[Fix] Chain the seq_lens publish event records so prebuilt seeding keeps the forward fence (#30435)

python/sglang/srt/managers/overlap_utils.py

@@ -322,8 +322,14 @@ class FutureMap:
         self.new_seq_lens_buf[indices] = new_seq_lens.to(self.new_seq_lens_buf.dtype)
         # Only spec_v2 needs the event; it gates the seq_lens D2H on the private stream.
         if self.spec_algo.is_some():
+            device_module = torch.get_device_module(self.device)
             if self.publish_ready is None:
-                self.publish_ready = torch.get_device_module(self.device).Event()
+                self.publish_ready = device_module.Event()
+            else:
+                # Chain the records: event fire implies every prior publish is
+                # visible, so an off-forward-stream publish (PD-decode prebuilt
+                # seeding) cannot drop the in-flight forward's fence.
+                device_module.current_stream().wait_event(self.publish_ready)
             self.publish_ready.record()
 
     def stash(self, future_indices: torch.Tensor, payload: RelayPayload) -> None:

python/sglang/srt/speculative/eagle_disaggregation.py

@@ -61,6 +61,8 @@ def build_eagle_disagg_draft_input(
 
     if batch.enable_overlap:
         spec_info.future_indices = batch.req_pool_indices
+        # Seed the relay buf with the known seq_lens; publish's chained record
+        # keeps the in-flight forward's fence intact (see FutureMap.publish).
         future_map.publish(spec_info.future_indices, batch.seq_lens)
         future_map.stash(
             spec_info.future_indices, RelayPayload.from_draft_input(spec_info)