[Test][MLA] Cover padded spec-decode context lengths in the DSA indexer

[200lz]

Purpose

What. Minimal regression coverage for the padded spec-decode context-length invariant fixed in #51538: for MTP (next_n = 2) with three live requests and one CUDA-graph padding request (seq_len = 0), the DSA indexer's per-token context lengths for the padded request must be [0, 0], not [-1, 0]. One parametrised test (58 lines) in the existing tests/v1/attention/test_indexer_native_next_n.py, built on the real DeepseekV32IndexerMetadataBuilder, covering both the native (B, next_n) path and the flattened uniform-decode kernel path. No production change.

Why. A downstream fork that predates #51538 reproduced the old defect while serving with FlashInfer sparse MLA on SM12x, where it presented as a FlashInfer decode hang (flashinfer-ai/flashinfer#5015). The investigation showed the FlashInfer launch was queued on the same stream behind vLLM's already-wedged persistent_topk, which had read the unclamped -1 as uint32 (#51593). The producer clamps landed in #51538 without a direct test; this pins them so the invariant cannot silently regress again.

Duplicate check

• [Bugfix] Make DSV4 sparse MLA work end-to-end for plain decode, MTP, and DSpark #51538 (merged) already contains the production fix (producer clamps in indexer.py and the persistent_topk length guard). This PR does not duplicate that fix; it adds the regression coverage that was missing from the merged change.
• Searched gh pr list --state open for persistent_topk, indexer padded, 51593 in:body, 51538 in:body, flashinfer 5015. The only overlap is the kernel-side invariant: open [Kernel] Make persistent_topk deterministic #55122 ([Kernel] Make persistent_topk deterministic) adds test_persistent_topk_degenerate_lengths (row length 0 / -1 → all -1). I therefore deliberately did not add a kernel-side negative-length test here; this PR covers only the producer side, which no open PR tests. [Bugfix] Align MLA indexer block table with MTP speculative decode #43970 touches indexer.py for block-table alignment and does not cover the clamp.

Test Plan

.venv/bin/python -m pytest -q tests/v1/attention/test_indexer_native_next_n.py
.venv/bin/python -m pytest -q tests/v1/attention/test_indexer_deepseek_v4_slot_mapping.py tests/v1/attention/test_flashinfer_sparse_mla_sm120_api.py
.venv/bin/python -m pytest -q tests/kernels/test_top_k_per_row.py -k persistent_topk
pre-commit run --files tests/v1/attention/test_indexer_native_next_n.py

Test Result

RTX PRO 6000 Blackwell Server Edition (SM120, cc 12.0), driver 580.173.02, torch 2.13.0+cu132, vLLM main fc6b6e1feb with the precompiled _C (VLLM_USE_PRECOMPILED=1 uv pip install -e .).

command	result
pytest -q tests/v1/attention/test_indexer_native_next_n.py	11 passed (9 existing + 2 new, 16.9 s)
negative control: same file with the two #51538 producer clamps temporarily removed from indexer.py	2 failed, both new cases: At index 6 diff: -1 != 0 (then restored)
pytest -q tests/v1/attention/test_flashinfer_sparse_mla_sm120_api.py tests/v1/attention/test_indexer_dcp_localize.py tests/v1/attention/test_indexer_deepseek_v4_slot_mapping.py	45 passed, 1 failed — test_indexer_builder_deepseek_v4_compressed_slot_mapping_uses_num_states fails in this environment on the gated meta-llama/Meta-Llama-3-8B config download (HTTP 401, no HF token on the box); unrelated to this change
pytest -q tests/kernels/test_top_k_per_row.py -k persistent_topk	54 passed, 136 deselected
pytest -q tests/kernels/test_top_k_per_row.py (full)	120 passed, 19 skipped, 51 failed — all 51 are cooperative_topk parametrisations (0 persistent_topk), a backend vLLM excludes on the SM120 family; identical on unmodified main, unrelated to this change
pre-commit run --files tests/v1/attention/test_indexer_native_next_n.py	all hooks passed (ruff check/format, typos, mypy 3.10, SPDX, …)

Model eval

Not applicable: test-only change, no serving or model-output path is modified.

AI assistance

AI assistance (Claude Code) was used to investigate the issue, write the test and run the validation on the hardware above. The human submitter reviewed every changed line and the test results before submission.

[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.

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[200lz]

@pavanimajety @zyongye — this is intentionally test-only follow-up coverage for #51538 commits 6–7. The downstream FlashInfer #5015 incident reproduced the old padded-MTP -1 context-length path, and the test fails under a negative control with those producer clamps removed. #55122 already covers the consumer-side degenerate length, so this PR only pins the producer invariant. Would appreciate a review when convenient.

[200lz]

External SM121 validation is now available from the reporter of flashinfer-ai/flashinfer#5015:

• GB10 component-validation report
• Reporter-provided component logs, source pins and method

Using the handoff components at 210c64614bc8ab954a4fa2357fc6b10f28485238 on one GB10 (SM121, cc 12.1), the reporter observed:

• Standalone top-k: pre-[Bugfix] Make DSV4 sparse MLA work end-to-end for plain decode, MTP, and DSpark #51538 poisoned padding hung for both k=1 and k=2; post-fix passed. Clean pre/post controls passed, with 2,000 iterations each for the pre-fix clean and post-fix poisoned controls.
• Native producer/top-k regression: the reporting image had the three expected failures (uniform-decode clamp, compiled next_n clamp, and negative-length top-k wedge), while the newer compiled vLLM v0.28.0 control passed all five cases without skips.
• The separate FlashInfer decode matrix passed all 11 cases, including 3,000 replays each for cases C/E/J.

These externally reported results corroborate the failure mechanism motivating this PR and the need to preserve the producer clamps in both indexer paths. They supplement the SM120 validation and clamp-removal negative control already documented here; they are not a claim that this exact PR revision was independently tested on SM121.

Scope remains unchanged: this PR adds producer-side regression coverage for the existing #51538 fix, with no production-code change. The native post-fix control was a newer image, not an otherwise-identical two-hunk rebuild. The reporter's seven-day serving result used the complete-capture workaround; production rebase and combined patched eight-rank qualification remain outstanding.

Review of the focused producer-invariant coverage would be appreciated.