fix: reset leader session state and withdraw backlog gauge on shutdown (ENG-1670)

[xernobyl]

What does this PR do?

Follow-up to #111 (ENG-1670), which merged before a last round of review comments were addressed. Six fixes, one with real (if bounded) production impact.

The one that matters — the backlog poller's leader leaked a session setting onto a pooled connection. To bound how long a leader lost to a node failure or network partition can hold its advisory lock, the leader runs SET idle_session_timeout = '30min' on its connection. But SET is session state, and pgxpool hands that exact backend to unrelated callers once the leader releases it — verified with a MaxConns=1 pool: same backend pid, setting still in effect, reset_val = 0. Postgres would then terminate whichever component next borrowed that connection and left it idle for 30 minutes.

release() now RESETs it before returning the connection. Pinned by a test that fails with actual: "30min" if the reset is removed.

Impact today is bounded rather than severe: pgxpool destroys connections past MaxConnLifetime (1h) and pings anything idle >1s on acquire, so the realistic worst case is one extra connection churn — and there is currently no long-idle pooled consumer to inherit the setting, because the API's River client is constructed but never Started. Starting it (or adding any consumer that holds a pooled connection idle >30min outside a transaction) would make this bite, so it is worth fixing now rather than later.

The rest:

• The gauge was not withdrawn on graceful shutdown — the most frequent handover of all. Close released the advisory lock while the values stayed, so the meter provider's final collect-and-export published a backlog this process no longer owned. Added the matching defer.
• Corrected the alerting guidance, which feat: expose per-tenant enrichment status and backlog metric (ENG-1670) #111's own withdraw-on-demotion change had inverted. A failed poll now removes the series rather than freezing it, so the symptom is a missing gauge; the interface doc, metric description and escalated log all still described a stale value, which would have led someone to build a value-based staleness rule that cannot fire.
• Two stale comments: one in feedback_records_repository.go claiming the marker is a post-filter on the 016 index (migration 020 realigned the index predicate, so it isn't), and one in the leader test left over from the transaction-scoped lock.
• The withdraw test passed vacuously — after the clear the SDK drops the empty metric and then the empty scope, so both loops ran zero times and the assertion never executed. It now counts data points, so it would also catch a wrong metric name or an empty Collect.
• Documented why migration 020's index rebuild accepts a brief no-index window (its only consumer is the manual classify-backfill command, which degrades to a sequential scan; no serving path uses it) rather than adding a temp-name-and-rename dance.

Note on the migration file: this touches migrations/020_add_emotions_classified_at.sql, which is already applied — the change is comments only, no DDL, so no re-run or checksum concern. Verified: no non-comment line differs.

No API, response-shape, OpenAPI or SDK change.

Linear: https://linear.app/formbricks/issue/ENG-1670

How should this be tested?

Automated

• make tests — TestEnrichmentBacklogLeaderLeavesNoSessionState is the load-bearing one: it takes leadership on a MaxConns=1 pool, releases, then borrows the same connection as any other caller would and asserts current_setting('idle_session_timeout') is back to 0. Temporarily removing the RESET makes it fail with actual: "30min" (checked).
• The existing leader test still covers election, stickiness and handover; TestEnrichmentBacklogMetricsClearWithdrawsSeries now counts data points instead of iterating an empty collection.
• go test ./internal/... ./cmd/..., make migrate-validate, make lint-openapi, golangci-lint — all clean on current main (which has moved on since feat: expose per-tenant enrichment status and backlog metric (ENG-1670) #111).

Manual

The behaviour was smoke-tested end to end before #111 merged and re-verified after these changes: three API replicas against one database hold exactly one advisory lock (pg_locks), killing non-leaders leaves it at 1, and killing the leader drops it to 0 immediately via the graceful Close release — no leaked lock. Emotion completion was verified with real providers: records that classify to no emotions are stored emotions=NULL, emotions_classified_at=set and correctly count as done, so the backlog drains to zero.

Checklist

Required

• Filled out the "How to test" section in this PR
• Read Repository Guidelines
• Self-reviewed my own code
• Commented on my code in hard-to-understand bits
• Ran make build
• Ran make tests (integration tests in tests/)
• Ran make fmt and make lint; no new warnings
• Removed debug prints / temporary logging
• Merged the latest changes from main onto my branch with git pull origin main (branched off current main)
• If database schema changed: no schema change — migrations/020 is touched for comments only (verified no DDL diff); make migrate-validate passes and the up/down round-trip was re-checked

Appreciated

• If API changed: n/a — no API change
• If API behavior changed: n/a
• Updated docs in docs/ — n/a; the hand-authored hub-api-docs pages for ENG-1670 (metrics reference + core-concepts) remain a separate follow-up
• Ran make tests-coverage for meaningful logic changes

[coderabbitai]

Walkthrough

The enrichment backlog poller now withdraws its pending gauge when it exits, and related logs, documentation, and tests describe the missing-series behavior. Leader release resets PostgreSQL session timeout state before returning connections to the pool, with a test covering reuse. Migration 020 updates the emotions backfill partial index to require both null classification fields and restores the previous predicate on rollback. Repository comments were aligned with the revised index behavior.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title is concise, Conventional Commits-style, and accurately summarizes the main session-state and backlog-gauge fixes.
Description check	✅ Passed	The description includes the change summary, issue reference, testing steps, and checklist items required by the template.
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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[coderabbitai]

Actionable comments posted: 3

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@internal/repository/enrichment_status_repository.go`:
- Around line 288-294: Update the connection cleanup in release to use a fresh
deadline/context for resetLeaderIdleTimeoutSQL rather than reusing unlockCtx.
Check errors from both the unlock and RESET executions; if either fails or
cannot complete, close/discard the connection instead of calling l.conn.Release.
When cleanup succeeds, propagate any unlock failure as required by the existing
release contract.

In `@tests/enrichment_status_test.go`:
- Around line 367-373: In the test around NewEnrichmentBacklogLeader, register
defer leader.Close(ctx) immediately after constructing the leader so cleanup
runs if a later assertion fails. Retain the existing explicit leader.Close(ctx)
before the reuse assertion.
- Around line 369-378: Update the leadership test around tryAcquire and
leader.CountIfLeader to query and store the connection’s original
idle_session_timeout before acquisition. Assert acquisition changes it to
"30min", then close the leader and assert the borrowed connection restores the
captured original value instead of hard-coding "0".

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: d7dd9ccf-3335-44ae-b660-05fbdd082088

📥 Commits

Reviewing files that changed from the base of the PR and between 4993dfb and ddc1a21.

📒 Files selected for processing (7)

• cmd/api/app.go
• internal/observability/enrichment_backlog.go
• internal/observability/enrichment_backlog_test.go
• internal/repository/enrichment_status_repository.go
• internal/repository/feedback_records_repository.go
• migrations/020_add_emotions_classified_at.sql
• tests/enrichment_status_test.go

[BhagyaAmarasinghe]

Requesting changes for the shutdown-ordering issue below. The remaining reviewed paths and validation were clean.