Fix shutdown hang on session processor thread lock

[Copilot]

Summary

After a generation completes, the server hangs on ^C because Python's threading._shutdown() blocks waiting for the non-daemon session_processor thread to exit — which is sleeping in poll_now_event.wait(polling_interval) and won't notice _stop_event until the 1-second timeout elapses. If the processor was paused via the API, resume_event.wait() (no timeout) would block indefinitely.

Changes to session_processor_default.py:

• daemon=True on the session processor thread — Python's interpreter shutdown will no longer block on this thread even if it's stuck (e.g., mid-generation)
• stop() now sets _poll_now_event and _resume_event — immediately wakes the thread from any sleeping/blocked state so it checks stop_event and exits without waiting for the polling interval to elapse

# Before
def stop(self, *args, **kwargs) -> None:
    self._stop_event.set()

# After
def stop(self, *args, **kwargs) -> None:
    self._stop_event.set()
    # Wake the thread if it is sleeping in poll_now_event.wait() or blocked in resume_event.wait() (paused).
    self._poll_now_event.set()
    self._resume_event.set()

Related Issues / Discussions

QA Instructions

1. Generate an image to completion
2. Press ^C to shut down the server
3. Verify the process exits cleanly immediately after "InvokeAI shutting down..." without requiring a second ^C

Also verify with a paused processor: pause via the API, then shut down — process should still exit immediately.

Merge Plan

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[bug]: Shutdown is hanging on a thread lock</issue_title>
<issue_description>### Is there an existing issue for this problem?

• I have searched the existing issues

Install method

Invoke's Launcher

Operating system

Linux

GPU vendor

Nvidia (CUDA)

GPU model

No response

GPU VRAM

No response

Version number

lstein/feature/nicer-shutdown

Browser

No response

System Information

No response

What happened

After generating an image I tried to shut down the invokeai server by pressing ^C. It printed "InvokeAI shutting down..." and then hung indefinitely. I hit ^C again and it exited after printing a warning message about lock.acquire().

Here is the log:

^C
[2026-03-01 21:52:51,384]::[ModelInstallService]::INFO --> Installer thread 138019081299648 exiting
[2026-03-01 21:52:52,388]::[InvokeAI]::INFO --> InvokeAI shutting down...

^C
Exception ignored in: <module 'threading' from '/home/lstein/.local/share/uv/python/cpython-3.12.12-linux-x86_64-gnu/lib/python3.12/threading.py'>
Traceback (most recent call last):
  File "/home/lstein/.local/share/uv/python/cpython-3.12.12-linux-x86_64-gnu/lib/python3.12/threading.py", line 1624, in _shutdown
    lock.acquire()
KeyboardInterrupt: 

What you expected to happen

I expect InvokeAI to shut down on its own soon after the "shutting down..." message. I suspect that there is a thread somewhere that is holding onto a lock.

How to reproduce the problem

No response

Additional context

No response

Discord username

No response</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [bug]: Shutdown is hanging on a thread lock #107

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[lstein]

Works as advertised.