Add typechecking of Any values for state object

An ugly solution for an ugly little problem. This patch uses JSON's type
checker to validate Any values in stateobject, in order to avoid a circular
import.

Fixes #3180

mitmproxy/stateobject.py

@@ -1,6 +1,7 @@
 import typing
 from typing import Any  # noqa
 from typing import MutableMapping  # noqa
+import json
 
 from mitmproxy.coretypes import serializable
 from mitmproxy.utils import typecheck
@@ -77,8 +78,14 @@ def _process(typeinfo: typecheck.Type, val: typing.Any, make: bool) -> typing.An
             for k, v in val.items()
         }
     elif typename.startswith("typing.Any"):
-        # FIXME: Remove this when we remove flow.metadata
+        # This requires a bit of explanation. We can't import our IO layer here,
-        assert isinstance(val, (int, str, bool, bytes))
+        # because it causes a circular import. Rather than restructuring the
+        # code for this, we use JSON serialization, which has similar primitive
+        # type restrictions as tnetstring, to check for conformance.
+        try:
+            json.dumps(val)
+        except TypeError:
+            raise ValueError(f"Data not serializable: {val}")
         return val
     else:
         return typeinfo(val)

test/mitmproxy/test_stateobject.py

@@ -125,7 +125,7 @@ def test_any():
     assert a.x == b.x
 
     a = TAny(object())
-    with pytest.raises(AssertionError):
+    with pytest.raises(ValueError):
         a.get_state()