Use new canonicaljson serisalistion hook

From matrix-org/python-canonicaljson#59

pyproject.toml

@@ -158,9 +158,8 @@ jsonschema = ">=3.0.0"
 immutabledict = ">=2.0"
 # We require 2.1.0 or higher for type hints. Previous guard was >= 1.1.0
 unpaddedbase64 = ">=2.1.0"
-# We require 1.5.0 to work around an issue when running against the C implementation of
-# frozendict: https://github.com/matrix-org/python-canonicaljson/issues/36
-canonicaljson = "^1.5.0"
+# We require 2.0.0 for immutabledict support.
+canonicaljson = "^2.0.0"
 # we use the type definitions added in signedjson 1.1.
 signedjson = "^1.1.0"
 # validating SSL certs for IP addresses requires service_identity 18.1.

synapse/__init__.py

@@ -17,9 +17,9 @@
 """ This is an implementation of a Matrix homeserver.
 """
 
-import json
 import os
 import sys
+from typing import Any, Dict
 
 from synapse.util.rust import check_rust_lib_up_to_date
 from synapse.util.stringutils import strtobool
@@ -61,11 +61,20 @@
 except ImportError:
     pass
 
-# Use the standard library json implementation instead of simplejson.
+# Teach canonicaljson how to serialise immutabledicts.
 try:
-    from canonicaljson import set_json_library
-
-    set_json_library(json)
+    from canonicaljson import register_preserialisation_callback
+    from immutabledict import immutabledict
+
+    def _immutabledict_cb(d: immutabledict) -> Dict[str, Any]:
+        try:
+            return d._dict
+        except Exception:
+            # Paranoia: fall back to a `dict()` call, in case a future version of
+            # immutabledict removes `_dict` from the implementation.
+            return dict(d)
+
+    register_preserialisation_callback(immutabledict, _immutabledict_cb)
 except ImportError:
     pass