[Bug]: ValueError: data_parallel_rank is not applicable in headless mode in commit 657f2f3

[bbenshab]

Your current environment

I'm working on a dev setup, so collect_env.py is not an option; however, I found the root cause in a commit from 2 hours ago, which explains why I had no issues 2 hours ago.

🐛 Describe the bug

When running vLLM in headless mode on a worker pod in a Kubernetes cluster, the command fails with the following error:

ValueError: data_parallel_rank is not applicable in headless mode

despite not explicitly setting the --data-parallel-rank or --data-parallel-start-rank flags in the command. The error originates from the run_headless function in vllm/entrypoints/cli/serve.py, specifically at this check:

https://github.com/vllm-project/vllm/blame/657f2f301a431542a731719fa8c6326deacc317d/vllm/entrypoints/cli/serve.py#L130
`

if parallel_config.data_parallel_rank is not None:
    raise ValueError("data_parallel_rank is not applicable in "
                     "headless mode")

parallel_config.data_parallel_rank is being set to a non-None value internally, even though no rank-related flags are provided in the command. This prevents the headless worker from starting and connecting to the master node (vllm-inference-pytorchjob-final-master-0:29500) for data-parallel coordination. The issue appears to have been introduced or exacerbated by commit 657f2f3, as you noted that the setup worked two hours prior to this change being pushed.

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[njhill]

@bbenshab apologies for this, fix PR is running through CI now: #20398