fix: treat CP HiCache metadata as backup state

test/registered/unit/mem_cache/test_cp_hicache_metadata.py

@@ -10,6 +10,7 @@ for _mod in ("sgl_kernel", "sgl_kernel.kvcacheio"):
     if _mod not in sys.modules:
         sys.modules[_mod] = MagicMock()
 
+from sglang.srt.mem_cache.base_prefix_cache import EvictParams
 from sglang.srt.mem_cache.hiradix_cache import CpHiCacheNodeMetadata, HiRadixCache
 from sglang.srt.mem_cache.radix_cache import RadixKey, TreeNode
 from sglang.test.ci.ci_register import register_cpu_ci
@@ -183,6 +184,13 @@ class FakeEvictionStrategy:
         return 0
 
 
+class FakeEvictDeviceController:
+    write_policy = "write_through"
+
+    def evict_device(self, device_indices):
+        return len(device_indices)
+
+
 class TestHiRadixCacheCPBackup(CustomTestCase):
     def test_node_backuped_uses_cp_metadata(self):
         cache = HiRadixCache.__new__(HiRadixCache)
@@ -260,6 +268,43 @@ class TestHiRadixCacheCPBackup(CustomTestCase):
         self.assertNotIn(1, root.children)
         self.assertEqual(node.host_len, 16)
 
+    def test_evict_demotes_cp_backed_node_without_deleting_radix_child(self):
+        cache = HiRadixCache.__new__(HiRadixCache)
+        cache._uses_cp_hicache = True
+        cache.cache_controller = FakeEvictDeviceController()
+        cache.evictable_leaves = set()
+        cache.evictable_host_leaves = set()
+        cache.eviction_strategy = FakeEvictionStrategy()
+        cache.evictable_size_ = 4
+        cache.protected_size_ = 0
+        cache._record_remove_event = lambda node: (_ for _ in ()).throw(
+            AssertionError("must not delete backed radix child")
+        )
+
+        root = TreeNode()
+        root.key = RadixKey(token_ids=[], extra_key=None)
+        root.value = []
+        cache.root_node = root
+        node = TreeNode()
+        node.parent = root
+        node.key = RadixKey(token_ids=[1, 2, 3, 4], extra_key=None)
+        node.value = torch.arange(4, dtype=torch.int64)
+        node.host_len = 4
+        node.cp_hicache = CpHiCacheNodeMetadata(
+            logical_len=4,
+            owned_positions=torch.tensor([0], dtype=torch.int64),
+            host_indices=torch.tensor([55], dtype=torch.int64),
+        )
+        root.children[1] = node
+        cache.evictable_leaves.add(node)
+
+        cache.evict(EvictParams(num_tokens=4))
+
+        self.assertIn(1, root.children)
+        self.assertIsNone(node.value)
+        self.assertIsNotNone(node.cp_hicache)
+        self.assertEqual(node.cp_hicache.host_indices.tolist(), [55])
+
 
 if __name__ == "__main__":
     unittest.main()