[Unified] Fix UnifiedRadixCache write_backup issue in write-back mode(#27108)

Co-authored-by: hzh0425 <hzh0425@apache.org>

python/sglang/srt/mem_cache/unified_radix_cache.py

@@ -1424,7 +1424,9 @@ class UnifiedRadixCache(KVCacheEventMixin, BasePrefixCache):
                 self.cache_controller is not None
                 and self.cache_controller.write_policy == "write_back"
             ):
-                self.write_backup(node, write_back=True)
+                written = self.write_backup(node, write_back=True)
+                if written == 0:
+                    return
                 self.writing_check(write_back=True)
                 self._evict_to_host(node, tracker)
                 return

test/registered/unit/mem_cache/test_unified_radix_cache_unittest.py

@@ -2452,6 +2452,36 @@ class UnifiedRadixCacheSuite:
             [conv[:, mamba_indices].float().cpu().clone() for conv in mamba_cache.conv],
         )
 
+    def test_hicache_evict_device_leaf_aborts_demote_when_backup_fails(self):
+        """when write_backup cannot allocate host pool,
+        _evict_device_leaf should not evict it to host."""
+        if self._skip_unsupported_hicache_test():
+            return
+        tree, allocator, req_to_token_pool = build_fixture(self.cfg)
+        self._init_hicache(tree, write_policy="write_back")
+        ct = ComponentType.FULL
+
+        seq = self._make_seq(1, 2)
+        self._insert(tree, allocator, req_to_token_pool, seq)
+        m = tree.match_prefix(MatchPrefixParams(key=RadixKey(array("q", seq))))
+        node = m.last_device_node
+        self.assertIsNot(node, tree.root_node)
+        self.assertFalse(node.backuped)
+        self.assertFalse(node.evicted)
+
+        tracker = {c: 0 for c in tree.tree_components}
+        with mock.patch.object(tree, "write_backup", return_value=0):
+            tree._evict_device_leaf(node, tracker)
+
+        self.assertFalse(node.evicted)
+        self.assertIsNotNone(node.component_data[ct].value)
+        self.assertIsNone(node.component_data[ct].host_value)
+
+        with self.assertRaises(AssertionError):
+            tree._evict_to_host(node, {c: 0 for c in tree.tree_components})
+
+        tree.sanity_check()
+
     def test_hicache_node_states(self):
         """Verify device-only to device+host transition after real backup."""
         if self._skip_unsupported_hicache_test():