[Fix] Serialize FanOutCommunicator queueing calls with a FIFO-fair asyncio.Lock (#30606)

test/registered/unit/managers/test_fanout_communicator.py

@@ -0,0 +1,51 @@
+"""Unit tests for FanOutCommunicator -- no server, no model loading."""
+
+import asyncio
+import unittest
+
+from sglang.srt.managers.communicator import FanOutCommunicator
+from sglang.test.ci.ci_register import register_cpu_ci
+from sglang.test.test_utils import CustomTestCase
+
+register_cpu_ci(est_time=5, suite="base-a-test-cpu")
+
+
+class TestQueueingCall(CustomTestCase):
+    def test_concurrent_caller_cannot_bypass_queue(self):
+        """A new caller arriving in the wakeup window must not overtake a
+        queued caller (this interleaving used to raise AssertionError and
+        return 500 on concurrent /server_info requests)."""
+
+        async def scenario():
+            sent = []
+            comm = FanOutCommunicator(send=sent.append, fan_out=1, mode="queueing")
+
+            # A in-flight, B queued behind it.
+            task_a = asyncio.create_task(comm("A"))
+            await asyncio.sleep(0)
+            task_b = asyncio.create_task(comm("B"))
+            await asyncio.sleep(0)
+
+            # Complete A, then create C before A's wakeup runs, so C's first
+            # step lands between A's cleanup and B's wakeup.
+            comm.handle_recv("resp-A")
+            task_c = asyncio.create_task(comm("C"))
+
+            # Drive to completion: feed a response whenever one is in flight.
+            tasks = [task_a, task_b, task_c]
+            for _ in range(100):
+                if all(t.done() for t in tasks):
+                    break
+                if comm._result_event is not None and not comm._result_event.is_set():
+                    comm.handle_recv(f"resp-{len(sent)}")
+                await asyncio.sleep(0)
+
+            # All callers complete without error, in strict FIFO order.
+            await asyncio.gather(*tasks)
+            self.assertEqual(sent, ["A", "B", "C"])
+
+        asyncio.run(scenario())
+
+
+if __name__ == "__main__":
+    unittest.main()