perf(responses): move response snapshot writes off the request path

[SantiagoDePolonia]

Non-streaming POST /v1/responses previously blocked the client on a synchronous SQLite upsert of the response snapshot before returning. On SQLite backends that write also contended with audit-log and usage writes on the shared single connection, so tail latency reflected unrelated flushes. /v1/chat/completions has no such write, which showed up as a latency gap between the two endpoints.

The snapshot is now written in the background:

• The handler deep-copies the snapshot before returning, so the write shares no memory with the response being serialized to the client (race-detector verified).
• The write uses a cancellation-detached context with a 15s cap (covers Create plus the Update fallback, each bounded by SQLite's 5s busy timeout).
• Server.Shutdown drains in-flight snapshot writes before closing the response store, so graceful shutdown loses nothing.
• Failure handling is unchanged: same gomodel_response_snapshot_store_failures_total metric and warning log with request_id. store: false behavior is unchanged.

User-visible impact: /v1/responses latency no longer includes storage time. A client that issues GET /v1/responses/{id} immediately after the POST may race the background write for a few milliseconds; a hard crash in that window loses the snapshot (falls back to native provider lookup or 404). Both were judged acceptable since a failed snapshot write already did not fail the request.

Design note: this uses one goroutine per request rather than the bounded worker pool responsecache uses — snapshots are small and database/sql already serializes writes on the single SQLite connection. If snapshot volume ever warrants it, converting to the bounded-queue pattern is a clean follow-up.

Docs updated to state that snapshots are written in the background and to name the failure metric. The small passthrough_support_test.go commit applies a go fix modernization required by the pre-commit hook.

Summary by CodeRabbit

• New Features

  • Response snapshots are saved asynchronously, allowing responses to return without waiting for storage.
  • Snapshot data now includes normalized response bodies and input items.
  • Pending snapshot writes are completed during graceful server shutdown.

• Bug Fixes

  • Snapshot write failures are logged and tracked with a metric.
  • Improved handling of snapshot creation conflicts and persistence errors.
  • Snapshot persistence is safely skipped for writes queued after shutdown draining begins.

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📒 Files selected for processing (4)

• docs/advanced/responses-api.mdx
• internal/server/handlers.go
• internal/server/handlers_test.go
• internal/server/translated_inference_service.go

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---

📝 Walkthrough

Walkthrough

Non-streaming response snapshots are now cloned and persisted asynchronously with a 15-second timeout. Failures are logged and measured. Server shutdown drains pending writes before closing stores. Tests cover asynchronous persistence and lifecycle behavior.

Changes

Response snapshot persistence

Layer / File(s)	Summary
Asynchronous snapshot scheduling and persistence internal/responsestore/store.go, internal/server/translated_inference_service.go	Eligible snapshots are deep-cloned and written in the background. Writes use bounded contexts, create-then-update fallback, failure metrics, and in-flight tracking.
Shutdown draining internal/server/handlers.go, internal/server/http.go	Shutdown delegates to the translated inference service and drains snapshot writes before closing response and conversation stores.
Lifecycle validation and documentation internal/server/handlers_test.go, internal/server/passthrough_support_test.go, docs/advanced/responses-api.mdx	Tests cover asynchronous persistence, failures, disabled storage, and deletion. Documentation describes asynchronous writes and failure reporting. The provider test uses slices.Contains.

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

Merge Risk: ⚪ Minimal · up to 0b45e

Snapshot persistence now occurs after the response returns, reducing request latency while allowing a brief race for immediate retrieval; no actionable merge-blocking risk remains.

Sequence Diagram(s)

sequenceDiagram
  participant HTTPClient
  participant translatedInferenceService
  participant responseStore
  participant Metrics
  translatedInferenceService-->>HTTPClient: return response
  translatedInferenceService->>translatedInferenceService: clone snapshot and start background write
  translatedInferenceService->>responseStore: create or update snapshot
  responseStore-->>translatedInferenceService: write result
  translatedInferenceService->>Metrics: record write failure when applicable

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Possibly related PRs

• ENTERPILOT/GoModel#488: Adds the response persistence implementation that this change extends with cloning, asynchronous writes, and shutdown draining.
• ENTERPILOT/GoModel#576: Also modifies response snapshot persistence in translated_inference_service.go.

Poem

A rabbit guards the snapshot store,
While cloned responses race the door.
Background writes complete in flight,
Shutdown waits before the night.
Failed writes leave a counted trace.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes moving response snapshot writes off the request path.
Description check	✅ Passed	The description explains the change, rationale, behavior, trade-offs, testing, and documentation updates in sufficient detail.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
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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• Commit unit tests in branch perf/sql-sync-fix

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Actionable comments posted: 2

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Inline comments:
In `@docs/advanced/responses-api.mdx`:
- Around line 35-39: Update the non-streaming POST /v1/responses documentation
to state that snapshots may not yet be available to an immediate GET
/v1/responses/{id}, pending snapshots can be lost on a hard process exit, and
graceful shutdown drains pending snapshot writes.

In `@internal/server/handlers_test.go`:
- Around line 5187-5193: The test around blockingResponseStore.Create must not
call srv.ServeHTTP synchronously because it can block indefinitely. Run
ServeHTTP in a goroutine, wait for its completion with a bounded timeout, and
ensure the store release occurs via cleanup-safe logic so the test cannot hang
or leak blocked work.

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ℹ️ Review info

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Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: 4709b1a1-1ba4-4e3a-95c5-51049d24acfc

📥 Commits

Reviewing files that changed from the base of the PR and between 3c9cb61 and d977bda.

📒 Files selected for processing (7)

• docs/advanced/responses-api.mdx
• internal/responsestore/store.go
• internal/server/handlers.go
• internal/server/handlers_test.go
• internal/server/http.go
• internal/server/passthrough_support_test.go
• internal/server/translated_inference_service.go

Included review availability: Your plan provides up to 4 included reviews per hour; 1 remains after this review.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 81.08108% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/server/translated_inference_service.go	87.50%	3 Missing and 1 partial ⚠️
internal/responsestore/store.go	0.00%	2 Missing ⚠️
internal/server/http.go	0.00%	1 Missing ⚠️

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[greptile-apps]

Confidence Score: 5/5

No blocking failure remains.

The shutdown path was exercised with concurrent Responses traffic and correctly prevented post-drain writes from reaching closed response storage.

T-Rex Logs

What T-Rex did

• I ran a deterministic shutdown-race reproduction test using go test -race with a blocking tracked snapshot Create, a Server.Shutdown, and a second /v1/responses request after draining; all 25 runs passed under the race detector.
• I also executed the focused asynchronous and post-drain lifecycle tests under the race detector; the focused test suite and the lifecycle tests passed.
• I examined the synchronization around snapshot writes, showing that drainSnapshotWrites is performed before closing the response store, and that shutdown drains before store close, with a blocking reproduction demonstrating the store staying open while the tracked write is blocked.
• I confirmed that the actual POST /v1/responses path clones the snapshot before starting the background writer, and that the focused race and lifecycle tests pass, disproving the claimed race condition in the mutation path.

_{Ran code and verified through T-Rex}

_{Reviews (2): Last reviewed commit: "fix(responses): gate snapshot writes aga..." | Re-trigger Greptile}