[BugFix] Handle errors when preprocessing added requests (#30895)

Signed-off-by: Nick Hill <nhill@redhat.com>

.buildkite/test-pipeline.yaml

@@ -319,7 +319,10 @@ steps:
     # TODO: accuracy does not match, whether setting
     # VLLM_USE_FLASHINFER_SAMPLER or not on H100.
     - pytest -v -s v1/e2e
-    - pytest -v -s v1/engine
+    # Run this test standalone for now;
+    # need to untangle use (implicit) use of spawn/fork across the tests.
+    - pytest -v -s v1/engine/test_preprocess_error_handling.py
+    - pytest -v -s v1/engine --ignore v1/engine/test_preprocess_error_handling.py
 
 - label: V1 Test entrypoints # 35min
   timeout_in_minutes: 50

tests/v1/engine/test_preprocess_error_handling.py

@@ -0,0 +1,56 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+import pytest
+import torch.cuda
+
+from vllm import LLM, SamplingParams
+from vllm.v1.engine import EngineCoreRequest
+from vllm.v1.engine.core import EngineCore
+
+MODEL_NAME = "hmellor/tiny-random-LlamaForCausalLM"
+
+
+def test_preprocess_error_handling(monkeypatch: pytest.MonkeyPatch):
+    """Test that preprocessing errors are handled gracefully."""
+
+    assert not torch.cuda.is_initialized(), (
+        "fork needs to be used for the engine "
+        "core process and this isn't possible if cuda is already initialized"
+    )
+
+    # Store original method to call for non-failing requests
+    original_preprocess = EngineCore.preprocess_add_request
+
+    # Monkeypatch to make preprocess_add_request raise an exception
+    # only for requests with "FAIL" in the first token
+    def conditional_failing_preprocess(self, request: EngineCoreRequest):
+        # Fail if the first token id is 333
+        if request.prompt_token_ids and request.prompt_token_ids[0] == 333:
+            raise ValueError("Simulated preprocessing error!")
+        return original_preprocess(self, request)
+
+    monkeypatch.setattr(
+        EngineCore, "preprocess_add_request", conditional_failing_preprocess
+    )
+
+    llm = LLM(model=MODEL_NAME)
+
+    # Create a failing request by crafting a request with an invalid token
+    # We need to use a direct approach since LLM.generate tokenizes for us
+    from vllm.inputs import TokensPrompt
+
+    # This should raise an exception due to the preprocessing failure
+    # Special token id to trigger the failure
+    failing_prompt = TokensPrompt(prompt_token_ids=[333])
+    outputs = llm.generate(failing_prompt, SamplingParams(max_tokens=10))  # type: ignore
+    assert len(outputs) == 1
+    assert len(outputs[0].outputs[0].token_ids) == 0
+    assert outputs[0].finished
+    assert outputs[0].outputs[0].finish_reason == "error"
+
+    # Verify the engine is still functional with a normal request
+    outputs = llm.generate("Hello, my name is", SamplingParams(max_tokens=10))
+    assert len(outputs) == 1
+    assert len(outputs[0].outputs[0].token_ids) > 0
+    assert outputs[0].outputs[0].finish_reason in ("stop", "length")

vllm/v1/engine/core.py

@@ -43,9 +43,11 @@ from vllm.v1.core.kv_cache_utils import (
 from vllm.v1.core.sched.interface import SchedulerInterface
 from vllm.v1.core.sched.output import SchedulerOutput
 from vllm.v1.engine import (
+    EngineCoreOutput,
     EngineCoreOutputs,
     EngineCoreRequest,
     EngineCoreRequestType,
+    FinishReason,
     ReconfigureDistributedRequest,
     ReconfigureRankType,
     UtilityOutput,
@@ -1055,9 +1057,14 @@ class EngineCoreProc(EngineCore):
                     request_type = EngineCoreRequestType(bytes(type_frame.buffer))
 
                     # Deserialize the request data.
+                    request: Any
                     if request_type == EngineCoreRequestType.ADD:
-                        request = add_request_decoder.decode(data_frames)
+                        req: EngineCoreRequest = add_request_decoder.decode(data_frames)
-                        request = self.preprocess_add_request(request)
+                        try:
+                            request = self.preprocess_add_request(req)
+                        except Exception:
+                            self._handle_request_preproc_error(req)
+                            continue
                     else:
                         request = generic_decoder.decode(data_frames)
 
@@ -1141,6 +1148,30 @@ class EngineCoreProc(EngineCore):
                     # Limit the number of buffers to reuse.
                     reuse_buffers.append(buffer)
 
+    def _handle_request_preproc_error(self, request: EngineCoreRequest) -> None:
+        """Log and return a request-scoped error response for exceptions raised
+        from the add request preprocessing in the input socket processing thread.
+        """
+        logger.exception(
+            "Unexpected error pre-processing request %s", request.request_id
+        )
+        self.output_queue.put_nowait(
+            (
+                request.client_index,
+                EngineCoreOutputs(
+                    engine_index=self.engine_index,
+                    finished_requests={request.request_id},
+                    outputs=[
+                        EngineCoreOutput(
+                            request_id=request.request_id,
+                            new_token_ids=[],
+                            finish_reason=FinishReason.ERROR,
+                        )
+                    ],
+                ),
+            )
+        )
+
 
 class DPEngineCoreProc(EngineCoreProc):
     """ZMQ-wrapper for running EngineCore in background process