Fix PP is_fully_idle missing in-flight microbatches (#27446)

test/registered/chunked_prefill/test_scripted_core_4gpu.py

@@ -8,6 +8,7 @@ from sglang.test.scripted_runtime_chunked_helpers import (
     SMALL_MODEL,
     base_engine_kwargs,
     run_until_all_finished,
+    run_until_finished,
 )
 
 register_cuda_ci(est_time=900, stage="extra-b", runner_config="4-gpu-h100")
@@ -55,6 +56,40 @@ class TestScriptedPpChunkSweep(ScriptedTestCase):
                 f"req {r.rid!r} did not finish"
             )
 
+    def test_pp_flush_cache_during_inflight_chunk_results(self):
+        """flush_cache landing after the last chunk dispatch but before its batch result is processed must not corrupt the radix tree."""
+        self.server.execute_script(self._script_flush_during_inflight_chunk_results)
+
+    @staticmethod
+    def _script_flush_during_inflight_chunk_results(t: ScriptedContext):
+        scheduler = t.scheduler
+        r = t.start_req(prompt_len=2 * _CHUNK_SIZE - 3, max_new_tokens=2)
+
+        # Wait for the window where the queues and the current microbatch slot
+        # bindings are all clear, yet dispatched chunk batch results are still
+        # in flight in the PP pipeline, then post flush_cache into that window.
+        flushed = False
+        for _ in range(DEFAULT_MAX_STEPS):
+            in_flight = any(
+                mb is not None and not mb.is_empty() for mb in scheduler.mbs
+            )
+            queues_clear = (
+                scheduler.chunked_req is None
+                and len(scheduler.waiting_queue) == 0
+                and all(x.is_empty() for x in scheduler.running_mbs)
+                and (scheduler.cur_batch is None or scheduler.cur_batch.is_empty())
+                and (scheduler.last_batch is None or scheduler.last_batch.is_empty())
+            )
+            if in_flight and queues_clear:
+                t.flush_cache()
+                flushed = True
+                break
+            yield
+        assert flushed, "never observed in-flight chunk results with clear queues"
+
+        yield from run_until_finished(r, max_steps=DEFAULT_MAX_STEPS)
+        assert r.finished, f"req {r.rid!r} did not finish after flush_cache"
+
 
 if __name__ == "__main__":
     unittest.main()