@remotion/renderer: Fix parallel browser downloads

[JonnyBurger]

Summary

• serialize browser downloads and extraction across Remotion processes
• recover stale download locks and recheck the installed browser after waiting
• add a regression test that launches two processes against the same browser cache

Testing

• bun run build
• bun run stylecheck
• bun test packages/renderer/src (343 tests passed)

Closes #9875

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Actions	Updated (UTC)
bugs	Ready	Preview, Comment	Jul 29, 2026 10:57am
remotion	Ready	Preview, Comment	Jul 29, 2026 10:57am

[pullfrog]

✅ No new issues found.

Reviewed changes — serializes concurrent browser downloads with a file-based lock to prevent corruption when multiple Remotion processes race to download/extract Chrome into the same cache directory.

• Add browser-download-lock.ts — file-based cross-process lock using O_EXCL|O_CREAT (wx), with stale lock recovery (process liveness check + 30s mtime threshold), heartbeat every 5s, and token-based ownership verification.
• Wrap downloadBrowser in the lock — acquires the lock before downloading, rechecks whether the browser was installed by a winner while waiting, and releases the lock in finally.
• Add integration test — spawns two worker processes competing for the same lock, asserts the second is blocked while the first holds the lock, and verifies cleanup.

  ^{｜ View workflow run ｜ Using DeepSeek Pro (free via Pullfrog for OSS) (Claude Opus not used — the program covers this model; add its provider key to run your pick) ｜ 𝕏}