Simplify event loop handling and remove deprecated get_event_loop

[GDYendell]

• Merge Add some docs #338 first

---

In the process of writing documentation it became clear that

• We are using get_event_loop, which is deprecated and planned for removal in 3.16
• The event loop handling is over complicated

Summary:

• Remove the loop parameter to FastCS and instead require clients to call serve from an async context with the loop they want it to run in
• Don't pass a loop to transports and instead retrieve it via get_running_loop only where required (EpicaCA and Tango)
• Use asyncio.create_task from an async context consistently in tests, rather than run_until_complete or ensure_future
• Improve tango test handling of DeviceTestContext to fix resource warnings

Summary by CodeRabbit

• Refactor
  • Simplified event loop configuration by removing explicit loop parameters from FastCS initialization and transport connection methods
  • Event loop management is now handled automatically by the framework
  • Updated async task handling patterns throughout the codebase for improved reliability

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📝 Walkthrough

Walkthrough

This PR removes explicit event loop parameters from the FastCS API and internal implementations, replacing manual loop passing with implicit asyncio patterns such as asyncio.get_running_loop() and asyncio.run(). The refactoring simplifies constructor signatures across all transport classes and the main control system.

Changes

Cohort / File(s)	Summary
Core Control System src/fastcs/control_system.py	Removed loop parameter from __init__, replaced explicit loop/task management with asyncio.run(), switched to asyncio.create_task(), enhanced task cancellation in _stop_scan_tasks(), and updated signal handler installation to use the current running loop.
Launch and CLI src/fastcs/launch.py	Simplified FastCS instantiation by removing loop parameter from constructor call; removed asyncio import.
EPICS CA Transport src/fastcs/transports/epics/ca/ioc.py, src/fastcs/transports/epics/ca/transport.py	EpicsCAIOC.run() signature changed to remove loop parameter and now uses asyncio.get_running_loop(); EpicsCATransport.connect() simplified to accept only controller_api and removed loop storage.
EPICS PVA Transport src/fastcs/transports/epics/pva/transport.py	Removed loop parameter from EpicsPVATransport.connect() signature; simplified initialization to rely on implicit event loop handling.
GraphQL & REST Transports src/fastcs/transports/graphql/transport.py, src/fastcs/transports/rest/transport.py	Both transport classes had loop parameter removed from connect() method signatures; simplified server initialization.
Tango Transport src/fastcs/transports/tango/transport.py	Removed loop parameter from connect() and updated TangoDSR initialization to use asyncio.get_running_loop(); simplified serve() to directly await thread execution.
Transport Base src/fastcs/transports/transport.py	Updated abstract connect() method to remove loop parameter and removed asyncio import; updated documentation to indicate implicit loop retrieval.
Test Updates tests/benchmarking/controller.py, tests/test_control_system.py, tests/transports/epics/ca/test_initial_value.py, tests/transports/epics/pva/test_p4p.py, tests/transports/graphQL/test_graphql.py, tests/transports/rest/test_rest.py, tests/transports/tango/test_dsr.py	Updated all test instantiations to remove loop parameters; modernized async test patterns using @pytest.mark.asyncio and asyncio.create_task() instead of ensure_future; adjusted event loop lifecycle management in test setup and teardown.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Possibly related PRs

• PR #332: Modifies src/fastcs/control_system.py to remove legacy build_controller_api path and transition to Controller.create_api_and_tasks, which may intersect with event loop initialization changes in this PR.

Suggested reviewers

• shihab-dls

Poem

🐰 Loop, loop, whence did you go?
Event loops freed from chains below,
Asyncio patterns now take the lead,
Implicit flows are all we need!
Refactored paths run light and free,
The rabbit's work brings harmony! ✨

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 19.15% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main objective of the PR: simplifying event loop handling and removing deprecated get_event_loop usage. The changes across all modified files consistently reflect this primary goal.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

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• Post copyable unit tests in a comment
• Commit unit tests in branch simplify-event-loop

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[GDYendell]

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Actionable comments posted: 3

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@src/fastcs/control_system.py`:
- Around line 54-55: The code currently installs SIGINT/SIGTERM handlers inside
serve(), which hijacks signal handling when serve() is called from an existing
async runtime; move the signal-handler installation out of serve() and into
run() so handlers are only installed when run() is explicitly invoked, and guard
the installation with a main-thread check (threading.current_thread() is
threading.main_thread()) and ensure the event loop supports add_signal_handler
before calling it (e.g., get the loop from asyncio.get_event_loop() and call
add_signal_handler only when permitted). Modify serve() to no longer call
add_signal_handler, and update run() to install the handlers conditionally for
SIGINT/SIGTERM using the same handler functions currently referenced in serve().
- Around line 164-167: The interactive_shell task created inside run()/wrapper
must be tracked and its exceptions handled: when creating the coroutine
(interactive_shell / asyncio.to_thread(partial(shell.mainloop, ...))) retain the
Task returned by asyncio.create_task and attach a done callback that
catches/logs exceptions and always calls stop_event.set() on error; also ensure
tasks spawned by run() are appended to a local registry so failures are visible
(e.g., add the created Task to a list in run() and on completion remove it), and
update wrapper to create the task via that helper so any raised exceptions won't
silently block await stop_event.wait().

In `@tests/transports/tango/test_dsr.py`:
- Around line 58-71: The current finally block's cleanup iterates
gc.get_objects() and closes any asyncio.AbstractEventLoop it finds, which is
unsafe; change the cleanup to only close loops created by this test/context by
recording references to event loops you create (e.g., the 'loop' variable and
any loops returned or owned by DeviceTestContext) or by snapshotting existing
loops before entering DeviceTestContext and closing only loops that appear
afterwards, and remove the gc.get_objects() iteration; ensure you still call
loop.run_until_complete(asyncio.sleep(0)), loop.close(),
asyncio.set_event_loop(None) and gc.collect() but limit any additional .close()
calls to the specific loop objects you tracked rather than all AbstractEventLoop
instances.

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: cc07bde6-a8ed-470a-a36d-83ba46c30250

📥 Commits

Reviewing files that changed from the base of the PR and between 6839f1d and b86c206.

📒 Files selected for processing (16)

• src/fastcs/control_system.py
• src/fastcs/launch.py
• src/fastcs/transports/epics/ca/ioc.py
• src/fastcs/transports/epics/ca/transport.py
• src/fastcs/transports/epics/pva/transport.py
• src/fastcs/transports/graphql/transport.py
• src/fastcs/transports/rest/transport.py
• src/fastcs/transports/tango/transport.py
• src/fastcs/transports/transport.py
• tests/benchmarking/controller.py
• tests/test_control_system.py
• tests/transports/epics/ca/test_initial_value.py
• tests/transports/epics/pva/test_p4p.py
• tests/transports/graphQL/test_graphql.py
• tests/transports/rest/test_rest.py
• tests/transports/tango/test_dsr.py

💤 Files with no reviewable changes (1)

• tests/transports/epics/ca/test_initial_value.py

[coretl]

@ajgdls is now our resident Tango expert so I'll defer to him to review the tango bits. Everything else looks fine

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.98%. Comparing base (04d7d24) to head (d9d0620).

Files with missing lines	Patch %	Lines
src/fastcs/control_system.py	68.57%	11 Missing ⚠️
src/fastcs/transports/tango/transport.py	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             docs     #340      +/-   ##
==========================================
- Coverage   91.21%   90.98%   -0.23%     
==========================================
  Files          70       70              
  Lines        2594     2596       +2     
==========================================
- Hits         2366     2362       -4     
- Misses        228      234       +6     

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