fix(sglang): emit prefill handoff before completion in sidecar

[connorcarpenter15]

Summary

The in-process SGLang worker yields bootstrap details before prefill completes, but the sidecar currently attaches them to its terminal response. Callers waiting for those details cannot start decode rendezvous while prefill is running.

The sidecar now emits one nonterminal handoff containing the bootstrap host, port, and room ID after the backend gRPC response opens or HTTP success headers arrive. The terminal response carries completion metadata without repeating the handoff. This boundary confirms that the transport opened; it does not acknowledge scheduler readiness.

Early handoff also requires preserving failures that arrive afterward. The router previously discarded outputs from the background prefill drain. It now observes that task while decode connects and streams:

• A prefill failure is returned to the caller and signals decode to stop, rather than leaving it waiting for its own timeout.
• Decode tokens can stream while prefill is pending. Final success waits for prefill completion so a late error cannot follow a successful response; decode failures return immediately.
• Dropping the response observer leaves the prefill task running, preserving the existing lifetime needed for KV transfer.
• Native HTTP prefill abort/error responses are converted into errors before the raw payload is discarded.

The scope is early sidecar prefill handoff and the error handling required after that handoff.

Validation

• Sidecar and mocker tests passed for the initial handoff change, including HTTP handoff before the response body arrives, rejection before handoff, and a gRPC prefill/decode round trip.
• The full Rust test matrix passed on f5582fde6def7, including the added failure-handling tests. These cover failures during decode dispatch and streaming, late failure before final success, and prefill surviving a dropped client response.
• cargo fmt --all -- --check and cargo clippy -p dynamo-llm --lib --tests -- -D warnings passed on 727638e1d6f40, the subsequent lint-only fix.
• CI is rerunning on 727638e1d6f40.

A live multi-node GPU reproduction has not been validated for this revision. The change addresses sidecar PD handoff ordering; the exact reason the original deployment exposed it remains unverified.

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[coderabbitai]

Walkthrough

The PR propagates Eagle capability through engine registration and model runtime configuration. It adds per-rank KV-event host mapping for multi-node SGLang data parallelism. Prefill streams now emit validated handoff parameters before backend connection.

Changes

SGLang runtime integration

Layer / File(s)	Summary
Eagle capability registration lib/backend-common/src/engine.rs, lib/bindings/python/rust/backend.rs, lib/backend-common/src/worker.rs, lib/sidecar/sglang/src/engine.rs, lib/backend-common/examples/mocker/src/engine.rs	LlmRegistration exposes enable_eagle. Python and SGLang registration paths populate it. Local model construction copies it into ModelRuntimeConfig.
Multi-node KV-event host mapping lib/sidecar/sglang/src/args.rs, lib/sidecar/sglang/src/engine.rs	SGLang parses kv_event_hosts, passes the values to discovery, maps hosts to data-parallel ranks, and tests rank-specific endpoints.
Prefill handoff bootstrap lib/sidecar/sglang/src/engine.rs, lib/sidecar/sglang/src/native_http.rs	Prefill generation validates handoff parameters and emits them before opening the backend stream. Terminal output no longer carries the handoff parameters.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Merge Risk: 🔵 Low · up to d7c53

Adding Eagle configuration can misinterpret positional registration arguments used by existing Python integrations, potentially causing invalid data-parallel settings or registration failures. Preserve the prior argument order before merging.

🚥 Pre-merge checks | ✅ 2 | ❌ 3

❌ Failed checks (3 warnings)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 55.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 20 functions across 7 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.
Linked Issues check	⚠️ Warning	The pull request description does not include the required issue link or the required confirmation that no related issue exists.	Add a Related Issues section with a closing or relating issue reference, or check the confirmation that no related issue exists.
Description check	⚠️ Warning	The description provides detailed scope and validation information, but it omits the required Related Issues section and the Where should the reviewer start? section from the repository template.	Add the required Related Issues section and select either a linked issue, such as Closes #XXXX, or confirm that no related issue exists. Add a Where should the reviewer start? section that identifies the key files or changes for review. Use…

✅ Passed checks (2 passed)

Check name	Status	Explanation
Out of Scope Changes check	✅ Passed	The changes match the stated objectives: Eagle detection, multi-node KV-event host mapping, and early prefill handoff emission. No unrelated changes are identified.
Title check	✅ Passed	The title clearly identifies the main change: emitting the SGLang prefill handoff before completion in the sidecar.

Full details: Description check

Resolution

Add the required Related Issues section and select either a linked issue, such as Closes #XXXX, or confirm that no related issue exists. Add a Where should the reviewer start? section that identifies the key files or changes for review. Use the template headings Overview and Details, or clearly map the existing Summary content to those sections.

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Actionable comments posted: 1

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Inline comments:
In `@lib/bindings/python/rust/backend.rs`:
- Line 171: Update the LlmRegistration constructor parameter ordering so
existing six-or-more positional calls continue binding their sixth argument to
data_parallel_size: move enable_eagle after bootstrap_port. Do not make the
constructor keyword-only unless this is an explicitly documented breaking
release.

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📥 Commits

Reviewing files that changed from the base of the PR and between 27fcd7b and d7c5326.

📒 Files selected for processing (7)

• lib/backend-common/examples/mocker/src/engine.rs
• lib/backend-common/src/engine.rs
• lib/backend-common/src/worker.rs
• lib/bindings/python/rust/backend.rs
• lib/sidecar/sglang/src/args.rs
• lib/sidecar/sglang/src/engine.rs
• lib/sidecar/sglang/src/native_http.rs

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[jain-ria]

Updates to the scope of this PR:

1. Multi-node KV-event hosts: remove --kv-event-hosts from this PR. The new planned approach is to give each node that owns a DP scheduler a lightweight/headless sidecar that consumes its local KV events, avoiding cross-node ZMQ connections and explicit rank-to-host configuration. This will be handled as a follow-up.
2. Eagle capability advertisement: this fix has already landed independently in #14374.
3. Early prefill handoff: retain this as the remaining focus of the PR.

[jain-ria]

/ok to test 727638e

[jain-ria]

/ok to test 3369d2b