[rollout, trainer, cfg] feat: per-request abort hooks and AbortableLLMServerClient

[cr-gao]

What does this PR do?

Adds selective (per-request) abort support to the rollout client path. Today
LLMServerClient exposes no way to interrupt a single in-flight request — only the
broadcast abort_all_requests on the rollout replica exists. This PR introduces
lightweight extension hooks plus a concrete AbortableLLMServerClient that tracks
in-flight requests so a custom AgentLoopWorker/AgentLoopManager can abort one specific
request (e.g. a timed-out rollout, or a custom trajectory-collection policy that decides to
drop a particular sequence).

AI assistance (Claude) was used to write this change; the author has reviewed every line.

Checklist Before Starting

• Search for similar PRs. Query: https://github.com/verl-project/verl/pulls?q=is%3Apr+abort+LLMServerClient (none found)

Resolves #6866. Feature discussion / motivation: #6866.

• Format the PR title as [{modules}] {type}: {description}

Test

Added tests/workers/rollout/test_abortable_llm_client_on_cpu.py (CPU-only, mocks the Ray
load balancer + server handles — no GPU/vLLM needed):

• test_record_abort_and_cleanup — dispatch records the request; abort() targets the
  inner vLLM request id (not the outer sticky-session id); completion clears the entry.
• test_inflight_cleared_on_normal_completion — normal finish also clears the table.
• test_abort_unknown_request_is_noop — aborting an unknown/finished id neither raises nor
  hits the server.
• test_cancellation_aborts_server_and_clears_inflight — cancelling generate() (e.g.
  asyncio.wait_for/timeout) fires _on_cancel, which aborts the server-side request so it
  does not leak, and _on_complete still clears the in-flight entry.

pytest tests/workers/rollout/test_abortable_llm_client_on_cpu.py -v

tests/workers/rollout/test_abortable_llm_client_on_cpu.py::test_record_abort_and_cleanup PASSED                  [ 25%]
tests/workers/rollout/test_abortable_llm_client_on_cpu.py::test_inflight_cleared_on_normal_completion PASSED     [ 50%]
tests/workers/rollout/test_abortable_llm_client_on_cpu.py::test_abort_unknown_request_is_noop PASSED             [ 75%]
tests/workers/rollout/test_abortable_llm_client_on_cpu.py::test_cancellation_aborts_server_and_clears_inflight PASSED [100%]

API and Usage Example

New config field actor_rollout_ref.rollout.agent.llm_client_class (FQN, dynamically
imported — mirrors the existing agent_loop_manager_class), wired in ray_trainer.py:

actor_rollout_ref:
  rollout:
    agent:
      llm_client_class: verl.workers.rollout.llm_server.AbortableLLMServerClient

# Inside a custom AgentLoopWorker/Manager holding the client:
# Explicit per-request abort:
await llm_client.abort(request_id)

# Cancellation (e.g. timeout) is handled automatically — the _on_cancel hook aborts the
# server-side request, so this does NOT leak server compute:
await asyncio.wait_for(llm_client.generate(request_id=request_id, ...), timeout=T)

Design & Code Changes

• llm_server.py:
  • Hoist the inner per-turn vLLM request_id into a named inner_request_id so subclasses
    can record it.
  • Add no-op hooks _on_dispatch(request_id, inner_request_id, server) (before awaiting
    generation), _on_complete(request_id) (in finally), and _on_cancel(request_id)
    (in except asyncio.CancelledError, before the finally, so the in-flight entry still
    exists).
  • Add AbortableLLMServerClient(LLMServerClient): tracks request_id -> (inner_request_id, server) in _inflight; _on_complete is the sole cleanup point; abort(request_id)
    and _on_cancel share a _send_abort helper that sends a precise abort_request to the
    right server (abort awaits it; _on_cancel is fire-and-forget, since awaiting during
    cancellation is unsafe). Inherits the base (non-resuming) generate, i.e. hard-abort
    semantics.
• workers/config/rollout.py: add llm_client_class field to AgentLoopConfig.
• trainer/ppo/ray_trainer.py: load llm_client_class (if set) and pass it as client_cls
  to get_client().
• agent_loop.py: fix typo chunkes -> chunks.

Checklist Before Submitting

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• Apply pre-commit checks. (All hooks pass; the local check-naming-conventions false
  positive only triggers on vendored files under .venv, which is gitignored and absent in CI.)
• Add / Update the documentation.
• Add unit or end-to-end test(s) to cover the code.
• Send a message in the ci-request channel once ready for CI.

🤖 Generated with Claude Code

[CLAassistant]

All committers have signed the CLA.

[gemini-code-assist]

Code Review

This pull request introduces AbortableLLMServerClient to support per-request aborts and in-flight request tracking, adds corresponding CPU unit tests, and allows configuring a custom LLMServerClient class. It also fixes a minor typo in the agent loop. The review feedback highlights a critical issue where client-side task cancellation (such as timeouts) would silently leak GPU resources on the vLLM server because the server-side request is not aborted; the reviewer suggests introducing and implementing an _on_cancel hook to safely abort the request upon cancellation.

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[gemini-code-assist]

If the client-side generate task is cancelled (e.g., due to a timeout via asyncio.timeout or asyncio.wait_for), the finally block runs immediately and removes the request from _inflight via _on_complete. However, the server-side request is never aborted, leading to silent GPU resource leaks on the vLLM server.

To fix this, we should catch asyncio.CancelledError in generate, trigger a new _on_cancel hook to abort the request on the server, and then re-raise the cancellation.

Suggested change

	finally:
	self._on_complete(request_id)
	self._release_server(server_id)
	def _on_dispatch(self, request_id, inner_request_id, server):
	"""Hook fired after the inner vLLM request_id is assigned, before awaiting generation.
	Default no-op; subclasses may record (request_id/inner_request_id, server) to enable abort."""
	def _on_complete(self, request_id):
	"""Hook fired when generation finishes or raises. Default no-op."""
	except asyncio.CancelledError:
	self._on_cancel(request_id)
	raise
	finally:
	self._on_complete(request_id)
	self._release_server(server_id)
	def _on_dispatch(self, request_id, inner_request_id, server):
	"""Hook fired after the inner vLLM request_id is assigned, before awaiting generation.
	Default no-op; subclasses may record (request_id/inner_request_id, server) to enable abort."""
	def _on_complete(self, request_id):
	"""Hook fired when generation finishes or raises. Default no-op."""
	def _on_cancel(self, request_id):
	"""Hook fired when generation is cancelled. Default no-op."""

[cr-gao]

Good catch. asyncio.wait_for/timeout cancellation indeed leaked the server-side request, and that's exactly one of the motivating use cases. Added an _on_cancel(request_id) hook fired on CancelledError (before the finally so the in-flight entry still exists), implemented as a fire-and-forget abort in AbortableLLMServerClient (we can't safely await during cancellation), with abort() and _on_cancel sharing a _send_abort helper. Also added a regression test for the cancellation path. Done in aeadf2f.

[gemini-code-assist]

Implement the _on_cancel hook in AbortableLLMServerClient to automatically abort the request on the server when the client-side task is cancelled. This prevents GPU resource leaks on the vLLM server during timeouts or cancellations.

    def _on_complete(self, request_id):
        # Sole owner of removal: fires from generate()'s finally on success, abort, or error.
        self._inflight.pop(request_id, None)

    def _on_cancel(self, request_id):
        entry = self._inflight.get(request_id)
        if entry is not None:
            inner_request_id, server = entry
            server.abort_request.remote(inner_request_id)

[cr-gao]

Implemented _on_cancel in AbortableLLMServerClient as fire-and-forget (_send_abort, no await during cancellation); removal stays owned by _on_complete. Done in aeadf2f.