fix hicache write back (#5543)

python/sglang/srt/mem_cache/hiradix_cache.py

@@ -92,7 +92,7 @@ class HiRadixCache(RadixCache):
             self.ongoing_write_through[node.id] = node
             self.inc_lock_ref(node)
         else:
-            return None
+            return 0
 
         return len(host_indices)
 
@@ -153,6 +153,7 @@ class HiRadixCache(RadixCache):
             if x.host_value is None:
                 if self.cache_controller.write_policy == "write_back":
                     num_evicted += self.write_backup(x)
+                    pending_nodes.append(x)
                 elif self.cache_controller.write_policy == "write_through_selective":
                     num_evicted += self._evict_write_through_selective(x)
                 else:
@@ -177,6 +178,9 @@ class HiRadixCache(RadixCache):
             while len(self.ongoing_write_through) > 0:
                 self.writing_check()
                 time.sleep(0.1)
+            for node in pending_nodes:
+                assert node.host_value is not None
+                self._evict_write_through(node)
 
     def _evict_write_through(self, node: TreeNode):
         # evict a node already written to host