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Allow Synapse modules to specify their own threadpool when calling defer_to_thread #19024

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Allow Synapse modules to specify their own threadpool when calling defer_to_thread #19024

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Allow Synapse modules to specify their own threadpool when calling `d…
anoadragon453 Oct 7, 2025
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anoadragon453 Oct 7, 2025
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1 change: 1 addition & 0 deletions changelog.d/19024.feature
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Original file line number Diff line number Diff line change
@@ -0,0 +1 @@
Allow Synapse modules to specify a custom threadpool when calling `defer_to_thread`.
13 changes: 11 additions & 2 deletions synapse/module_api/__init__.py
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Expand Up @@ -43,6 +43,7 @@

from twisted.internet import defer
from twisted.internet.interfaces import IDelayedCall
from twisted.python.threadpool import ThreadPool
from twisted.web.resource import Resource

from synapse.api import errors
Expand Down Expand Up @@ -78,7 +79,7 @@
from synapse.http.servlet import parse_json_object_from_request
from synapse.http.site import SynapseRequest
from synapse.logging.context import (
defer_to_thread,
defer_to_threadpool,
make_deferred_yieldable,
run_in_background,
)
Expand Down Expand Up @@ -1716,6 +1717,7 @@ def run_as_background_process(
async def defer_to_thread(
self,
f: Callable[P, T],
threadpool: Optional[ThreadPool] = None,
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This will mess with positional args that people are already using with defer_to_thread.

For example: module_api.defer_to_thread(add, 1, 2) will be have 1 interpreted as the threadpool positional argument.

We could use a keyword argument and hope there aren't arg naming collisions with threadpool with the f function. We could further namespace the variable name like defer_to_thread_threadpool

But I think it might be better if we just introduce defer_to_threadpool to the ModuleApi. We need to rely on a new version of the ModuleApi anyway.

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Argh, yes, good point.

While writing this PR, it also felt like a code smell to have the ModuleApi's defer_to_thread actually be defer_to_threadpool under the hood.

I'll rework this PR to expose defer_to_threadpool instead.

*args: P.args,
**kwargs: P.kwargs,
) -> T:
Expand All @@ -1731,7 +1733,14 @@ async def defer_to_thread(
Returns:
The return value of the function once ran in a thread.
"""
return await defer_to_thread(self._hs.get_reactor(), f, *args, **kwargs)
# If a threadpool is not provided by the module, then use the default
# reactor threadpool of the homeserver.
if threadpool is None:
threadpool = self._hs.get_reactor().getThreadPool()

return await defer_to_threadpool(
self._hs.get_reactor(), threadpool, f, *args, **kwargs
)

async def check_username(self, username: str) -> None:
"""Checks if the provided username uses the grammar defined in the Matrix
Expand Down
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