[BUGFIX] Raise an error for no draft token case when draft_tp>1 (#6369)

tests/spec_decode/e2e/test_integration_dist_tp4.py

@@ -58,3 +58,65 @@ def test_draft_model_tp_lt_target_model_tp4(test_llm_generator,
                                          batch_size,
                                          max_output_len=32,
                                          force_output_len=True)
+
+
+@pytest.mark.skipif(torch.cuda.device_count() < 4,
+                    reason="Need at least 4 GPUs to run the test.")
+@pytest.mark.parametrize(
+    "common_llm_kwargs",
+    [{
+        "model": "JackFram/llama-160m",
+
+        # Skip cuda graph recording for fast test.
+        "enforce_eager": True,
+
+        # Required for spec decode.
+        "use_v2_block_manager": True,
+        "tensor_parallel_size": 4,
+
+        # Use AsyncLLM engine, so that the engine runs in its own process.
+        # Otherwise, since vLLM does not follow true SPMD, the test runner
+        # process will have both the engine and the rank0 worker. NCCL is not
+        # cleaned up properly, and its server host thread leaks, causing the
+        # second run of the test to fail with internal NCCL error.
+        "use_async": True,
+    }])
+@pytest.mark.parametrize("per_test_common_llm_kwargs", [{}])
+@pytest.mark.parametrize("baseline_llm_kwargs", [{}])
+@pytest.mark.parametrize(
+    "test_llm_kwargs",
+    [
+        {
+            "speculative_model": "JackFram/llama-68m",
+            "num_speculative_tokens": 5,
+
+            # Artificially limit the draft model max model len; this forces vLLM
+            # to skip speculation once the sequences grow beyond 32-k tokens.
+            "speculative_max_model_len": 32,
+        },
+    ])
+@pytest.mark.parametrize("batch_size", [8])
+@pytest.mark.parametrize(
+    "output_len",
+    [
+        # This must be a good bit larger than speculative_max_model_len so that
+        # we can test the case where all seqs are skipped, but still small to
+        # ensure fast test.
+        64,
+    ])
+@pytest.mark.parametrize("seed", [1])
+def test_skip_speculation(baseline_llm_generator, test_llm_generator,
+                          batch_size: int, output_len: int):
+    """Verify job failure with RuntimeError when all sequences skip speculation.
+    We do this by setting the max model len of the draft model to an
+    artificially low value, such that when the sequences grow beyond it, they
+    are skipped in speculative decoding.
+
+    TODO: fix it to pass without raising Error. (#5814)
+    """
+    with pytest.raises(RuntimeError):
+        run_greedy_equality_correctness_test(baseline_llm_generator,
+                                             test_llm_generator,
+                                             batch_size,
+                                             max_output_len=output_len,
+                                             force_output_len=True)