Retry completion responses missing 'choices' instead of crashing the actor

[jlamypoirier]

Authored by Claude Opus 4.8 (1M context) via Claude Code.

Problem

The actor can die mid-run with:

File "pipelinerl/async_llm.py", line 173, in llm_async_generate
    content = response_data["choices"][0]["message"]["content"]
KeyError: 'choices'

A single response lacking choices crashes the whole rollout loop.

Root cause

llm_async_generate fetches the completion in a retry loop. _is_retryable_abort_response already inspects choices but returns False for a missing one (correctly — a missing choices is not an abort), the loop breaks, and the post-loop parse dereferences response_data["choices"][0]... unguarded. When the body lacks choices (e.g. an HTTP 200 carrying an error object, or an empty payload) this raises KeyError; the surrounding try/except only logs and re-raises, and the actor retries RetryableAbortedCompletionError but not a bare KeyError, so it propagates and kills the actor.

Fix

After the loop, if response_data has no choices, raise RetryableAbortedCompletionError so the response is retried like other transient vLLM failures.

Scope — what this does and does NOT cover

• Covers: a transient bad response where the vLLM server stays alive (a one-off malformed / error payload). The actor retries and the run continues.
• Does NOT cover: a permanent EngineCore death. If the engine dies, its run_vllm1 process exits non-zero, and the launcher's monitor is fail-fast — launch.py tears down the entire run when any child exits non-zero (gently_stop_all_processes(); sys.exit(1); comment: "if just one dies non-zero, stop all"). So a dead engine ends the run regardless of this actor guard. Surviving that needs launcher-level resilience (restart the dead inference server + route the actor around it), which is out of scope here.
• This change also does not diagnose why a server returns a body without choices (a server-side error / OOM / EngineCore death are all plausible; in the case that motivated this, the engine had died and was returning a 200 InternalServerError body).

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