`AssertionError: get_device_updates_by_remote returned too many EDUs` causing federation problems

[richvdh]

We're seeing quite a lot of these, and they seem to be interrupting federation traffic:

2022-01-10 10:03:58,307 - synapse.federation.sender.per_destination_queue - 364 - ERROR - federation_transaction_transmission_loop-9933549 - TX [....] Failed to send transaction
Capture point (most recent call last):
  File "/usr/local/lib/python3.7/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/local/lib/python3.7/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/home/synapse/src/synapse/app/federation_sender.py", line 22, in <module>
    start(sys.argv[1:])
  File "/home/synapse/src/synapse/app/generic_worker.py", line 505, in start
    _base.start_worker_reactor("synapse-generic-worker", config)
  File "/home/synapse/src/synapse/app/_base.py", line 126, in start_worker_reactor
    run_command=run_command,
  File "/home/synapse/src/synapse/app/_base.py", line 178, in start_reactor
    run()
  File "/home/synapse/src/synapse/app/_base.py", line 162, in run
    run_command()
  File "/home/synapse/env-py37/lib/python3.7/site-packages/twisted/internet/base.py", line 1318, in run
    self.mainLoop()
  File "/home/synapse/env-py37/lib/python3.7/site-packages/twisted/internet/base.py", line 1328, in mainLoop
    reactorBaseSelf.runUntilCurrent()
  File "/home/synapse/src/synapse/metrics/__init__.py", line 645, in f
    ret = func(*args, **kwargs)
  File "/home/synapse/env-py37/lib/python3.7/site-packages/twisted/internet/base.py", line 967, in runUntilCurrent
    f(*a, **kw)
  File "/home/synapse/env-py37/lib/python3.7/site-packages/twisted/internet/defer.py", line 662, in callback
    self._startRunCallbacks(result)
  File "/home/synapse/env-py37/lib/python3.7/site-packages/twisted/internet/defer.py", line 764, in _startRunCallbacks
    self._runCallbacks()
  File "/home/synapse/env-py37/lib/python3.7/site-packages/twisted/internet/defer.py", line 859, in _runCallbacks
    current.result, *args, **kwargs
  File "/home/synapse/env-py37/lib/python3.7/site-packages/twisted/internet/defer.py", line 1751, in gotResult
    current_context.run(_inlineCallbacks, r, gen, status)
  File "/home/synapse/env-py37/lib/python3.7/site-packages/twisted/internet/defer.py", line 1661, in _inlineCallbacks
    result = current_context.run(gen.send, result)
  File "/home/synapse/src/synapse/metrics/background_process_metrics.py", line 242, in run
    return await func(*args, **kwargs)
Traceback (most recent call last):
  File "/home/synapse/src/synapse/federation/sender/per_destination_queue.py", line 278, in _transaction_transmission_loop
    async with _TransactionQueueManager(self) as (
  File "/home/synapse/src/synapse/federation/sender/per_destination_queue.py", line 631, in __aenter__
    limit
  File "/home/synapse/src/synapse/federation/sender/per_destination_queue.py", line 563, in _get_device_update_edus
    assert len(edus) <= limit, "get_device_updates_by_remote returned too many EDUs"
AssertionError: get_device_updates_by_remote returned too many EDUs

[richvdh]

Looks to be new in Synapse v1.50.0rc1: https://sentry.matrix.org/sentry/synapse-matrixorg/issues/239101

[squahtx]

May be due to #10520, which increased the size of edus by one per user.
edus comes from DeviceWorkerStore.get_device_updates_by_remote and given its non-trivial implementation, I don't believe the assert holds, even before that change.

[richvdh]

yes, that does look odd. I guess the first thing to do here is to figure out how it used to work.