scheduler.py: Only run thread-safe code in callbacks from watchers

Per
https://docs.python.org/3/library/asyncio-policy.html#asyncio.AbstractChildWatcher.add_child_handler,
the callback from a child handler must be thread safe. Not all our
callbacks were. This changes all our callbacks to schedule a call for
the next loop iteration instead of executing it directly.

1 files changed, 7 insertions, 1 deletions

diff --git a/buildstream/_scheduler/jobs/job.py b/buildstream/_scheduler/jobs/job.py
index cbe41bcd1..67ec75db5 100644
--- a/buildstream/_scheduler/jobs/job.py
+++ b/buildstream/_scheduler/jobs/job.py
@@ -138,7 +138,13 @@ class Job():
             with asyncio.get_child_watcher() as watcher:
                 self._process.start()
                 # Register the process to call `_parent_child_completed` once it is done
-                watcher.add_child_handler(self._process.pid, self._parent_child_completed)
+
+                # Here we delay the call to the next loop tick. This is in order to be running
+                # in the main thread, as the callback itself must be thread safe.
+                def on_completion(pid, returncode):
+                    asyncio.get_event_loop().call_soon(self._parent_child_completed, pid, returncode)
+
+                watcher.add_child_handler(self._process.pid, on_completion)
 
     # terminate()
     #