Break the DispatchSession ↔ worker-task reference cycle to reduce per-dispatch GC pressure — Closes #266

[conradbzura]

Summary

Break the DispatchSession → _worker_task → coro → session reference cycle so a completed dispatch's session, worker task, and per-fork contexts are reclaimed promptly by refcounting instead of lingering until the next cyclic-GC pass. The worker driver's _run coroutine closes over the session, so the session's strong reference to its worker task formed a cycle only the cyclic collector could reclaim; under sustained dispatch these accumulate between GC passes (~40× more live tasks / ~14× more contexts without forced GC). Clear self._worker_task in the task's done-callback to sever it. This is safe: the only post-completion reader, cancel(), already guards on the worker task being non-None, and teardown (drain/__aexit__) synchronizes on _worker_done, not _worker_task. Closes #266

Proposed changes

Clear the worker-task reference on completion (runtime/worker/session.py)

In DispatchSession._schedule_worker's _on_done done-callback, set self._worker_task = None after settling the completion future. This drops the session's strong reference to the finished worker task, so session ↔ _worker_task ↔ coro is no longer a cycle and refcounting reclaims the objects immediately.

Test cases

#	Test Suite	Given	When	Then	Coverage Target
1	test_session	A coroutine dispatch driven to natural (non-cancelled) completion	The dispatch finishes and the worker task's done-callback runs	The captured worker task is reclaimed (public weakref clears) with automatic GC disabled	Cycle severed on completion
2	test_session	A completed dispatch whose session is deliberately retained	Automatic cyclic GC is disabled and no gc.collect() is forced	A weakref to the finished worker task still clears — refcounting reclaims it, not the retained session pinning it	Refcount reclamation, not GC

Note: the tests identify the worker driver task via the private _run coroutine name — there is no public discriminator between it and in-flight step tasks on the worker loop (review finding B2, retained as the pragmatic last resort).