[Frontend][Rust] Bound chat-template evaluation to prevent DoS (#52025)

[jaideeppyne]

Purpose

Fixes #52025.

The Rust frontend renders a caller-supplied (or model-supplied) Jinja chat_template from POST /v1/chat/completions with no evaluation budget. Rendering cost is O(N^depth) in caller-controlled loop bounds, and minijinja's per-range() element limit is trivially walked around by nesting three individually-legal range() calls. A 116-byte request body can occupy a request-runtime worker thread for tens of seconds:

{% for x0 in range(999) %}{% for x1 in range(999) %}{% for x2 in range(999) %}{% endfor %}{% endfor %}{% endfor %}ok

The request runtime is a bounded pool (worker_threads = min(available_parallelism, 32)), so a small number of such requests can stall the chat-completions plane while /health stays green. The request-supplied gate (trust_request_chat_template, default off in Python vLLM) is not yet implemented in the Rust frontend, and the model-supplied template path has no gate at all.

This wires up minijinja's existing fuel feature and sets a total evaluation budget in build_environment (rust/src/chat/src/renderer/hf/template.rs). The budget bounds total work regardless of who supplied the template, so a hostile template is rejected quickly while legitimate templates — which only iterate over the request's messages/tools/documents — are unaffected. For reference, minijinja itself already refuses a single range() above ~1M elements; the budget here (20M fuel units) is ~20× that, well above any real chat template.

This is the primary remediation (item 1) from the issue. Implementing trust_request_chat_template and spawn_blocking-ing the render (items 2 and 4) are follow-ups and out of scope here.

Test Plan

cargo test -p vllm-chat — added two tests in template.rs:

• test_chat_template_evaluation_is_bounded: the nested-range() bomb from the issue is now rejected (and returns quickly instead of running for ~55s).
• test_chat_template_within_budget_renders: a legitimate template doing non-trivial work (range(50000)) still renders.

Test Result

$ cargo test -p vllm-chat
test result: ok. 267 passed; 0 failed
test result: ok. 17 passed; 0 failed
test result: ok. 16 passed; 0 failed

All existing vllm-chat tests pass (the budget does not affect any real template); cargo fmt --check and cargo clippy -p vllm-chat are clean.

[claude]

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[chatgpt-codex-connector]

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @jaideeppyne.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[jaideeppyne]

Rebased onto current main (branch was ~166 commits behind) and force-pushed — the mergify conflict/staleness should now be cleared; the branch is even with main again.

Re the codex P2 note on rust/src/chat/src/renderer/hf/template.rs: the fuel test no longer asserts on wall-clock time. test_chat_template_evaluation_is_bounded asserts only that an over-budget template render returns an error (result.is_err()), i.e. the fuel budget trips and evaluation is bounded — nothing about elapsed duration — so it won't flake on slow/contended CI workers. A companion test_chat_template_within_budget_renders confirms a legitimately-sized template still renders. Both pass locally (cargo test -p vllm-chat: 16 passed, 0 failed; cargo fmt --check and cargo clippy -p vllm-chat clean).