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fix: Hold pybmq::Session by shared_ptr in _ext.Session - #112
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`Session.stop()` stops the underlying `pybmq::Session` but does not destroy it. Destruction is left to `_ext.Session.__dealloc__`, which runs whenever the garbage collector gets to it, on whichever thread triggers the collection. Destroying a `pybmq::Session` blocks until the libbmq FSM thread finishes stopping it, and the FSM thread logs through the Python `logging` module before it can finish. If the collection runs on a thread that holds a `logging.Handler` lock, the destructor waits for the FSM thread and the FSM thread waits for the lock, and the process deadlocks. Any reference cycle hands a stopped `Session` to the collector; a traceback retained after an exception is a common one. This patch holds the `pybmq::Session` by `shared_ptr` and releases it in `stop()`, so a `Session` used as a context manager is destroyed when the `with` block exits. Each method takes its own copy of the pointer before calling into C++, so a concurrent `stop()` cannot destroy the session while it is in use. Calling a method after `stop()` now raises `Error`. Signed-off-by: Chris A. Evans <thecityofguanyu@outlook.com>
pniedzielski
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Sep 28, 2026
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Looks good, thanks for this!
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Summary
Session.stop()stops the underlyingpybmq::Sessionbut does not destroy it. Destruction is left to_ext.Session.__dealloc__, which runs whenever the garbage collector gets to it, on whichever thread triggers the collection.Destroying a
pybmq::Sessionblocks until the libbmq FSM thread finishes stopping it, and the FSM thread logs through the Pythonloggingmodule before it can finish.If the collection runs on a thread that holds a
logging.Handlerlock, the destructor waits for the FSM thread and the FSM thread waits for the lock, and the process deadlocks. Any reference cycle hands a stoppedSessionto the collector. A traceback retained after an exception is a common one.stop().Proposed Changes
Hold the
pybmq::Sessionviashared_ptrand release it instop(), so aSessionused as a context manager is destroyed when thewithblock exits.Each method takes its own copy of the pointer before calling into C++, so a concurrent
stop()cannot destroy the session while it is in use. A method called afterstop()still raises the sameErroras before.Unit tests have been added to validate this behavior.