[Bugfix] V1: fix allowed_token_ids_mask aliasing in InputBatch.swap_states

[huthvincent]

Purpose

InputBatch.swap_states() swaps the two allowed_token_ids_mask_cpu_tensor
rows with a tuple assignment on row views:

(
    self.allowed_token_ids_mask_cpu_tensor[i1],
    self.allowed_token_ids_mask_cpu_tensor[i2],
) = (
    self.allowed_token_ids_mask_cpu_tensor[i2],
    self.allowed_token_ids_mask_cpu_tensor[i1],
)

Integer indexing on a torch.Tensor returns a view, not a copy. The RHS
tuple holds two views into the live tensor, so the left-to-right assignment
first overwrites row i1 with row i2, then overwrites row i2 with the
row-i1 view that now already contains row i2's data. Both rows collapse to
the original i2 contents, and the request that ends up at index i2 silently
loses its allowed_token_ids restriction (or inherits the wrong one).

swap_states is invoked by reorder_batch
(vllm/v1/attention/backends/utils.py) on the MLA / Mamba / GDN / FlashInfer
decode–prefill split, so any request using SamplingParams.allowed_token_ids
that gets reordered samples from the wrong token set — wrong output, no error.

The fix uses fancy indexing (which copies on the RHS), matching the safe
is_token_ids swap two lines above. Only the CPU staging tensor needs swapping;
_make_sampling_metadata copies it to the GPU tensor every step.

Not a duplicate. Issue #43894 / PR #43931 address the sibling row-leak in
InputBatch.condense(). This PR fixes the separate swap_states aliasing that
none of them touch.

Test Plan

Added test_swap_states_preserves_allowed_token_ids_mask to
tests/v1/worker/test_gpu_input_batch.py (constrained request at row 0,
unconstrained at row 1, swap, assert the constraint follows the request).

pytest tests/v1/worker/test_gpu_input_batch.py::test_swap_states_preserves_allowed_token_ids_mask
pytest tests/v1/worker/test_gpu_input_batch.py

Test Result

Verified on 1× NVIDIA H200 (CUDA 13.0, torch 2.11.0).

# new test, BEFORE fix (bug reproduced):
>   assert int(mask[1].sum().item()) == VOCAB_SIZE - 1
E   assert 0 == (1024 - 1)
1 failed

# new test, AFTER fix:
1 passed

# whole file, AFTER fix:
10 passed

# lint:
ruff check      -> All checks passed!
ruff format     -> 2 files already formatted

The change only alters behavior for allowed_token_ids requests that are
reordered; it restores the intended masking, so it can only make constrained
output more correct. Covered by the deterministic unit test above.

---

AI assistance (Claude) was used to develop this change. The submitter has
reviewed every changed line and run the tests above.

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[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[ErenAta16]

Verified the root cause independently -- confirmed the aliasing with a quick numpy repro (1D arr[i1], arr[i2] = arr[i2], arr[i1] is safe since scalar indexing copies, but the 2D row-view case collapses both rows to the second one's data exactly as described):

>>> import numpy as np
>>> a = np.arange(12.).reshape(3, 4)
>>> a[0], a[1] = a[1], a[0]
>>> a
array([[ 4.,  5.,  6.,  7.],
       [ 4.,  5.,  6.,  7.],   # row 0 leaked into row 1
       [ 8.,  9., 10., 11.]])

Given swap_states has ~15 other tuple-swapped fields using the identical self.X[i1], self.X[i2] = self.X[i2], self.X[i1] pattern, I went through all of them to check whether any others share this bug:

• num_tokens_no_spec, num_prompt_tokens, num_computed_tokens_cpu, temperature_cpu, top_p_cpu, top_k_cpu, frequency_penalties_cpu, presence_penalties_cpu, repetition_penalties_cpu, num_accepted_tokens_cpu, request_lora_mapping are all 1D (max_num_reqs,) arrays -- basic integer indexing on a 1D array returns a scalar copy, not a view, so these are safe.
• req_output_token_ids, spec_token_ids are plain Python lists; req_id_to_index is a dict -- also unaffected (no view semantics).
• token_ids_cpu (2D) and is_token_ids (2D) already use the safe copy/fancy-index pattern, per the existing NOTE: the following is unsafe comment.

So allowed_token_ids_mask_cpu_tensor (2D: (max_num_reqs, vocab_size)) is the only remaining field with this bug class in swap_states -- the fix is complete, not partial. Good catch, and the fancy-indexing fix matches the existing is_token_ids pattern exactly, so no new failure mode introduced. Test looks right too: it pins one row to a single allowed token and the other unconstrained, which is exactly the case that collapses under the old tuple-of-views assignment.

[huthvincent]

Thanks for the thorough independent verification, @ErenAta16 — especially auditing the other ~15 swapped fields to confirm the 2D mask is the only affected one. That matches what I found: the 1D _cpu arrays return scalar copies and token_ids_cpu/is_token_ids already use the safe fancy-index pattern, so this completes that bug class in swap_states.

[ErenAta16]

Glad it lines up. Good find on your end too -- this is a genuinely easy pattern to miss since it only bites on 2D fields, and most of the file's swapped state is 1D.