Add vLLM v0.18.1 source tree with KV transfer abort fix

third_party/vllm/ now tracked in git for direct patch management.
Based on vLLM v0.18.1 release with one patch applied:

  vllm/v1/core/sched/scheduler.py:
    Replace fatal assert with graceful skip when KV transfer callback
    arrives for an already-aborted request during PD disaggregated serving.

Future vLLM modifications should be made directly in third_party/vllm/
and committed normally. The patches/ directory is kept as documentation
of what changed from upstream.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

third_party/vllm/tests/v1/engine/test_parallel_sampling.py

@@ -0,0 +1,83 @@
+# 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 import EngineCoreRequest
+from vllm.v1.engine.parallel_sampling import ParentRequest
+
+
+def test_parent_request_to_output_stream() -> None:
+    parent_request = ParentRequest(make_request(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 ([output_0], False) == parent_request.get_outputs("child_id_0", output_0)
+    assert ([output_1], False) == parent_request.get_outputs("child_id_1", output_1)
+    assert ([output_0], False) == parent_request.get_outputs("child_id_0", output_0)
+    assert ([output_1], False) == parent_request.get_outputs("child_id_1", output_1)
+
+    # output_1 finished
+    output_1.finish_reason = "ended"
+    assert ([output_0], False) == parent_request.get_outputs("child_id_0", output_0)
+    assert ([output_1], False) == parent_request.get_outputs("child_id_1", output_1)
+    # Finished output_1 had already returned, DO NOT returned again
+    assert ([output_0], False) == parent_request.get_outputs("child_id_0", output_0)
+    assert parent_request.get_outputs("child_id_1", output_1) == ([], False)
+
+    # output_0 finished
+    output_0.finish_reason = "ended"
+    assert ([output_0], True) == parent_request.get_outputs("child_id_0", output_0)
+    assert parent_request.get_outputs("child_id_1", output_1) == ([], True)
+    # Finished output_0 had already returned, DO NOT returned again
+    assert parent_request.get_outputs("child_id_0", output_0) == ([], True)
+    assert parent_request.get_outputs("child_id_1", output_1) == ([], True)
+
+
+def test_parent_request_to_output_final_only() -> None:
+    parent_request = ParentRequest(
+        make_request(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) == ([], False)
+    assert parent_request.get_outputs("child_id_1", output_1) == ([], 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) == ([], False)
+    assert parent_request.get_outputs("child_id_1", output_1) == ([], False)
+    # output_0 finished, as all child requests finished, the output would be returned
+    output_0.finish_reason = "ended"
+    assert ([output_0, output_1], True) == parent_request.get_outputs(
+        "child_id_0", output_0
+    )
+    assert ([output_0, output_1], True) == parent_request.get_outputs(
+        "child_id_1", output_1
+    )
+
+
+def make_request(sampling_params: SamplingParams) -> EngineCoreRequest:
+    return EngineCoreRequest(
+        request_id="parent_id",
+        external_req_id="ext_parent_id",
+        prompt_token_ids=None,
+        mm_features=None,
+        sampling_params=sampling_params,
+        pooling_params=None,
+        arrival_time=0.0,
+        lora_request=None,
+        cache_salt=None,
+        data_parallel_rank=None,
+    )