[n-gen] DO NOT repeatedly return finished child requests (#28591)

Signed-off-by: Jialin Ouyang <Jialin.Ouyang@gmail.com>

tests/v1/engine/test_parallel_sampling.py

+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+from vllm import SamplingParams
+from vllm.outputs import CompletionOutput
+from vllm.sampling_params import RequestOutputKind
+from vllm.v1.engine.parallel_sampling import ParentRequest
+
+
+def test_parent_request_to_output_stream() -> None:
+    parent_request = ParentRequest("parent_id", SamplingParams(n=2))
+    parent_request.child_requests = {"child_id_0", "child_id_1"}
+    output_0 = CompletionOutput(
+        index=0, text="child 0", token_ids=[], cumulative_logprob=None, logprobs=None
+    )
+    output_1 = CompletionOutput(
+        index=1, text="child 1", token_ids=[], cumulative_logprob=None, logprobs=None
+    )
+    # Request not finished
+    assert ("parent_id", [output_0], False) == parent_request.get_outputs(
+        "child_id_0", output_0
+    )
+    assert ("parent_id", [output_1], False) == parent_request.get_outputs(
+        "child_id_1", output_1
+    )
+    assert ("parent_id", [output_0], False) == parent_request.get_outputs(
+        "child_id_0", output_0
+    )
+    assert ("parent_id", [output_1], False) == parent_request.get_outputs(
+        "child_id_1", output_1
+    )
+
+    # output_1 finished
+    output_1.finish_reason = "ended"
+    assert ("parent_id", [output_0], False) == parent_request.get_outputs(
+        "child_id_0", output_0
+    )
+    assert ("parent_id", [output_1], False) == parent_request.get_outputs(
+        "child_id_1", output_1
+    )
+    # Finished output_1 had already returned, DO NOT returned again
+    assert ("parent_id", [output_0], False) == parent_request.get_outputs(
+        "child_id_0", output_0
+    )
+    assert parent_request.get_outputs("child_id_1", output_1) == (
+        "parent_id",
+        [],
+        False,
+    )
+
+    # output_0 finished
+    output_0.finish_reason = "ended"
+    assert ("parent_id", [output_0], True) == parent_request.get_outputs(
+        "child_id_0", output_0
+    )
+    assert parent_request.get_outputs("child_id_1", output_1) == ("parent_id", [], True)
+    # Finished output_0 had already returned, DO NOT returned again
+    assert parent_request.get_outputs("child_id_0", output_0) == ("parent_id", [], True)
+    assert parent_request.get_outputs("child_id_1", output_1) == ("parent_id", [], True)
+
+
+def test_parent_request_to_output_final_only() -> None:
+    parent_request = ParentRequest(
+        "parent_id", SamplingParams(n=2, output_kind=RequestOutputKind.FINAL_ONLY)
+    )
+    parent_request.child_requests = {"child_id_0", "child_id_1"}
+    output_0 = CompletionOutput(
+        index=0, text="child 0", token_ids=[], cumulative_logprob=None, logprobs=None
+    )
+    output_1 = CompletionOutput(
+        index=1, text="child 1", token_ids=[], cumulative_logprob=None, logprobs=None
+    )
+    # Request not finished, return nothing
+    assert parent_request.get_outputs("child_id_0", output_0) == (
+        "parent_id",
+        [],
+        False,
+    )
+    assert parent_request.get_outputs("child_id_1", output_1) == (
+        "parent_id",
+        [],
+        False,
+    )
+    # output_1 finished, but outputs won't be returned until all child requests finished
+    output_1.finish_reason = "ended"
+    assert parent_request.get_outputs("child_id_0", output_0) == (
+        "parent_id",
+        [],
+        False,
+    )
+    assert parent_request.get_outputs("child_id_1", output_1) == (
+        "parent_id",
+        [],
+        False,
+    )
+    # output_0 finished, as all child requests finished, the output would be returned
+    output_0.finish_reason = "ended"
+    assert ("parent_id", [output_0, output_1], True) == parent_request.get_outputs(
+        "child_id_0", output_0
+    )
+    assert ("parent_id", [output_0, output_1], True) == parent_request.get_outputs(
+        "child_id_1", output_1
+    )

vllm/v1/engine/parallel_sampling.py

@@ -97,12 +97,21 @@ class ParentRequest:
         child_request_id: str,
         completion_output: CompletionOutput,
     ) -> tuple[str, list[CompletionOutput], bool]:
+        already_finished_and_returned: bool = False
         if completion_output.finished():
+            if child_request_id in self.child_requests:
                 self.child_requests.remove(child_request_id)
+            else:
+                # child request ID is not available in child_requests
+                # which means the request had finished in previous
+                # batch step and returned to the client earlier
+                already_finished_and_returned = True
 
         if self.sampling_params.output_kind != RequestOutputKind.FINAL_ONLY:
-            # If streaming, just return the current output.
-            outputs = [completion_output]
+            # If streaming, just return the current output
+            #
+            # DO NOT output finished and already returned child request to client again
+            outputs = [] if already_finished_and_returned else [completion_output]
         else:
             # If not streaming, aggregate the n final outputs.
             self.output_aggregator[completion_output.index] = completion_output