fix(generation): report no OpenAI server as an empty list

[yupengtang]

What does this PR do ?

Makes all four generation backends report "no OpenAI server" the same way — an empty list — and narrows dp_openai_server_base_urls to list[str].

Megatron already returns [] and filters its Nones out. vLLM, TRT-LLM and Dynamo returned a list of Nones instead. As #3977 says, the placeholder does not stand for anything: in those branches there is no server at all, so the length carries no information, but [None] is truthy, so "we have URLs" reads as true when there are none.

Changed to match Megatron:

backend	before	after
vLLM (sync engine)	[None]	[]
vLLM (deferred load, sync engine)	[None]	[]
TRT-LLM (no HTTP server)	[None] * dp_size	[]
Dynamo (no token wrapper)	[None]	[]

NemoGymConfig.base_urls and spinup_nemo_gym_actor already declare list[str], so this makes the producers match what the consumers were asking for rather than widening anything.

One case the issue does not list

vLLM needed more than swapping the sentinel. A worker leaves base_url unset unless expose_http_server started a server (vllm_worker_async.py:104 assigns None, :206-210 only sets it under that flag), and get_reserved_url returns None unless a socket was reserved under the same flag. So async_engine: true with expose_http_server: false produced [None] * dp_size from the workers themselves, never reaching the guarded branch — a longer, still-truthy version of the same problem. Both _report_dp_openai_server_base_urls and _collect_reserved_urls now drop those, which is why they filter rather than only returning [] early.

Issues

Closes #3977.

Usage

No config or API change. if not urls is now a correct emptiness test on every backend.

# before, on a sync vLLM engine
generation.dp_openai_server_base_urls   # [None]  -> truthy, len 1
# after
generation.dp_openai_server_base_urls   # []      -> falsy, len 0

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Additional Information

Callers

I went through every reader before changing the producers. None needed edits, and none reads the length in the no-server case:

• _shard_base_urls (setup.py:777) collapses an all-None list to None via if not any(urls); an empty list takes the same branch.
• _maybe_start_generation_router filters falsy entries into backend_urls and raises when that is empty, so [None] and [] already produced the identical error.
• NeMo-Gym spinup is reachable only behind the should_expose_http_server assert in nemo_rl/environments/nemo_gym.py, so the no-server list never gets there.
• MegatronGeneration.verify_served_address only ever saw Megatron's list, which was already [].
• The one len() call, grpo.py:1550, is a print on the deferred vLLM path. It now says Reserved 0 vLLM server URLs: [] instead of Reserved 1 vLLM server URLs: [None], which is the symptom the issue calls out.

Per the issue I left the now-redundant any(...) guards alone — they stay correct either way and are cleaner to remove separately.

Tests

Five added, plus the existing Dynamo assertion at test_dynamo_generation.py:167 updated from [None] to [].

tests/unit/models/generation/test_vllm_generation.py:

• test_sync_engine_reports_no_openai_server_urls
• test_async_engine_without_http_server_reports_no_urls — the case above, workers answering all Nones
• test_async_engine_with_http_server_reports_served_urls — guards the happy path against over-filtering

tests/unit/models/generation/trtllm/test_trtllm_generation.py:

• test_no_http_server_reports_an_empty_url_list
• test_exposed_http_server_reports_served_addresses

Reverting each backend to its pre-fix version, the no-server tests fail and the served-address tests still pass:

# vllm_generation.py reverted
FAILED test_sync_engine_reports_no_openai_server_urls
FAILED test_async_engine_without_http_server_reports_no_urls
PASSED test_async_engine_with_http_server_reports_served_urls

# trtllm_generation.py reverted
FAILED test_no_http_server_reports_an_empty_url_list
PASSED test_exposed_http_server_reports_served_addresses

With the change, the touched suites and the consumer suites pass:

tests/unit/models/generation/test_dynamo_generation.py         23 passed
tests/unit/models/generation/test_vllm_generation.py -k url     3 passed
tests/unit/models/generation/trtllm/... --trtllm-only -k url     2 passed
tests/unit/single_controller/test_setup.py -k 'url or shard or gym or router'   14 passed
tests/unit/algorithms/test_grpo.py -k 'dynamo or trtllm or url'  5 passed
tests/unit/environments/test_nemo_gym.py -k url                  3 passed

CPU only — I don't have a GPU box, so the vLLM/TRT-LLM tests that need a real engine and the functional suite did not run here. The --trtllm-only selection needed a stub tensorrt_llm locally to get past the conftest gate; the two tests themselves only use _bare_generation and a mocked worker group.

ruff 0.9.9 check, import sort and format are clean on all seven files.

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