[Bugfix] Skip absent radix lock during cache cleanup (#37494)

python/sglang/srt/mem_cache/unified_radix_cache.py

@@ -903,6 +903,8 @@ class UnifiedRadixCache(BasePrefixCache):
         else:
             self.free_kv_row(req.kv, [(req.kv.cache_protected_len, kv_len_to_handle)])
 
+        # Synthetic profiling requests may own KV without locking a tree node.
+        if req.last_node is not None:
             self._dec_req_lock(req, skip_swa=req.swa_prefix_lock_released)
 
         if is_insert and result is not None and result.last_device_node is not None:

test/registered/unit/mem_cache/test_unified_radix_lock_ref.py

@@ -0,0 +1,45 @@
+"""CPU-only tests for UnifiedRadixCache request lock lifecycle."""
+
+import unittest
+from array import array
+from types import SimpleNamespace
+from unittest.mock import MagicMock
+
+import torch
+
+from sglang.srt.mem_cache.unified_radix_cache import UnifiedRadixCache
+from sglang.test.ci.ci_register import register_cpu_ci
+
+register_cpu_ci(est_time=2, suite="base-a-test-cpu")
+
+
+class TestUnifiedRadixLockRefScenarios(unittest.TestCase):
+    def test_no_insert_without_last_node_skips_lock_release(self):
+        cache = object.__new__(UnifiedRadixCache)
+        cache.session = MagicMock()
+        cache.session.try_cache_finished_req.return_value = False
+        cache.disable = False
+        cache.req_to_token_pool = MagicMock()
+        cache.req_to_token_pool.req_to_token = torch.arange(8).reshape(1, 8)
+        cache.free_kv_row = MagicMock()
+        cache._dec_req_lock = MagicMock()
+        cache._components_tuple = ()
+        cache.enable_session_radix_cache = False
+
+        kv = SimpleNamespace(req_pool_idx=0, cache_protected_len=0)
+        req = SimpleNamespace(
+            origin_input_ids=array("q", [1, 2, 3]),
+            output_ids=array("q"),
+            kv=kv,
+            last_node=None,
+            swa_prefix_lock_released=False,
+        )
+
+        cache.cache_finished_req(req, is_insert=False, kv_len_to_handle=3)
+
+        cache.free_kv_row.assert_called_once_with(kv, [(0, 3)])
+        cache._dec_req_lock.assert_not_called()
+
+
+if __name__ == "__main__":
+    unittest.main()