[HiCache] Publish a host store event for storage-prefetch refills (#38486)

Co-authored-by: Shuwen Wang <47200617+alphabetc1@users.noreply.github.com>

rust/sglang-radix-tree/src/tests/unified_tree_core.rs

@@ -3634,6 +3634,40 @@ fn insert_host_attaches_a_host_only_leaf_under_the_root() {
     tc.sanity_check(&[], &[]);
 }
 
+#[test]
+fn insert_host_publishes_a_host_store_event() {
+    // A storage-prefetch refill has no write-through ack to publish it.
+    let mut tc = events_core(2);
+    let root = tc.arena.root();
+    let key = vec![1i64, 2, 7, 8];
+    let hashes = crate::node::get_hash_str::<Vec<i64>>(&key, None, 2);
+    let result = tc
+        .insert_host(
+            tc.arena.node(root).id,
+            /* extra_key = */ None,
+            key.clone(),
+            Tensor::from_slice(&[100i64, 101, 102, 103]),
+            hashes.clone(),
+        )
+        .expect("live test node");
+    assert!(!result.host_insert_dropped);
+    assert!(result.inserted_host_node.is_some());
+    assert_eq!(
+        tc.take_events(),
+        vec![KvCacheEvent::BlockStored {
+            block_hashes: hashes
+                .iter()
+                .map(|hash| crate::node::hash_str_to_int64(hash))
+                .collect(),
+            parent_block_hash: None,
+            token_ids: key,
+            block_size: 2,
+            medium: StorageMedium::Cpu,
+            cache_salt: None,
+        }]
+    );
+}
+
 #[test]
 fn insert_host_allows_a_suffix_under_an_unbacked_write_back_parent() {
     let mut tc = core();

rust/sglang-radix-tree/src/unified_tree_core.rs

@@ -3107,6 +3107,7 @@ impl<K: ChildKeyType> UnifiedTreeCore<K> {
         self.update_evictable_leaf_sets_(new_node_id);
         self.update_evictable_leaf_sets_(node_id);
         result.inserted_host_node = Some(self.arena.node(new_node_id).id);
+        self.record_store_event_(new_node_id, StorageMedium::Cpu);
         Ok(result)
     }