Restore pickling of Pydantic data converters

CHANGELOG.md

@@ -29,6 +29,8 @@ to include examples, links to docs, or any other relevant information.
 
 ### Fixed
 
+- Restore pickling of Pydantic data converters, preserving the type adapter cache
+  size limit while excluding cached adapters.
 - Current workflow and activity payload converter accessors now return the configured converter
   without SDK-internal transfer type conversion.
 

temporalio/contrib/pydantic.py

@@ -51,6 +51,8 @@ class PydanticJSONPlainPayloadConverter(EncodingPayloadConverter):
     types from the datetime module, sets, UUID, etc, and custom types composed
     of any of these.
 
+    Pickling preserves the cache size limit but excludes cached type adapters.
+
     See https://docs.pydantic.dev/latest/api/standard_library_types/
     """
 
@@ -77,6 +79,19 @@ def __init__(
             TypeAdapter
         )
 
+    def __getstate__(self) -> dict[str, Any]:
+        """Pickle without the type adapter cache, whose wrapper is not picklable."""
+        state = self.__dict__.copy()
+        state["_type_adapter"] = self._type_adapter.cache_info().maxsize
+        return state
+
+    def __setstate__(self, state: dict[str, Any]) -> None:
+        """Restore state with an empty type adapter cache."""
+        self.__dict__.update(state)
+        self._type_adapter = functools.lru_cache(maxsize=state["_type_adapter"])(
+            TypeAdapter
+        )
+
     @property
     def encoding(self) -> str:
         """See base class."""

tests/contrib/pydantic/test_pydantic.py

@@ -2,13 +2,15 @@
 import datetime
 import os
 import pathlib
+import pickle
 import typing
 import uuid
 
 import pydantic
 import pytest
 from pydantic import BaseModel
 
+import temporalio.contrib.pydantic
 from temporalio.client import Client
 from temporalio.contrib.pydantic import (
     PydanticJSONPlainPayloadConverter,
@@ -55,6 +57,41 @@
 )
 
 
+@pytest.mark.parametrize("populate_cache", [False, True])
+def test_pydantic_json_converter_pickle(populate_cache: bool):
+    class Model(BaseModel):
+        value: int
+        default: int = 0
+
+    converter = PydanticJSONPlainPayloadConverter(
+        temporalio.contrib.pydantic.ToJsonOptions(exclude_unset=True),
+        max_cached_type_adapters=2,
+    )
+    value = Model(value=1)
+    payload = converter.to_payload(value)
+    assert payload is not None
+    if populate_cache:
+        assert converter.from_payload(payload, Model) == value
+    cache_info = converter._type_adapter.cache_info()
+
+    restored = pickle.loads(pickle.dumps(converter))
+
+    assert converter._type_adapter.cache_info() == cache_info
+    assert restored._type_adapter.cache_info().maxsize == 2
+    assert restored._type_adapter.cache_info().currsize == 0
+    assert restored.to_payload(value) == payload
+    assert payload.data == b'{"value":1}'
+    assert restored.from_payload(payload, Model) == value
+    assert restored.from_payload(payload, Model) == value
+    assert restored._type_adapter.cache_info().hits == 1
+
+
+async def test_pydantic_data_converter_pickle():
+    restored = pickle.loads(pickle.dumps(pydantic_data_converter))
+    value = {"a": 1}
+    assert await restored.decode(await restored.encode([value]), [dict]) == [value]
+
+
 @pytest.mark.parametrize(
     (
         "converter_kwargs",