[Bugfix] Fix GDN attention crash with mixed decode/spec-decode batches (#34871)

Signed-off-by: haosdent <haosdent@gmail.com>

tests/v1/attention/test_gdn_metadata_builder.py

+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+"""Tests for GDNAttentionMetadataBuilder.build() — specifically the
+reclassification of non-spec decodes as prefills when spec decodes exist.
+Covers the fix for https://github.com/vllm-project/vllm/issues/34845.
+"""
+
+from dataclasses import dataclass
+
+import pytest
+import torch
+
+from tests.v1.attention.utils import (
+    BatchSpec,
+    create_common_attn_metadata,
+    create_vllm_config,
+)
+from vllm.config import SpeculativeConfig
+from vllm.v1.attention.backends.gdn_attn import (
+    GDNAttentionMetadata,
+    GDNAttentionMetadataBuilder,
+)
+from vllm.v1.kv_cache_interface import MambaSpec
+
+BLOCK_SIZE = 16
+DEVICE = torch.device("cpu")
+
+
+@dataclass
+class GDNBuildTestCase:
+    """Specification for a GDN metadata builder classification test."""
+
+    seq_lens: list[int]
+    query_lens: list[int]
+    num_decode_draft_tokens: list[int] | None  # None = no spec config
+    num_speculative_tokens: int
+    expected_num_decodes: int
+    expected_num_prefills: int
+    expected_num_prefill_tokens: int
+    expected_num_spec_decodes: int
+
+
+GDN_BUILD_TEST_CASES = {
+    # The original #34845 crash: non-spec query_len=1 + spec decode
+    "mixed_decode_and_spec_decode": GDNBuildTestCase(
+        seq_lens=[65, 20],
+        query_lens=[1, 3],
+        num_decode_draft_tokens=[-1, 2],
+        num_speculative_tokens=2,
+        expected_num_decodes=0,
+        expected_num_prefills=1,
+        expected_num_prefill_tokens=1,
+        expected_num_spec_decodes=1,
+    ),
+    # All requests are spec decodes — no reclassification needed
+    "pure_spec_decode": GDNBuildTestCase(
+        seq_lens=[50, 30],
+        query_lens=[3, 3],
+        num_decode_draft_tokens=[2, 2],
+        num_speculative_tokens=2,
+        expected_num_decodes=0,
+        expected_num_prefills=0,
+        expected_num_prefill_tokens=0,
+        expected_num_spec_decodes=2,
+    ),
+    # No speculative config at all — standard decode path
+    "pure_regular_decode": GDNBuildTestCase(
+        seq_lens=[40, 30, 20],
+        query_lens=[1, 1, 1],
+        num_decode_draft_tokens=None,
+        num_speculative_tokens=0,
+        expected_num_decodes=3,
+        expected_num_prefills=0,
+        expected_num_prefill_tokens=0,
+        expected_num_spec_decodes=0,
+    ),
+    # Multi-token prefill alongside spec decode — no decode to reclassify
+    "spec_decode_with_real_prefill": GDNBuildTestCase(
+        seq_lens=[100, 20],
+        query_lens=[50, 3],
+        num_decode_draft_tokens=[-1, 2],
+        num_speculative_tokens=2,
+        expected_num_decodes=0,
+        expected_num_prefills=1,
+        expected_num_prefill_tokens=50,
+        expected_num_spec_decodes=1,
+    ),
+    # All three types in one batch — decode gets reclassified
+    "prefill_decode_and_spec_decode": GDNBuildTestCase(
+        seq_lens=[100, 65, 20],
+        query_lens=[50, 1, 3],
+        num_decode_draft_tokens=[-1, -1, 2],
+        num_speculative_tokens=2,
+        expected_num_decodes=0,
+        expected_num_prefills=2,
+        expected_num_prefill_tokens=51,
+        expected_num_spec_decodes=1,
+    ),
+    # Multiple non-spec query_len=1 requests all reclassified
+    "multiple_decodes_reclassified": GDNBuildTestCase(
+        seq_lens=[40, 50, 60, 20],
+        query_lens=[1, 1, 1, 3],
+        num_decode_draft_tokens=[-1, -1, -1, 2],
+        num_speculative_tokens=2,
+        expected_num_decodes=0,
+        expected_num_prefills=3,
+        expected_num_prefill_tokens=3,
+        expected_num_spec_decodes=1,
+    ),
+    # Zero-length padded sequence excluded from counts
+    "zero_length_padding_with_spec": GDNBuildTestCase(
+        seq_lens=[16, 65, 20],
+        query_lens=[0, 1, 3],
+        num_decode_draft_tokens=[-1, -1, 2],
+        num_speculative_tokens=2,
+        expected_num_decodes=0,
+        expected_num_prefills=1,
+        expected_num_prefill_tokens=1,
+        expected_num_spec_decodes=1,
+    ),
+}
+
+
+def _create_gdn_builder(
+    num_speculative_tokens: int = 0,
+) -> GDNAttentionMetadataBuilder:
+    """Create a GDNAttentionMetadataBuilder with minimal config."""
+    vllm_config = create_vllm_config(block_size=BLOCK_SIZE)
+    if num_speculative_tokens > 0:
+        vllm_config.speculative_config = SpeculativeConfig(
+            method="ngram",
+            num_speculative_tokens=num_speculative_tokens,
+        )
+    mamba_spec = MambaSpec(
+        block_size=BLOCK_SIZE,
+        shapes=((16, 64),),
+        dtypes=(torch.float16,),
+    )
+    return GDNAttentionMetadataBuilder(
+        kv_cache_spec=mamba_spec,
+        layer_names=["layer.0"],
+        vllm_config=vllm_config,
+        device=DEVICE,
+    )
+
+
+def _build(
+    builder: GDNAttentionMetadataBuilder,
+    batch_spec: BatchSpec,
+    num_decode_draft_tokens: list[int] | None = None,
+) -> GDNAttentionMetadata:
+    """Build GDN attention metadata, optionally with spec-decode kwargs."""
+    common = create_common_attn_metadata(batch_spec, BLOCK_SIZE, DEVICE)
+    kwargs: dict = {}
+    if num_decode_draft_tokens is not None:
+        kwargs["num_decode_draft_tokens_cpu"] = torch.tensor(
+            num_decode_draft_tokens, dtype=torch.int32
+        )
+        kwargs["num_accepted_tokens"] = torch.ones(
+            batch_spec.batch_size, dtype=torch.int32, device=DEVICE
+        )
+    return builder.build(common_prefix_len=0, common_attn_metadata=common, **kwargs)
+
+
+@pytest.mark.parametrize(
+    "test_case", GDN_BUILD_TEST_CASES.values(), ids=GDN_BUILD_TEST_CASES.keys()
+)
+def test_gdn_build_classification(test_case: GDNBuildTestCase):
+    """Test that GDN metadata builder classifies requests correctly."""
+    builder = _create_gdn_builder(test_case.num_speculative_tokens)
+    batch = BatchSpec(seq_lens=test_case.seq_lens, query_lens=test_case.query_lens)
+    meta = _build(builder, batch, test_case.num_decode_draft_tokens)
+
+    assert meta.num_decodes == test_case.expected_num_decodes
+    assert meta.num_prefills == test_case.expected_num_prefills
+    assert meta.num_prefill_tokens == test_case.expected_num_prefill_tokens
+    assert meta.num_spec_decodes == test_case.expected_num_spec_decodes
+
+
+def test_has_initial_state_after_reclassification():
+    """After reclassification, num_prefills > 0 so the prefill kernel path
+    should compute has_initial_state. For the reclassified request with
+    context_lens > 0, the corresponding entry must be True."""
+    builder = _create_gdn_builder(num_speculative_tokens=2)
+    batch = BatchSpec(seq_lens=[65, 20], query_lens=[1, 3])
+    meta = _build(builder, batch, num_decode_draft_tokens=[-1, 2])
+
+    assert meta.num_prefills > 0, "reclassification should produce prefills"
+    assert meta.has_initial_state is not None
+    # req0 has context_lens = 65 - 1 = 64 > 0, so has_initial_state[0] = True
+    assert meta.has_initial_state[0].item() is True

vllm/v1/attention/backends/gdn_attn.py

@@ -220,6 +220,16 @@ class GDNAttentionMetadataBuilder(AttentionMetadataBuilder[GDNAttentionMetadata]
                 query_lens_cpu.sum().item() - num_prefill_tokens - num_decode_tokens
             )
 
+            # num_decodes and num_spec_decodes are mutually exclusive.
+            # Reclassify non-spec decodes as prefills when spec decodes
+            # exist — the prefill kernel handles 1-token sequences with
+            # initial state correctly, producing identical results.
+            if num_decodes > 0 and num_spec_decodes > 0:
+                num_prefills += num_decodes
+                num_prefill_tokens += num_decode_tokens
+                num_decodes = 0
+                num_decode_tokens = 0
+
             if num_prefills == 0 and num_decodes == 0:
                 spec_token_size = min(
                     num_spec_decodes * (self.num_spec + 1),