feat(webapp): read realtime run rows from the primary, not the replica

[ericallam]

Summary

The realtime runs feed hydrates run rows from read replicas, which means it needs a replica-lag gate to avoid serving a run's previous state right after a write. Setting REALTIME_BACKEND_NATIVE_RUN_READS_FROM_PRIMARY=1 reads those rows from each run store's primary instead, so there is no lag to gate against: no probe, no wake delay, no stale-read retries. Off by default, so nothing changes unless you set it.

Design

The run stores already decide replica-vs-primary from the brand on the read client they are handed: a branded replica keeps the read on the owning store's replica, an unbranded writer escalates it to that store's own primary. So this is a one-line choice at the hydrator, and it stays correct across topologies. With the run-ops split on, each leg lands on its own writer and the caller's client is never forwarded across databases; with the split off, it is the single database's primary.

const runReader = new RunHydrator({
  readClient: runReadsFromPrimary ? prisma : $replica,
  runStore,
});

The same flag skips constructing the lag estimator, since probing a replica the feed no longer reads would be measuring the wrong thing.

Independently, AuroraReplicaLagSource detected Aurora by letting aurora_replica_status() fail, on the assumption that the app-level catch made that free. It isn't: an unresolvable function is a query error the driver reports to the error log on every sample, so a non-Aurora replica produced a continuous stream of error events while the estimator quietly fell through to its next candidate. It now resolves the function with to_regproc and memoizes the answer, so the unparseable call never reaches the wire.

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[coderabbitai]

Walkthrough

Realtime run hydration now uses a generic readClient option, with native and shadow client wiring and tests updated accordingly. A new environment flag allows native realtime reads to use the primary database instead of the replica, disabling replica-lag gating in that mode. Aurora replica-lag detection now probes and caches function availability before sampling, with tests covering unavailable functions and cached probes.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description covers summary and design, but it omits the template's Closes, checklist, testing, changelog, and screenshots sections.	Add the missing template sections, including Closes #issue, checklist items, testing steps, changelog, and screenshots or a note that none are needed.

✅ Passed checks (4 passed)

Check name	Status	Explanation
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.
Title check	✅ Passed	The title clearly summarizes the main change: reading realtime run rows from primary instead of the replica.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/realtime-run-reads-from-primary

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.