fa3: sync-free eagle spec via fixed-window draft-extend metadata (#31364)

python/sglang/srt/layers/attention/flashattention_backend.py

@@ -824,10 +824,15 @@ class FlashAttentionBackend(AttentionBackend):
                 forward_batch.req_pool_indices, : metadata.max_seq_len_k
             ]
 
-            if (
-                any(forward_batch.extend_prefix_lens_cpu)
-                or forward_batch.forward_mode.is_draft_extend_v2()
-            ):
+            if forward_batch.forward_mode.is_draft_extend_v2():
+                # Fixed-q window: the host max is a config constant, and
+                # extend_seq_lens_cpu may be None on the GPU-only spec path.
+                extend_seq_lens = forward_batch.extend_seq_lens
+                metadata.max_seq_len_q = self.speculative_num_draft_tokens
+                metadata.cu_seqlens_q = torch.nn.functional.pad(
+                    torch.cumsum(extend_seq_lens, dim=0, dtype=torch.int32), (1, 0)
+                )
+            elif any(forward_batch.extend_prefix_lens_cpu):
                 extend_seq_lens = forward_batch.extend_seq_lens
                 metadata.max_seq_len_q = max(forward_batch.extend_seq_lens_cpu)
                 metadata.cu_seqlens_q = torch.nn.functional.pad(
@@ -3022,6 +3027,10 @@ class FlashAttentionBackend(AttentionBackend):
 
 
 class FlashAttentionMultiStepBackend:
+    # Read by decide_needs_cpu_seq_lens (a missing flag defaults to True);
+    # the multi-step draft and draft-extend paths are device-side.
+    needs_cpu_seq_lens: bool = False
+
     def __init__(
         self,
         model_runner: ModelRunner,

python/sglang/srt/model_executor/runner/decode_cuda_graph_runner.py

@@ -162,7 +162,11 @@ def build_replay_fb_view(
             if forward_batch.seq_lens_sum is None
             else forward_batch.seq_lens_sum + (bs - raw_bs) * seq_len_fill_value
         ),
-        seq_lens_cpu=buffers.seq_lens_cpu[:bs],
+        # Propagate mirror absence: the pinned buffer is not refreshed when the
+        # batch has no CPU mirror; a stale non-None tensor defeats None-guards.
+        seq_lens_cpu=(
+            None if forward_batch.seq_lens_cpu is None else buffers.seq_lens_cpu[:bs]
+        ),
         num_padding=bs - raw_bs,
         encoder_lens=buffers.encoder_lens[:bs] if is_encoder_decoder else None,
         out_cache_loc=getattr(forward_batch, "out_cache_loc", None),

python/sglang/srt/speculative/eagle_draft_extend_cuda_graph_runner.py

@@ -584,7 +584,10 @@ class EAGLEDraftExtendCudaGraphRunner(DecodeCudaGraphRunner):
             req_pool_indices=buffers.req_pool_indices,
             seq_lens=buffers.seq_lens,
             seq_lens_sum=seq_lens_sum,
-            seq_lens_cpu=buffers.seq_lens_cpu,
+            # Mirror absence must survive replay (stale buffer defeats None-guards).
+            seq_lens_cpu=(
+                None if forward_batch.seq_lens_cpu is None else buffers.seq_lens_cpu
+            ),
             encoder_lens=None,
             out_cache_loc=buffers.out_cache_loc[:num_tokens],
             out_cache_loc_dsv4=getattr(forward_batch, "out_cache_loc_dsv4", None),