[Fix] memory leak by overlap + retract (#11981)

Co-authored-by: Liangsheng Yin <lsyincs@gmail.com>

python/sglang/srt/mem_cache/chunk_cache.py

@@ -32,6 +32,8 @@ class ChunkCache(BasePrefixCache):
         else:
             self.device = torch.device("cpu")
 
+        self.protected_size_ = 0
+
     # NOTE (csy): this is to determine if a cache has prefix matching feature.
     # Chunk cache always return True to indicate no prefix matching.
     # TODO (csy): Using a prefix cache trait to replace this
@@ -57,11 +59,13 @@ class ChunkCache(BasePrefixCache):
         ]
         self.req_to_token_pool.free(req.req_pool_idx)
         self.token_to_kv_pool_allocator.free(kv_indices)
+        self.protected_size_ -= len(req.prefix_indices)
 
     def cache_unfinished_req(self, req: Req, chunked=False):
         kv_indices = self.req_to_token_pool.req_to_token[
             req.req_pool_idx, : len(req.fill_ids)
         ]
+        self.protected_size_ += len(kv_indices) - len(req.prefix_indices)
 
         # `req.prefix_indices` will be used in `PrefillAdder::add_chunked_req` later
         req.prefix_indices = kv_indices.to(dtype=torch.int64, copy=True)
@@ -75,6 +79,9 @@ class ChunkCache(BasePrefixCache):
     def dec_lock_ref(self, node: Any, swa_uuid_for_lock: Optional[str] = None):
         return 0
 
+    def protected_size(self):
+        return self.protected_size_
+
     def pretty_print(self):
         return ""