[worker] fix: create a new event loop if none exists

[baymax591]

What does this PR do?

If asyncio.run and asyncio.get_event_loop are called sequentially in the same thread, a 'RuntimeError: There is no current event loop in thread %r' is thrown.

This PR fixes the above-mentioned issue. same issue as pr#3703.

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[gemini-code-assist]

Code Review

This pull request correctly addresses a RuntimeError that occurs when asyncio.get_event_loop() is called in a thread without a current event loop, which can happen after asyncio.run() has been used. The fix involves a try...except block to create and set a new event loop when one is not found. My review includes suggestions to use the more modern and explicit asyncio.get_running_loop() for better code clarity and future compatibility. I've also noted the code duplication across files and suggested creating a helper function to improve maintainability.

[gemini-code-assist]

For better clarity and future-proofing, it's recommended to use asyncio.get_running_loop() instead of asyncio.get_event_loop(). get_event_loop() has complex behavior that can be surprising and is deprecated since Python 3.10. get_running_loop() is more explicit as it returns the running loop in the current thread and raises a RuntimeError if no loop is running, which is the exact scenario you are handling.

This logic is also duplicated in verl/workers/megatron_workers.py. See my comment there for a suggestion to refactor this into a helper function to improve maintainability.

Suggested change

	loop = asyncio.get_event_loop()
	loop = asyncio.get_running_loop()

[vermouth1992]

Shall we create a common utils for this?

[gemini-code-assist]

Similar to my comment in recipe/one_step_off_policy/fsdp_workers.py, it's recommended to use asyncio.get_running_loop() here for better clarity and to align with modern asyncio best practices. get_event_loop() is deprecated and can have surprising behavior.

Since this logic for getting or creating an event loop is now duplicated, consider creating a helper function in a shared utility module to improve maintainability. For example:

import asyncio

def get_or_create_event_loop():
    """Gets the current event loop or creates a new one if it doesn't exist."""
    try:
        return asyncio.get_running_loop()
    except RuntimeError:
        loop = asyncio.new_event_loop()
        asyncio.set_event_loop(loop)
        return loop

Suggested change

	loop = asyncio.get_event_loop()
	loop = asyncio.get_running_loop()

[ji-huazhong]

LGTM

[wuxibin89]

Better to put it in verl/utils/ray_utils.py?