[PY] Drop FFI String cache when pickling

Pickled FFI strings should only persist their Python string payload, avoiding serialization of the hidden cached FFI backing object.

Author: Ubospica

python/tvm_ffi/cython/string.pxi

@@ -57,6 +57,9 @@ class String(str, PyNativeObject):
         val._tvm_ffi_cached_object = None
         return val
 
+    def __reduce_ex__(self, protocol):
+        return (type(self), (str(self),))
+
     # pylint: disable=no-self-argument
     def __from_tvm_ffi_object__(cls, obj: Any) -> "String":
         """Construct a ``String`` from an FFI object (internal)."""

tests/python/test_string.py

@@ -18,6 +18,7 @@
 import pickle
 
 import tvm_ffi
+from tvm_ffi.core import TypeSchema, _to_py_class_value
 
 
 def test_string() -> None:
@@ -34,6 +35,15 @@ def test_string() -> None:
     s4 = pickle.loads(pickle.dumps(s))
     assert s4 == "hello"
 
+    cached = _to_py_class_value(TypeSchema("str").convert("x" * 200))
+    assert isinstance(cached, tvm_ffi.core.String)
+    assert cached._tvm_ffi_cached_object is not None
+
+    cached_roundtrip = pickle.loads(pickle.dumps(cached))
+    assert isinstance(cached_roundtrip, tvm_ffi.core.String)
+    assert cached_roundtrip == cached
+    assert cached_roundtrip._tvm_ffi_cached_object is None
+
 
 def test_bytes() -> None:
     fecho = tvm_ffi.get_global_func("testing.echo")