envs: allow VLLM_WORKER_MULTIPROC_METHOD=forkserver

[terafin]

Summary

Widens the VLLM_WORKER_MULTIPROC_METHOD env Literal + choices list to include "forkserver", making the existing surviving forkserver code path at vllm/entrypoints/openai/api_server.py:83-90 reachable for users who opt in.

Why

PR #40331 (merged 2026-04-21) added forkserver support; PR #40438 reverted hours later because of an unrelated BG-thread import transformers preload that broke tests/entrypoints/pooling/basic/test_truncation.py. The forkserver plumbing itself survived the revert (multiprocessing.set_start_method("forkserver"), set_forkserver_preload(["vllm.v1.engine.async_llm"]), forkserver.ensure_running()), but became unreachable because the env value was rejected.

This PR restores ONLY the env-widening — no BG preload, no default change. Behavior is unchanged unless a user explicitly sets VLLM_WORKER_MULTIPROC_METHOD=forkserver.

Test plan

• tests/entrypoints/pooling/basic/test_truncation.py passes (the test that broke under [Startup] Parallelize torch/transformers import + weight prefetch + forkserver prewarm #40331's preload — should be untouched here)
• VLLM_WORKER_MULTIPROC_METHOD=forkserver vllm serve <small-model> starts successfully and uses the surviving forkserver path at api_server.py:83-90
• VLLM_WORKER_MULTIPROC_METHOD=invalid is still rejected (validator still works)
• Default behavior (no env var set) is unchanged — still uses fork or _maybe_force_spawn fallback

Followups (NOT in this PR)

• Default flip (cli_env_setup → forkserver) — separate PR with cuda_is_initialized() guard
• The BG transformers preload from [Startup] Parallelize torch/transformers import + weight prefetch + forkserver prewarm #40331 — needs more careful delivery, NOT restored here

🤖 Generated with Claude Code

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[terafin]

Self-review pass — the original 4-line env widen made four latent bugs in the surviving forkserver code path reachable. Pushed an amended commit (61bd4b3, force-with-lease, single-commit history preserved) that ships the env widen plus four targeted fixes and regression tests.

Latent bugs the original env widen exposed

1. _maybe_force_spawn() clobbered forkserver→spawn under CUDA-init (vllm/utils/system_utils.py)
   The early-return only checked =="spawn". With the env widened, an opt-in user with CUDA already initialized in the parent (or Ray-actor / WSL / --numa-bind) would have their "forkserver" silently rewritten to "spawn". Extended the early-return to short-circuit on both "spawn" and "forkserver". The forkserver helper is started by forkserver.ensure_running() before CUDA touch in api_server, so the CUDA-init hazard that motivates forcing spawn doesn't apply.

2. set_start_method("forkserver") raises RuntimeError on re-entry (vllm/entrypoints/openai/api_server.py)
   Called without force=True. Test fixtures, importlib reloads, or any path where the start method is already set would crash before reaching forkserver.ensure_running(). Added force=True.

3. os.getenv vs envs.VLLM_WORKER_MULTIPROC_METHOD source-of-truth split (api_server.py)
   The forkserver gate read os.getenv(...) directly, while _maybe_force_spawn and the env_with_choices validator read via envs. Switched the gate to envs.VLLM_WORKER_MULTIPROC_METHOD so all three read the same canonical value (envs.__getattr__ re-evaluates the lambda per access, so post-_maybe_force_spawn mutations are visible).

4. Removed surviving set_forkserver_preload(["vllm.v1.engine.async_llm"])
   This is the exact line that triggered the Revert "[Startup] Parallelize torch/transformers import + weight prefetch + forkserver prewarm" #40438 revert — preloading async_llm pulled in a background-thread import transformers that broke tests/entrypoints/pooling/basic/test_truncation.py. The eager-import optimization is fully separable from the env-widen; forkserver works correctly without it (cold imports per fork). If anyone wants the warm-import speedup back, it should land in a separate PR with a regression-tested non-eager preload list.

Tests added

tests/utils_/test_system_utils.py (uses monkeypatch only, no forking):

• test_forkserver_opt_in_survives_cuda_init — simulates cuda_is_initialized() returning True, asserts forkserver is preserved (the bug-1 regression guard).
• test_forkserver_opt_in_survives_numa_bind — --numa-bind argv path.
• test_spawn_opt_in_still_short_circuits — guard that the original "spawn" early-return still works.
• test_unset_with_cuda_init_still_forces_spawn — negative case: unset env + CUDA-init still gets force-promoted to spawn (unchanged behavior preserved).

Diff stat

tests/utils_/test_system_utils.py     | 46 ++++++++++++++++++++++++++++++++++++
vllm/entrypoints/openai/api_server.py | 21 +++++++++++++----
vllm/envs.py                          |  4 ++--
vllm/utils/system_utils.py            | 10 +++++++-
4 files changed, 73 insertions(+), 8 deletions(-)

Force-pushed onto fresh upstream/main so the PR is rebase-clean.