There is a slight chance this will be fixed when matrix-synapse is upgraded to a more recent version (bug 1671106) although I have not checked. It looks like upstream still only tests against Python 3.6.

matrix-synapse	1.0.0-1.fc31:

===============================================================================
[SKIPPED]
Disabled by default because it's slow

tests.test_visibility.FilterEventsForServerTestCase.test_large_room
===============================================================================
[FAIL]
Traceback (most recent call last):
  File "/usr/lib64/python3.8/site-packages/twisted/internet/defer.py", line 1418, in _inlineCallbacks
    result = g.send(result)
  File "/builddir/build/BUILD/synapse-1.0.0/tests/storage/test__base.py", line 308, in test_double_get
    self.assertEquals(a.func2.cache.cache.pop.call_count, 3)
  File "/usr/lib64/python3.8/site-packages/twisted/trial/_synctest.py", line 432, in assertEqual
    super(_Assertions, self).assertEqual(first, second, msg)
  File "/usr/lib64/python3.8/unittest/case.py", line 907, in assertEqual
    assertion_func(first, second, msg=msg)
  File "/usr/lib64/python3.8/unittest/case.py", line 900, in _baseAssertEqual
    raise self.failureException(msg)
twisted.trial.unittest.FailTest: 4 != 3

tests.storage.test__base.CacheDecoratorTestCase.test_double_get
-------------------------------------------------------------------------------
Ran 687 tests in 283.260s

FAILED (skips=1, failures=1, successes=685)

This bug appears to have been reported against 'rawhide' during the Fedora 31 development cycle.
Changing version to '31'.

The coordinated rebuild of Python 3.8 has started in the `f32-python` side tag.

If you figure out how to rebuild this package, please don't rebuild it in regular rawhide, but use the side tag instead:

    on branch master:
    $ fedpkg build --target=f32-python

To wait for a build to show up in the side tag, do:

    $ koji wait-repo f32-python --build=<nvr>

Where <nvr> is name-version-release of the source package, e.g. python-foo-1.1-2.fc32.

An updated mock config is posted at:
http://copr.fedorainfracloud.org/coprs/g/python/python3.8/

Note that it will take a while before the essential packages are rebuilt, so don't expect all your dependencies to be available right away.

Thanks. Let us know if you need up to date info, or if you have any questions.



PS this message is mass posted to all the bugs that block the PYTHON38 bug. If this is also a Fedora 31 FTBFS bug and you manage to fix it, you can do a f31 build as usual:

    on branch f31:
    $ fedpkg build

The f32-python side tag has been merged. In order to rebuild the package, do it in regular rawhide, but please wait until python3-3.8 is tagged:

  $ koji wait-repo f32-build --build python3-3.8.0~b3-3.fc32


If your built already started in f32-python, after it is finished, please tag it to rawhide with:

  $ koji tag-build f32-pending <nvr>

For example:

  $ koji tag-build f32-pending libreoffice-6.3.0.4-3.fc32

Thanks!

(This comment is mass posted to all bugzillas blocking the PYTHON38 tracking bug.)

(Python 3.8 has landed in the rawhide buildroot.)

This issue is blocking the Python 3.8 rebuilds. If this package won't build with 3.8, it won't be installable, along with all its dependent packages, in Fedora 32.
Furthermore, as it fails to install, its dependent packages will fail to install and/or build as well.

Could you please look into it?

This problem still exists with the latest synapse release (1.4.0) and upstream is still only testing with Python 3.7. The issue hasn't been reported upstream yet as far as I could see, so I filed it:

https://github.com/matrix-org/synapse/issues/6200

Thanks for the fix.