fix(updater): send no-cache header on update check to avoid stale manifest

[wesbillman]

Problem

"Check for updates" is flaky: the new version is published but the check often reports "up to date" until you click 3–4 times or restart the app.

Root cause (full writeup in RESEARCH/UPDATE_CHECK_FLAKINESS.md): the updater fetches latest.json with a plain GET and no cache directive. The manifest is overwritten in place at a stable URL and served from a multi-node cache (Fastly for OSS, Artifactory HA replicas for the internal build) with no Cache-Control. A check can hit a node still serving the old manifest → version not greater than current → "up to date". A retry/restart eventually lands on a fresh node — the 4-clicks symptom.

Fix

Pass Cache-Control: no-cache request headers on the updater check(), so every poll explicitly revalidates instead of trusting a stale cached copy.

const update = await check({ headers: { "Cache-Control": "no-cache" } });

This sits in runUpdateCheck, the single code path behind both the manual "Check for updates" button and the launch/6h background check, so one edit covers every check.

Scope / honesty

This is app-side hardening — a backstop, not a standalone fix. A shared/origin cache can ignore request headers, so this is most effective paired with the response-side fix landing in squareup/sprout-releases (no-cache on the Artifactory manifest PUT). It meaningfully helps the internal build.

For the public/OSS path to be deterministic, a follow-up is still needed: point the OSS check at a source that isn't asset-CDN-cached (the GitHub API release endpoint returns cache-control: max-age=60 + ETag, bounded staleness, no node roulette). github.com strips the query string before redirecting to the signed asset, so a cache-buster can't beat the OSS asset CDN.

Testing

No hook-test harness exists in this repo yet, and this is a pure pass-through of an options object verified against tauri-plugin-updater's typed API (CheckOptions.headers?: HeadersInit). Full pre-push suite (desktop-test, rust-tests, mobile-test, clippy, tauri-test) passes green. Adding a Tauri-plugin mock + RTL scaffold for a one-liner felt disproportionate — flagging the tradeoff rather than hiding it.

[wesbillman]

@codex please review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Swish!

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