythontest
+    def test_make_cache_correct_length(self):
+        """Returned array has the right number of entries."""
+        types = [cqltypes.Int32Type, cqltypes.UTF8Type, cqltypes.BooleanType]
+        result = make_deserializers(types)
+        self.assertEqual(len(result), 3)
+
+    # -- clear_deserializer_caches -----------------------------------------
+
+    @cythontest
+    def test_clear_invalidates_find_cache(self):
+        """After clearing, find_deserializer returns a new instance."""
+        d1 = find_deserializer(cqltypes.Int32Type)
+        clear_deserializer_caches()
+        d2 = find_deserializer(cqltypes.Int32Type)
+        # New instance, but same deserializer class
+        self.assertIsNot(d1, d2)
+        self.assertEqual(type(d1).__name__, type(d2).__name__)
+
+    @cythontest
+    def test_clear_invalidates_make_cache(self):
+        """After clearing, make_deserializers returns a new array."""
+        types = [cqltypes.Int32Type, cqltypes.UTF8Type]
+        r1 = make_deserializers(types)
+        clear_deserializer_caches()
+        r2 = make_deserializers(types)
+        self.assertIsNot(r1, r2)
+
+    # -- get_deserializer_cache_sizes --------------------------------------
+
+    @cythontest
+    def test_cache_sizes_empty_after_clear(self):
+        """Sizes are (0, 0) immediately after clearing."""
+        find_size, make_size = get_deserializer_cache_sizes()
+        self.assertEqual(find_size, 0)
+        self.assertEqual(make_size, 0)
+
+    @cythontest
+    def test_cache_sizes_increment(self):
+        """Sizes reflect the number of cached entries."""
+        find_deserializer(cqltypes.Int32Type)
+        find_deserializer(cqltypes.UTF8Type)
+        make_deserializers([cqltypes.Int32Type, cqltypes.UTF8Type])
+
+        find_size, make_size = get_deserializer_cache_sizes()
+        self.assertEqual(find_size, 2)
+        self.assertEqual(make_size, 1)
+
+    # -- bounded eviction --------------------------------------------------
+
+    @cythontest
+    def test_find_cache_bounded_size(self):
+        """find_deserializer cache should not exceed 256 entries."""
+        # Create 300 distinct cqltype objects via apply_parameters.
+        # Each ListType.apply_parameters() call creates a fresh class.
+        inner_types = [
+            cqltypes.Int32Type,
+            cqltypes.UTF8Type,
+            cqltypes.BooleanType,
+            cqltypes.DoubleType,
+            cqltypes.LongType,
+        ]
+        distinct_types = []
+        for i in range(300):
+            # Create ListType(inner) — each apply_parameters returns a new
+            # class object, so these are all distinct cache keys.
+            inner = inner_types[i % len(inner_types)]
+            ct = cqltypes.ListType.apply_parameters([inner])
+            distinct_types.append(ct)
+
+        for ct in distinct_types:
+            find_deserializer(ct)
+
+        find_size, _ = get_deserializer_cache_sizes()
+        self.assertLessEqual(
+            find_size,
+            256,
+            "find_deserializer cache should be bounded to 256, got %d" % find_size,
+        )
+
+    @cythontest
+    def test_make_cache_bounded_size(self):
+        """make_deserializers cache should not exceed 256 entries."""
+        inner_types = [
+            cqltypes.Int32Type,
+            cqltypes.UTF8Type,
+            cqltypes.BooleanType,
+            cqltypes.DoubleType,
+            cqltypes.LongType,
+        ]
+        for i in range(300):
+            inner = inner_types[i % len(inner_types)]
+            ct = cqltypes.ListType.apply_parameters([inner])
+            make_deserializers([ct])
+
+        _, make_size = get_deserializer_cache_sizes()
+        self.assertLessEqual(
+            make_size,
+            256,
+            "make_deserializers cache should be bounded to 256, got %d" % make_size,
+        )