fix(rollout): isolate per-trajectory exceptions in generate_and_rm_group

[aoshen02]

Problem

generate_and_rm_group gathers per-trajectory tasks with a bare asyncio.gather(*tasks) (no return_exceptions=True). If any single trajectory raises an unhandled exception, gather cancels the siblings and propagates, crashing the entire rollout via CancelledError — which also swallows the root exception (logs show only CancelledError, not what actually failed).

This is benign for plain RLVR rollouts (where generate_and_rm reliably catches its own errors), which is why it has not surfaced before. But agentic rollouts can raise after the custom generate() returns — e.g. trajectory token-merge / prefix-drift edge cases that run outside generate()'s own try/except. Observed on a 500-instance SWE-bench eval: a single bad trajectory (~1 in 350-400) reproducibly took down all 500 (crashed ~321 and ~373 on two runs), with only a CancelledError in the logs.

Fix

Catch per-trajectory exceptions at the group gather:

• return_exceptions=True so one failure no longer cancels the batch.
• logger.error(..., exc_info=res) to surface the real traceback (currently swallowed by CancelledError).
• Substitute an ABORTED / resolved=False placeholder with the same fan-out list shape, reusing the existing _abort() sample contract (tokens=[0,0], loss_mask=[0], status=ABORTED, reward=0.0).

ABORTED is already a first-class status that downstream short-circuits (reward-model skip at generate_and_rm, routing-replay skip), so the placeholder introduces no new sample shape.

Notes

• Mirror of vllm-project/vime#200.
• AI-assisted (Claude). Test: ran 500-instance SWE-bench eval; pre-fix crashed at ~321/~373, post-fix completed 500/500 clean.

🤖 Generated with Claude Code