[HiCache] Fix hicache host memory leak by bounding PP-sync work_list (#28916)

test/registered/unit/mem_cache/test_hiradix_pp_sync_drain.py

@@ -0,0 +1,50 @@
+"""Unit test for HiRadixCache._drain_async_work PP-sync backpressure."""
+
+import unittest
+
+from sglang.srt.mem_cache.hiradix_cache import HiRadixCache
+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=1, suite="base-a-test-cpu")
+
+
+class _FakeWork:
+    def __init__(self):
+        self.waited = False
+
+    def wait(self):
+        self.waited = True
+
+
+class _Holder:
+    """Minimal carrier exposing only what _drain_async_work touches."""
+
+
+class TestPPSyncDrain(unittest.TestCase):
+    def _drain_fns(self):
+        return (HiRadixCache._drain_async_work, UnifiedRadixCache._drain_async_work)
+
+    def test_drain_waits_all_and_clears(self):
+        for drain in self._drain_fns():
+            holder = _Holder()
+            works = [_FakeWork(), _FakeWork(), _FakeWork()]
+            holder.work_list = list(works)
+
+            drain(holder)
+
+            self.assertTrue(all(w.waited for w in works))
+            self.assertEqual(holder.work_list, [])
+
+    def test_drain_empty_is_noop(self):
+        for drain in self._drain_fns():
+            holder = _Holder()
+            holder.work_list = []
+
+            drain(holder)
+
+            self.assertEqual(holder.work_list, [])
+
+
+if __name__ == "__main__":
+    unittest.main()