Fix overlap prebuilt row reuse race (#35748)

python/sglang/srt/disaggregation/decode.py

@@ -2632,6 +2632,10 @@ class SchedulerDisaggregationDecodeMixin:
 
         # construct fake completed prefill
         new_batch.prepare_for_prebuilt()
+        if self.enable_overlap:
+            # A finished request can still have one redundant forward in flight.
+            # Drain it before a prebuilt request seeds a potentially reused row.
+            self.schedule_stream.wait_stream(self.forward_stream)
         new_batch.process_prebuilt(self.future_map)
 
         return new_batch

python/sglang/srt/managers/overlap_utils.py

@@ -523,14 +523,8 @@ class FutureMap:
             self.confidence_relay.scatter(indices, confidence)
         # 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 = 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 = torch.get_device_module(self.device).Event()
             self.publish_ready.record()
             self._publish_fresh = True
         if publish_confidence: