Revert "[worker] fix: create a new event loop if none exists when building rollouts"

[vermouth1992]

Reverts #3803

[gemini-code-assist]

Code Review

This pull request reverts a change related to asyncio event loop creation. While the revert might fix an issue with asyncio.run() being called from an already running event loop, it introduces a critical risk of a RuntimeError if no event loop is present. My review includes a suggestion to adopt a more robust pattern for handling the event loop, similar to what's used elsewhere in the same file, to prevent potential crashes.

[gemini-code-assist]

This change reverts the use of asyncio.run(), likely because it can cause a RuntimeError if an event loop is already running. However, the new implementation using asyncio.get_event_loop() will raise a RuntimeError if no event loop is running, which can happen depending on the execution context.

A more robust approach is to handle both cases, as is done in the generate_sequences method within this same file (lines 911-915). This ensures an event loop is always available.

I suggest adopting that pattern here to prevent potential crashes.

Suggested change

	loop = asyncio.get_event_loop()
	loop.run_until_complete(self.trainer_mode())
	try:
	loop = asyncio.get_event_loop()
	except RuntimeError:
	loop = asyncio.new_event_loop()
	asyncio.set_event_loop(loop)
	loop.run_until_complete(self.trainer_mode())

[vermouth1992]

Related to this: sgl-project/sglang#5162