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scheduler.py: Replace asyncio child watcher with our own watcher thread #2064

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Merged
juergbi merged 1 commit into from
Sep 19, 2025
Merged

scheduler.py: Replace asyncio child watcher with our own watcher thread #2064

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38 changes: 17 additions & 21 deletions src/buildstream/_scheduler/scheduler.py
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Original file line number Diff line number Diff line change
Expand Up @@ -25,6 +25,7 @@
import datetime
import multiprocessing.forkserver
import sys
import threading
from concurrent.futures import ThreadPoolExecutor

# Local imports
Expand Down Expand Up @@ -138,12 +139,6 @@ def run(self, queues, casd_process_manager):
# Hold on to the queues to process
self.queues = queues

# NOTE: Enforce use of `SafeChildWatcher` as we generally don't want
# background threads.
# In Python 3.8+, `ThreadedChildWatcher` is the default watcher, and
# not `SafeChildWatcher`.
asyncio.set_child_watcher(asyncio.SafeChildWatcher()) # pylint: disable=deprecated-class

# Ensure that we have a fresh new event loop, in case we want
# to run another test in this thread.
self.loop = asyncio.new_event_loop()
Expand All @@ -160,12 +155,7 @@ def run(self, queues, casd_process_manager):

# Watch casd while running to ensure it doesn't die
self._casd_process = casd_process_manager.process
_watcher = asyncio.get_child_watcher()

def abort_casd(pid, returncode):
asyncio.get_event_loop().call_soon(self._abort_on_casd_failure, pid, returncode)

_watcher.add_child_handler(self._casd_process.pid, abort_casd)
threading.Thread(target=self._watch_casd, name="watch-casd", daemon=True).start()

# Start the profiler
with PROFILER.profile(Topics.SCHEDULER, "_".join(queue.action_name for queue in self.queues)):
Expand All @@ -185,8 +175,6 @@ def abort_casd(pid, returncode):
# Invoke the ticker callback a final time to render pending messages
self._ticker_callback()

# Stop watching casd
_watcher.remove_child_handler(self._casd_process.pid)
self._casd_process = None

# Stop handling unix signals
Expand Down Expand Up @@ -309,16 +297,24 @@ def job_completed(self, job, status):
# This will terminate immediately all jobs, since buildbox-casd is dead,
# we can't do anything with them anymore.
#
# Args:
# pid (int): the process id under which buildbox-casd was running
# returncode (int): the return code with which buildbox-casd exited
#
def _abort_on_casd_failure(self, pid, returncode):
def _abort_on_casd_failure(self):
self.context.messenger.bug("buildbox-casd died while the pipeline was active.")

self._casd_process.returncode = returncode
self.terminate()

# _watch_casd()
#
# This runs in a separate thread to detect casd exiting while the loop is
# still running.
#
def _watch_casd(self):
loop = self.loop
proc = self._casd_process
if loop and proc:
# This sets the `returncode` attribute
proc.wait()
if not loop.is_closed():
loop.call_soon_threadsafe(self._abort_on_casd_failure)

# _start_job()
#
# Spanws a job
Expand Down
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