This PR reverts https://github.com/element-hq/synapse/pull/18751
### Why revert?
@reivilibre
[found](https://matrix.to/#/!vcyiEtMVHIhWXcJAfl:sw1v.org/$u9OEmMxaFYUzWHhCk1A_r50Y0aGrtKEhepF7WxWJkUA?via=matrix.org&via=node.marinchik.ink&via=element.io)
that our CI was failing in bizarre ways (thanks for stepping up to dive
into this 🙇). Examples:
- `twisted.internet.error.ProcessTerminated: A process has ended with a
probable error condition: process ended by signal 9.`
- `twisted.internet.error.ProcessTerminated: A process has ended with a
probable error condition: process ended by signal 15.`
<details>
<summary>More detailed part of the log</summary>
https://github.com/element-hq/synapse/actions/runs/16758038107/job/47500520633#step:9:6809
```
tests.util.test_wheel_timer.WheelTimerTestCase.test_single_insert_fetch
===============================================================================
Error:
Traceback (most recent call last):
File "/home/runner/.cache/pypoetry/virtualenvs/matrix-synapse-pswDeSvb-py3.9/lib/python3.9/site-packages/twisted/trial/_dist/disttrial.py", line 371, in task
await worker.run(case, result)
File "/home/runner/.cache/pypoetry/virtualenvs/matrix-synapse-pswDeSvb-py3.9/lib/python3.9/site-packages/twisted/trial/_dist/worker.py", line 305, in run
return await self.callRemote(workercommands.Run, testCase=testCaseId) # type: ignore[no-any-return]
File "/home/runner/.cache/pypoetry/virtualenvs/matrix-synapse-pswDeSvb-py3.9/lib/python3.9/site-packages/twisted/internet/defer.py", line 1187, in __iter__
yield self
File "/home/runner/.cache/pypoetry/virtualenvs/matrix-synapse-pswDeSvb-py3.9/lib/python3.9/site-packages/twisted/internet/defer.py", line 1092, in _runCallbacks
current.result = callback( # type: ignore[misc]
File "/home/runner/.cache/pypoetry/virtualenvs/matrix-synapse-pswDeSvb-py3.9/lib/python3.9/site-packages/twisted/protocols/amp.py", line 1968, in _massageError
error.trap(RemoteAmpError)
File "/home/runner/.cache/pypoetry/virtualenvs/matrix-synapse-pswDeSvb-py3.9/lib/python3.9/site-packages/twisted/python/failure.py", line 431, in trap
self.raiseException()
File "/home/runner/.cache/pypoetry/virtualenvs/matrix-synapse-pswDeSvb-py3.9/lib/python3.9/site-packages/twisted/python/failure.py", line 455, in raiseException
raise self.value.with_traceback(self.tb)
twisted.internet.error.ProcessTerminated: A process has ended with a probable error condition: process ended by signal 9.
tests.util.test_macaroons.MacaroonGeneratorTestCase.test_guest_access_token
-------------------------------------------------------------------------------
Ran 4325 tests in 669.321s
FAILED (skips=159, errors=62, successes=4108)
while calling from thread
Traceback (most recent call last):
File "/home/runner/.cache/pypoetry/virtualenvs/matrix-synapse-pswDeSvb-py3.9/lib/python3.9/site-packages/twisted/internet/base.py", line 1064, in runUntilCurrent
f(*a, **kw)
File "/home/runner/.cache/pypoetry/virtualenvs/matrix-synapse-pswDeSvb-py3.9/lib/python3.9/site-packages/twisted/internet/base.py", line 790, in stop
raise error.ReactorNotRunning("Can't stop reactor that isn't running.")
twisted.internet.error.ReactorNotRunning: Can't stop reactor that isn't running.
joining disttrial worker #0 failed
Traceback (most recent call last):
File "/home/runner/.cache/pypoetry/virtualenvs/matrix-synapse-pswDeSvb-py3.9/lib/python3.9/site-packages/twisted/internet/defer.py", line 1853, in _inlineCallbacks
result = context.run(
File "/home/runner/.cache/pypoetry/virtualenvs/matrix-synapse-pswDeSvb-py3.9/lib/python3.9/site-packages/twisted/python/failure.py", line 467, in throwExceptionIntoGenerator
return g.throw(self.value.with_traceback(self.tb))
File "/home/runner/.cache/pypoetry/virtualenvs/matrix-synapse-pswDeSvb-py3.9/lib/python3.9/site-packages/twisted/trial/_dist/worker.py", line 406, in exit
await endDeferred
File "/home/runner/.cache/pypoetry/virtualenvs/matrix-synapse-pswDeSvb-py3.9/lib/python3.9/site-packages/twisted/internet/defer.py", line 1187, in __iter__
yield self
twisted.internet.error.ProcessTerminated: A process has ended with a probable error condition: process ended by signal 15.
```
</details>
With more debugging (thanks @devonh for also stepping in as maintainer),
we were finding that the CI was consistently failing at
`test_exposed_to_prometheus` which was a bit of smoke because of all of
the [metrics
changes](https://github.com/element-hq/synapse/issues/18592) that were
merged recently.
Locally, although I wasn't able to reproduce the bizarre errors, I could
easily see increased memory usage (~20GB vs ~2GB) and the
`test_exposed_to_prometheus` test taking a while to complete when
running a full test run (`SYNAPSE_TEST_LOG_LEVEL=INFO poetry run trial
tests`).
<img width="1485" height="78" alt="Lots of memory usage"
src="https://github.com/user-attachments/assets/811e2a96-75e5-4a3c-966c-00dc0512cea9"
/>
After updating `test_exposed_to_prometheus` to dump the
`latest_metrics_response = generate_latest(REGISTRY)`, I could see that
it's a massive 3.2GB response. Inspecting the contents, we can see 4.1M
(4,137,123) entries for just
`synapse_background_update_status{server_name="test"} 3.0` which is a
`LaterGauge`. I don't think we have 4.1M test cases so it's also unclear
why we end up with so many samples but it does make sense that we do see
a lot of duplicates because each `HomeserverTestCase` will create a
homeserver for each test case that will `LaterGauge.register_hook(...)`
(part of the https://github.com/element-hq/synapse/pull/18751 changes).
`tests/storage/databases/main/test_metrics.py`
```python
latest_metrics_response = generate_latest(REGISTRY)
with open("/tmp/synapse-test-metrics", "wb") as f:
f.write(latest_metrics_response)
```
After reverting the https://github.com/element-hq/synapse/pull/18751
changes, running the full test suite locally doesn't result in memory
spikes and seems to run normally.
### Dev notes
Discussion in the
[`#synapse-dev:matrix.org`](https://matrix.to/#/!vcyiEtMVHIhWXcJAfl:sw1v.org/$vkMATs04yqZggVVd6Noop5nU8M2DVoTkrAWshw7u1-w?via=matrix.org&via=node.marinchik.ink&via=element.io)
room.
### Pull Request Checklist
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https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->
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* [ ] Pull request includes a [changelog
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The entry should:
- Be a short description of your change which makes sense to users.
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instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
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* [ ] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct (run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))
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