Serve the update site through a global CDN so updates work from mainland China

[guiyanakuang]

Problem

The Conveyor update site (Sparkle appcast, metadata.properties, packages, apt repo) is served from GitHub Releases (releases/latest/download/...). From mainland China, GitHub is frequently unreachable without a proxy, so:

• macOS users cannot fetch the Sparkle appcast — in-app updates never happen (reported in In-client software update is not available #4551);
• the Windows installer (Conveyor) channel cannot fetch update metadata;
• the apt repository is unreachable for Linux users;
• new-version discovery only works thanks to the crosspaste.com/api/desktop.json fallback, but the update itself still fails.

Conveyor's app.site.base-url is a single address with no client-side mirror/failover support, so the fix must live server-side: serve the update site from a domain we control, distributed globally.

Verified facts (2026-06-11)

• oss.crosspaste.com is already behind Alibaba Cloud CDN (CNAME *.w.cdngslb.com) and is reachable from overseas (verified from a US egress);
• release CI already uploads all artifacts to OSS (oss://crosspaste-desktop/<version>.<revision>/);
• appcast enclosure URLs are absolute (baked from base-url), so metadata and packages must live under the same base URL;
• Conveyor supports S3-compatible upload targets and app.site.move-from for migrating existing installs.

Plan

• Phase 1 — mirror the update site to OSS (zero client impact): CI uploads the complete Conveyor site to a stable site/ prefix, packages first and metadata last; confirm CDN acceleration region covers overseas; set cache policy (short TTL for metadata, immutable for packages)
• Phase 2 — switch app.site.base-url to https://oss.crosspaste.com/site with move-from pointing at the old GitHub URL; ship with a regular release
• Phase 3 — observe: CDN hit rate/cost, overseas installs migrating via move-from, mainland macOS users updating via Sparkle end to end
• Phase 4 — wrap up: website guidance for existing mainland macOS users (one manual download), GitHub Releases demoted to archive/download-page role

Notes

• Existing mainland macOS installs cannot migrate automatically (their Sparkle feed points at GitHub); they need one manual download from the website.
• The Microsoft Store and Windows portable-zip channels are unaffected (the zip updater already downloads from oss.crosspaste.com as its second source).

[guiyanakuang]

Progress update:

• Phase 1 merged (👷 mirror the Conveyor update site to a stable OSS prefix #4559): release CI now mirrors the complete Conveyor site to oss://crosspaste-desktop/site/ — packages first, metadata last, with Cache-Control set at upload time (30 days for version-named packages, 300s for update metadata).
• Phase 2 merged (👷 serve updates from the OSS-backed CDN site #4560): app.site.base-url and the app's check-metadata-url now point at https://oss.crosspaste.com/site (DCDN with global acceleration, confirmed enabled). The crosspaste.com/api/desktop.json fallback is unchanged; GitHub Releases keeps receiving the full asset set and serves as the migration bridge for existing installs — no move-from needed.

Both changes take effect at the next tag build. Verification checklist for that release:

• "Mirror update site to OSS" CI step succeeds
• https://oss.crosspaste.com/site/metadata.properties serves the new version with Cache-Control: max-age=300
• A second request shows edge caching (X-Cache: HIT / Age header); if every request goes back to origin, add a "follow origin" cache rule on the DCDN domain
• appcast-*.rss enclosure URLs point at https://oss.crosspaste.com/site/...
• A fresh install updates without touching GitHub
• Existing overseas installs update normally via the old GitHub feed (packages served from the CDN)

Known one-time cost: the first release after the switch may ship without Sparkle delta updates (no prior versions under the new site URL); deltas resume on the following release.