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Start background tasks after we fork the process (daemonize) #18886

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Merged
merged 11 commits into from
Sep 9, 2025
Merged

Start background tasks after we fork the process (daemonize) #18886

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38b67ca
Fix CPU time going backwards when `daemonize`
MadLittleMods Sep 2, 2025
e39a219
Add context
MadLittleMods Sep 5, 2025
313da3e
Add changelog
MadLittleMods Sep 5, 2025
ee29030
Use correct changelog number
MadLittleMods Sep 5, 2025
2f235e3
`start_background_tasks` where we start other background tasks
MadLittleMods Sep 5, 2025
99b99c2
`start_background_tasks` in tests
MadLittleMods Sep 5, 2025
11c39c5
Try `start_background_tasks` in `synapse/_scripts/update_synapse_data…
MadLittleMods Sep 9, 2025
ed42696
Revert "Try `start_background_tasks` in `synapse/_scripts/update_syna…
MadLittleMods Sep 9, 2025
74cd02d
Ensure storage classes are instantiated so the background updates are…
MadLittleMods Sep 9, 2025
02bb9b4
Merge branch 'develop' into madlittlemods/start-background-tasks-afte…
MadLittleMods Sep 9, 2025
2ab99d5
Merge branch 'develop' into madlittlemods/start-background-tasks-afte…
MadLittleMods Sep 9, 2025
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1 change: 1 addition & 0 deletions changelog.d/18886.misc
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Original file line number Diff line number Diff line change
@@ -0,0 +1 @@
Start background tasks after we fork the process (daemonize).
15 changes: 15 additions & 0 deletions synapse/app/_base.py
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Expand Up @@ -601,6 +601,21 @@ def run_sighup(*args: Any, **kwargs: Any) -> None:
hs.get_datastores().main.db_pool.start_profiling()
hs.get_pusherpool().start()

# Register background tasks required by this server. This must be done
# somewhat manually due to the background tasks not being registered
# unless handlers are instantiated.
#
# While we could "start" these before the reactor runs, nothing will happen
# until the reactor is running, so we may as well do it here in `start`.
#
# Additionally, this means we also start them after we daemonize and fork the
# process which means we can avoid any potential problems with cputime metrics getting
# confused about the per-thread resource usage appearing to go backwards because
# we're comparing the resource usage (`rusage`) from the original process to the
# forked process.
if hs.config.worker.run_background_tasks:
hs.start_background_tasks()

# Log when we start the shut down process.
hs.get_reactor().addSystemEventTrigger(
"before", "shutdown", logger.info, "Shutting down..."
Expand Down
8 changes: 1 addition & 7 deletions synapse/server.py
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Expand Up @@ -366,12 +366,6 @@ def setup(self) -> None:
self.datastores = Databases(self.DATASTORE_CLASS, self)
logger.info("Finished setting up.")

# Register background tasks required by this server. This must be done
# somewhat manually due to the background tasks not being registered
# unless handlers are instantiated.
if self.config.worker.run_background_tasks:
self.setup_background_tasks()
Comment on lines -369 to -373
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In terms of where we moved this code from and to:

Relevant starting point is here:

synapse/synapse/app/homeserver.py

Lines 407 to 417 in b2997a8

def main() -> None:
with LoggingContext("main"):
# check base requirements
check_requirements()
hs = setup(sys.argv[1:])
# redirect stdio to the logs, if configured.
if not hs.config.logging.no_redirect_stdio:
redirect_stdio_to_logs()
run(hs)

The main thing to see here is setup() vs run(). We fork the process in run() and then start the reactor after.

Previously, we started the background tasks in the setup() phase.

Now we start the background tasks in start which happens "once the reactor is running".


def __del__(self) -> None:
"""
Called when an the homeserver is garbage collected.
Expand Down Expand Up @@ -410,7 +404,7 @@ def start_listening(self) -> None: # noqa: B027 (no-op by design)
appropriate listeners.
"""

def setup_background_tasks(self) -> None:
def start_background_tasks(self) -> None:
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Just a rename to align with it's new home in start(). I think either could fit but this might be more straightforward to understand.

"""
Some handlers have side effects on instantiation (like registering
background updates). This function causes them to be fetched, and
Expand Down
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