fix(router): cap upstream body buffering and sanitize upstream error-code labels

[slin1237]

Description

Problem

Two response-hardening gaps against misbehaving or hostile backends:

1. send_typed_request buffered every non-streaming worker response with an unbounded res.bytes() read, and the rerank rebuild then re-read the buffered body with to_bytes(body, usize::MAX) — so a worker returning an oversized body could balloon router memory before JSON parsing even starts. The remaining usize::MAX body reads under routers/ and middleware/ are test-only.
2. record_http_response / record_router_upstream_response intern the X-SMG-Error-Code value as a metric label, and responses rebuilt from worker responses preserve upstream headers, so a hostile or buggy backend could mint unbounded error_code label values (and never-freed interner entries). fix(observability): bound client-controlled metric label cardinality #2093 bounded the client-controlled labels; the backend-controlled channel was still open.

Solution

1. Cap the buffered worker-body read at the configured max_payload_size (default 512MB) — the limit the gateway already enforces on ingress bodies, so a worker response is never buffered beyond what a client is allowed to send, and no new knob is needed. The cap sits in send_typed_request, the point where an upstream body first enters memory (capping only the rerank rebuild would bound a body that had already been fully buffered), and accumulation stops as soon as the limit is crossed. Exceeding it returns 502 upstream_response_too_large (the worker misbehaved, so a gateway-fault 500 would be wrong) and now also feeds the circuit breaker and retry path like any other worker fault; other read failures keep today's 500 read_response_body_failed, and the rerank rebuild keeps a bounded read with the same limit as its own contract. Streaming responses are untouched: they are relayed chunk-by-chunk under backpressure and never buffered whole.
2. Only trust error codes the gateway itself sets: create_error — the sole producer of X-SMG-Error-Code — now also stores the code in a process-local response extension, and extract_error_code_from_response reads only that extension. Upstream responses can never inject an extension, so no backend-controlled value reaches the label regardless of which proxy path rebuilt the response, and gateway-emitted codes are recorded exactly as before. Routing the label through the fix(observability): bound client-controlled metric label cardinality #2093 bounded interner was considered instead, but a backend flooding the cap would collapse later gateway codes to the other sentinel; the extension keeps them fully intact. Client-visible behavior is unchanged: gateway errors still carry the header, and upstream headers still pass through.

Changes

• routers/error.rs: GatewayErrorCode response extension set by create_error; extract_error_code_from_response reads only the extension, never the header.
• routers/http/router.rs: Router carries max_payload_size; send_typed_request buffers non-streaming worker bodies through read_worker_body_capped, which stops accumulating at the cap and maps overflow to 502 upstream_response_too_large; build_rerank_response keeps a bounded read instead of usize::MAX.
• middleware/metrics.rs: test proving per-response forged X-SMG-Error-Code values leave the interner flat.
• tests/common/mock_worker.rs: rerank handler echoes PAD:<n> documents as n-byte strings so a small request can produce an oversized worker response.
• tests/api/api_endpoints_test.rs: end-to-end oversized-rerank test.

Test Plan

• cargo test -p smg --lib -- extract_error_code build_rerank_response read_worker_body_capped upstream_forged — 9 new tests: gateway codes extracted intact with the client header still set; an upstream-supplied header is ignored; a forged header cannot override a gateway code; 1000 distinct forged codes leave the interner flat; the capped reader accepts a multi-chunk body exactly at the limit, rejects one over it with 502 upstream_response_too_large, and maps a mid-body read failure to 500 read_response_body_failed; a rerank body exactly at the limit passes through with top_k/document handling unchanged; a body over the limit returns 502.
• cargo test -p smg --test api_tests -- rerank — 7 passed, including new test_rerank_oversized_worker_body_returns_502: with a 16KB cap, a ~64KB mock worker body returns 502 with the code in header and body, and a ~1KB body is rebuilt normally.
• cargo test -p smg — all 23 targets green.

Checklist

• cargo +nightly fmt passes
• cargo clippy --all-targets --all-features -- -D warnings passes
• (Optional) Documentation updated
• (Optional) Please join us on Slack #sig-smg to discuss, review, and merge PRs

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 94ac6ee and 8e25661.

📒 Files selected for processing (2)

• model_gateway/src/middleware/metrics.rs
• model_gateway/src/routers/http/router.rs

🚧 Files skipped from review as they are similar to previous changes (2)

• model_gateway/src/middleware/metrics.rs
• model_gateway/src/routers/http/router.rs

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📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes
  • Rerank requests now reject upstream responses exceeding the configured payload limit with a clear 502 Bad Gateway error.
  • Responses within the configured limit continue to process successfully.
  • Upstream or tampered error headers can no longer alter gateway error reporting or metric labels.
  • Prevented forged error codes from causing unbounded growth in metrics tracking.

Walkthrough

The gateway now records error codes in private response extensions and ignores client-visible error-code headers for metrics. Rerank and typed upstream responses are bounded by configuration, with explicit handling for oversized, unreadable, or invalid bodies.

Changes

Gateway response safety

Layer / File(s)	Summary
Gateway error-code provenance model_gateway/src/routers/error.rs, model_gateway/src/middleware/metrics.rs	Gateway-generated codes are stored in response extensions. Metric extraction ignores upstream and tampered headers. Tests cover extraction rules and interner growth.
Bounded upstream response handling model_gateway/src/routers/http/router.rs	The router applies the configured payload limit while reading worker responses. Oversized bodies return 502 with upstream_response_too_large; read and parsing failures return gateway errors.
Rerank payload-limit validation model_gateway/src/routers/http/router.rs, model_gateway/tests/common/mock_worker.rs, model_gateway/tests/api/api_endpoints_test.rs	Tests cover boundary, oversized, read-failure, and successful responses. The mock worker generates padded documents for integration coverage.

Estimated code review effort: 4 (Complex) | ~45 minutes

Mergeability Score: ⚪ Minimal · up to 8e256

The PR adds bounded upstream response buffering and prevents backend-controlled error-code labels from reaching metrics; no actionable merge-blocking risk remains after normal checks and review.

Sequence Diagram(s)

sequenceDiagram
  participant RerankRouter
  participant UpstreamWorker
  participant ErrorResponse
  RerankRouter->>UpstreamWorker: request rerank response
  UpstreamWorker-->>RerankRouter: stream response body
  RerankRouter->>RerankRouter: enforce max_payload_size
  RerankRouter->>ErrorResponse: create 502 upstream_response_too_large

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Possibly related PRs

• smg-project/smg#1679: Both changes address metric interner growth in metrics.rs.
• smg-project/smg#2130: Both changes modify HttpMetricsLayer for distinct metric-handling issues.

Suggested reviewers: catherinesue, key4ng

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the two main changes: limiting upstream body buffering and sanitizing upstream error-code metric labels.
Description check	✅ Passed	The description directly explains the oversized-response and forged-error-code fixes, implementation details, and associated tests.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[claude]

Clean, well-tested fix for two response-hardening gaps. The extension-based approach for error code labeling is the right call — it's immune to upstream forgery regardless of which proxy path rebuilt the response, and it avoids the cap-collapse problem that the bounded-interner alternative would have. Rerank body capping reuses the existing ingress limit, so no new config surface. All remaining usize::MAX body reads confirmed test-only. Tests are thorough across unit, integration, and e2e layers.

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Inline comments:
In `@model_gateway/src/routers/http/router.rs`:
- Around line 1139-1168: Update the rerank request flow around
send_typed_request so successful worker response bodies are size-limited before
res.bytes().await, preventing full unbounded buffering; preserve the existing
502 error behavior for oversized responses and add coverage proving bounded
buffering rejects responses exceeding the configured limit.

In `@model_gateway/tests/common/mock_worker.rs`:
- Around line 1720-1724: Update the PAD control parsing in the surrounding mock
worker logic so an invalid suffix causes the test to fail instead of defaulting
to a zero-length body; replace the fallible parse fallback in the Some branch of
the doc.strip_prefix("PAD:") match with an explicit failure while preserving
valid repetition behavior.

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ℹ️ Review info

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Configuration used: Organization UI

Review profile: CHILL

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Run ID: cf0db5d6-db00-4069-943e-c8a91a65f155

📥 Commits

Reviewing files that changed from the base of the PR and between 5d080ac and faeb0f5.

📒 Files selected for processing (5)

• model_gateway/src/middleware/metrics.rs
• model_gateway/src/routers/error.rs
• model_gateway/src/routers/http/router.rs
• model_gateway/tests/api/api_endpoints_test.rs
• model_gateway/tests/common/mock_worker.rs