Use immutabledict instead of frozendict (#15113)

Additionally:

* Consistently use `freeze()` in test

---------

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>
Co-authored-by: 6543 <6543@obermui.de>

synapse/util/__init__.py

@@ -18,7 +18,7 @@ import typing
 from typing import Any, Callable, Dict, Generator, Optional, Sequence
 
 import attr
-from frozendict import frozendict
+from immutabledict import immutabledict
 from matrix_common.versionstring import get_distribution_version_string
 from typing_extensions import ParamSpec
 
@@ -41,22 +41,18 @@ def _reject_invalid_json(val: Any) -> None:
     raise ValueError("Invalid JSON value: '%s'" % val)
 
 
-def _handle_frozendict(obj: Any) -> Dict[Any, Any]:
-    """Helper for json_encoder. Makes frozendicts serializable by returning
+def _handle_immutabledict(obj: Any) -> Dict[Any, Any]:
+    """Helper for json_encoder. Makes immutabledicts serializable by returning
     the underlying dict
     """
-    if type(obj) is frozendict:
+    if type(obj) is immutabledict:
         # fishing the protected dict out of the object is a bit nasty,
         # but we don't really want the overhead of copying the dict.
         try:
             # Safety: we catch the AttributeError immediately below.
-            # See https://github.com/matrix-org/python-canonicaljson/issues/36#issuecomment-927816293
-            # for discussion on how frozendict's internals have changed over time.
-            return obj._dict  # type: ignore[attr-defined]
+            return obj._dict
         except AttributeError:
-            # When the C implementation of frozendict is used,
-            # there isn't a `_dict` attribute with a dict
-            # so we resort to making a copy of the frozendict
+            # If all else fails, resort to making a copy of the immutabledict
             return dict(obj)
     raise TypeError(
         "Object of type %s is not JSON serializable" % obj.__class__.__name__
@@ -64,11 +60,11 @@ def _handle_frozendict(obj: Any) -> Dict[Any, Any]:
 
 
 # A custom JSON encoder which:
-#   * handles frozendicts
+#   * handles immutabledicts
 #   * produces valid JSON (no NaNs etc)
 #   * reduces redundant whitespace
 json_encoder = json.JSONEncoder(
-    allow_nan=False, separators=(",", ":"), default=_handle_frozendict
+    allow_nan=False, separators=(",", ":"), default=_handle_immutabledict
 )
 
 # Create a custom decoder to reject Python extensions to JSON.