Fix msgspec mutable default values to use default_factory (#2666)

* Fix handling of mutable default values in msgspec to ensure safety

* Fix handling of default values in msgspec to support empty defaults and improve factory assignment

Author: koxudaxi

src/datamodel_code_generator/model/msgspec.py

@@ -274,6 +274,13 @@ def __str__(self) -> str:
                 data.pop("default")
                 data["default_factory"] = default_factory
 
+        if "default" in data and isinstance(data["default"], (list, dict, set)) and "default_factory" not in data:
+            default_value = data.pop("default")
+            if default_value:
+                data["default_factory"] = f"lambda: {default_value!r}"
+            else:
+                data["default_factory"] = type(default_value).__name__
+
         if not data:
             return ""
 

tests/data/expected/main/openapi/msgspec_empty_dict_default.py

@@ -0,0 +1,18 @@
+# generated by datamodel-codegen:
+#   filename:  empty_dict_default.yaml
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from typing import Union
+
+from msgspec import UNSET, Struct, UnsetType, field
+
+
+class ObjectMeta(Struct):
+    name: Union[str, UnsetType] = UNSET
+    namespace: Union[str, UnsetType] = UNSET
+
+
+class PodSpec(Struct):
+    metadata: Union[ObjectMeta, UnsetType] = field(default_factory=dict)

tests/data/expected/main/openapi/msgspec_union_default_object.py

@@ -19,4 +19,6 @@ class Container(Struct):
         default_factory=lambda: convert({'start': 2009, 'end': 2019}, type=Interval)
     )
     string_or_interval: Union[Interval, str, UnsetType] = 'some string value'
-    dict_or_interval: Union[Dict[str, str], Interval, UnsetType] = {'key': 'value'}
+    dict_or_interval: Union[Dict[str, str], Interval, UnsetType] = field(
+        default_factory=lambda: {'key': 'value'}
+    )

tests/main/openapi/test_main_openapi.py

@@ -1928,6 +1928,10 @@ def test_main_openapi_union_default_object(output_model: str, expected_output: s
             "pydantic_v2.BaseModel",
             "pydantic_v2_empty_dict_default.py",
         ),
+        (
+            "msgspec.Struct",
+            "msgspec_empty_dict_default.py",
+        ),
     ],
 )
 @pytest.mark.skipif(