[HiCache] Detect for double-free in HostKVCache (#29310)

python/sglang/srt/mem_cache/pool_host/base.py

@@ -230,6 +230,9 @@ class HostKVCache(abc.ABC):
             (self.size,), dtype=torch.uint8, device=self.device
         )
         self.free_slots = torch.arange(self.size, dtype=torch.int64)
+        # Per-slot flag used to detect double-free.
+        # slot_used[k] is true if slot k is allocated.
+        self.slot_used = torch.zeros(self.size, dtype=torch.bool)
 
     def available_size(self):
         return len(self.free_slots)
@@ -245,9 +248,21 @@ class HostKVCache(abc.ABC):
         select_index = self.free_slots[:need_size]
         self.free_slots = self.free_slots[need_size:]
 
+        assert not self.slot_used[select_index].any(), (
+            f"Double-alloc detected: slots already allocated: "
+            f"{select_index[self.slot_used[select_index]].tolist()}."
+        )
+        self.slot_used[select_index] = True
+
         return select_index
 
     @synchronized
     def free(self, indices: torch.Tensor) -> int:
-        self.free_slots = torch.cat([self.free_slots, indices.cpu()])
+        indices_cpu = indices.cpu()
+        assert self.slot_used[indices_cpu].all(), (
+            f"Double-free detected: slots not currently allocated: "
+            f"{indices_cpu[~self.slot_used[indices_cpu]].tolist()}."
+        )
+        self.slot_used[indices_cpu] = False
+        self.free_slots = torch.cat([self.free_slots, indices_cpu])
         return len(indices)