Fix Python 3.8 issues

Signed-off-by: Kai A. Hiller <V02460@gmail.com>

matrix-synapse.spec

@@ -11,6 +11,7 @@ URL:        https://github.com/matrix-org/%{srcname}
 Source0:    %{url}/archive/v%{version}/%{srcname}-%{version}.tar.gz
 Source1:    synapse.sysconfig
 Source2:    synapse.service
+Patch0:     python38_fix.patch
 BuildArch:  noarch
 
 BuildRequires:  python3-devel

python38_fix.patch

@@ -0,0 +1,67 @@
+diff --git a/synapse/util/caches/descriptors.py b/synapse/util/caches/descriptors.py
+index 5ac2530a6..bddadd629 100644
+--- a/synapse/util/caches/descriptors.py
++++ b/synapse/util/caches/descriptors.py
+@@ -17,8 +17,8 @@ import functools
+ import inspect
+ import logging
+ import threading
+-from collections import namedtuple
+ from typing import Any, cast
++from weakref import WeakValueDictionary
+ 
+ from six import itervalues
+ 
+@@ -430,7 +430,7 @@ class CacheDescriptor(_CacheDescriptorBase):
+             # Add our own `cache_context` to argument list if the wrapped function
+             # has asked for one
+             if self.add_cache_context:
+-                kwargs["cache_context"] = _CacheContext(cache, cache_key)
++                kwargs["cache_context"] = _CacheContext.get_instance(cache, cache_key)
+ 
+             try:
+                 cached_result_d = cache.get(cache_key, callback=invalidate_callback)
+@@ -625,14 +625,36 @@ class CacheListDescriptor(_CacheDescriptorBase):
+         return wrapped
+ 
+ 
+-class _CacheContext(namedtuple("_CacheContext", ("cache", "key"))):
+-    # We rely on _CacheContext implementing __eq__ and __hash__ sensibly,
+-    # which namedtuple does for us (i.e. two _CacheContext are the same if
+-    # their caches and keys match). This is important in particular to
+-    # dedupe when we add callbacks to lru cache nodes, otherwise the number
+-    # of callbacks would grow.
++class _CacheContext:
++    """Holds cache information from the cached function higher in the calling order.
++
++    Can be used to invalidate the higher level cache entry if something changes
++    on a lower level.
++    """
++
++    _cache_context_objects = WeakValueDictionary()
++
++    def __init__(self, cache, cache_key):
++        self._cache = cache
++        self._cache_key = cache_key
++
+     def invalidate(self):
+-        self.cache.invalidate(self.key)
++        """Invalidates the cache entry referred to by the context."""
++        self._cache.invalidate(self._cache_key)
++
++    @classmethod
++    def get_instance(cls, cache, cache_key):
++        """Returns an instance constructed with the given arguments.
++
++        A new instance is only created if none already exists.
++        """
++
++        # We make sure there are no identical _CacheContext instances. This is
++        # important in particular to dedupe when we add callbacks to lru cache
++        # nodes, otherwise the number of callbacks would grow.
++        return cls._cache_context_objects.setdefault(
++            (cache, cache_key), cls(cache, cache_key)
++        )
+ 
+ 
+ def cached(