[Bugfix] Spec decode + structured output + spec model max len edge case (#28298)

Signed-off-by: Andy Lo <andy@mistral.ai>

tests/v1/spec_decode/test_max_len.py

@@ -7,6 +7,7 @@ import pytest
 from tests.utils import get_attn_backend_list_based_on_platform
 from vllm import LLM, SamplingParams
 from vllm.platforms import current_platform
+from vllm.sampling_params import StructuredOutputsParams
 
 _PROMPTS = [
     "1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1",
@@ -56,8 +57,34 @@ def test_eagle_max_len(
                 "method": "eagle",
                 "model": "yuhuili/EAGLE-LLaMA3-Instruct-8B",
                 "num_speculative_tokens": num_speculative_tokens,
+                "max_model_len": 80,
             },
-            max_model_len=100,
+            max_model_len=200,
         )
-        sampling_params = SamplingParams(max_tokens=100, ignore_eos=True)
-        llm.generate(_PROMPTS, sampling_params)
+        sampling_params = SamplingParams(max_tokens=200, ignore_eos=True)
+        outputs = llm.generate(_PROMPTS, sampling_params)
+        for o in outputs:
+            assert o.outputs[0].finish_reason == "length", (
+                "This test is only meaningful if the output "
+                "is truncated due to max length"
+            )
+
+        sampling_params = SamplingParams(
+            max_tokens=200,
+            structured_outputs=StructuredOutputsParams(
+                regex="^" + "a b c d e " * 15 + "$"
+            ),
+        )
+        output = llm.generate(_PROMPTS, sampling_params)
+        for o in output:
+            assert o.prompt_token_ids is not None
+            assert (
+                len(o.prompt_token_ids)
+                < 80
+                < len(o.prompt_token_ids) + len(o.outputs[0].token_ids)
+                < 200
+            ), (
+                "This test is only meaningful if the output "
+                "is longer than the eagle max length"
+            )
+            assert o.outputs[0].text == "a b c d e " * 15