`log_kv` within `_handle_new_device_update_async` is not associated with an opentracing span, causing sentry logspam and large memory usage

[DMRobertson]

This caused rapid memory usage rises on matrix.org.

We suspect it's only noticable if you're using sentry to report errors.

2022-04-26 13:34:43,569 - synapse.logging.opentracing - 347 - ERROR - _handle_new_device_update_async-0 - There was no active span when trying to log. Did you forget to start one or did a context slip?
Stack (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/homeserver.py", line 460, in <module>
    main()
  File "/home/synapse/src/synapse/app/homeserver.py", line 456, in main
    run(hs)
  File "/home/synapse/src/synapse/app/homeserver.py", line 442, in run
    logger=logger,
  File "/home/synapse/src/synapse/app/_base.py", line 180, 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 1315, in run
    self.mainLoop()
  File "/home/synapse/env-py37/lib/python3.7/site-packages/twisted/internet/base.py", line 1325, in mainLoop
    reactorBaseSelf.runUntilCurrent()
  File "/home/synapse/env-py37/lib/python3.7/site-packages/twisted/internet/base.py", line 964, in runUntilCurrent
    f(*a, **kw)
  File "/home/synapse/env-py37/lib/python3.7/site-packages/twisted/internet/defer.py", line 661, in callback
    self._startRunCallbacks(result)
  File "/home/synapse/env-py37/lib/python3.7/site-packages/twisted/internet/defer.py", line 763, in _startRunCallbacks
    self._runCallbacks()
  File "/home/synapse/env-py37/lib/python3.7/site-packages/twisted/internet/defer.py", line 858, in _runCallbacks
    current.result, *args, **kwargs
  File "/home/synapse/env-py37/lib/python3.7/site-packages/twisted/internet/defer.py", line 1750, in gotResult
    current_context.run(_inlineCallbacks, r, gen, status)
  File "/home/synapse/env-py37/lib/python3.7/site-packages/twisted/internet/defer.py", line 1663, in _inlineCallbacks
    status.deferred.callback(getattr(e, "value", None))
  File "/home/synapse/env-py37/lib/python3.7/site-packages/twisted/internet/defer.py", line 661, in callback
    self._startRunCallbacks(result)
  File "/home/synapse/env-py37/lib/python3.7/site-packages/twisted/internet/defer.py", line 763, in _startRunCallbacks
    self._runCallbacks()
  File "/home/synapse/env-py37/lib/python3.7/site-packages/twisted/internet/defer.py", line 858, in _runCallbacks
    current.result, *args, **kwargs
  File "/home/synapse/env-py37/lib/python3.7/site-packages/twisted/internet/defer.py", line 661, in callback
    self._startRunCallbacks(result)
  File "/home/synapse/env-py37/lib/python3.7/site-packages/twisted/internet/defer.py", line 763, in _startRunCallbacks
    self._runCallbacks()
  File "/home/synapse/env-py37/lib/python3.7/site-packages/twisted/internet/defer.py", line 858, in _runCallbacks
    current.result, *args, **kwargs
  File "/home/synapse/env-py37/lib/python3.7/site-packages/twisted/internet/defer.py", line 1750, in gotResult
    current_context.run(_inlineCallbacks, r, gen, status)
  File "/home/synapse/env-py37/lib/python3.7/site-packages/twisted/internet/defer.py", line 1660, in _inlineCallbacks
    result = current_context.run(gen.send, result)
  File "/home/synapse/src/synapse/metrics/background_process_metrics.py", line 243, in run
    return await func(*args, **kwargs)
  File "/home/synapse/src/synapse/handlers/device.py", line 687, in _handle_new_device_update_async
    #    {"message": "sent device update to host", "host": host}
  File "/home/synapse/src/synapse/logging/opentracing.py", line 347, in ensure_active_span_inner_2
    stack_info=True,

[DMRobertson]

Related: #12321 #12365. In particular we suspect the latter is responsible.

[DMRobertson]

@erikjohnston says:

select min(stream_id), max(stream_id) - min(stream_id), count(*) from matrix.device_lists_changes_in_room where NOT converted_to_destinations is the query I'm using to track progress

[anoadragon453]

What's happening here is that log_kv is trying to log to the active OpenTracing span, but there was no active span. Hence the error. Note spans are started automatically for requests, but we don't do it automatically for background jobs.

OpenTracing spans are propagated by log contexts (which is how they're somewhat closely related). The shown error can happen if you didn't start a span, or the log context slipped. In this case it's the former.

We can either:

1. Create a big span for this whole background job, but long-lived spans are against the philosophy of OpenTracing, and we'd only report it once it's finished (and thus you get a disconnected parent).

2. Otherwise we can do a span per loop, but the issue there is that the log_kv is logging the hosts we're sending a device update to. We'd like that to be logged against a span that includes the device update itself - otherwise the span is pretty much useless.

   So what we could do is start a span based on the opentracing_context given back to us by get_uncoverted_output_room_pokes. Though... now we've noticed that a type error has caused opentracing_context to be broken - so that needs to be fixed first.

[DMRobertson]

There is also a problem propagating opentracing spans between whitelisted homeservers. Details of the underlying cause here: #12485 (comment)

(Oh, Andrew says just as much above.)

#12554 mitigates the logging pain.
#12555 fixes the opentracing_context not being parsed.

I'm not sure if we ought to keep this issue open to track fixing the opencontext span?

[DMRobertson]

I'm not sure if we ought to keep this issue open to track fixing the opencontext span?

After discussion, will raise a new issue to track fixing the logging.