fix/concurrent-ffmpeg-loader-race

[Pranav-IIITM]

Description

Fixes a race condition in src/lib/ffmpeg.ts where concurrent calls to
loadFFmpeg() during the async initialization window could trigger multiple
simultaneous ffmpeg.load() calls on the same WASM instance, corrupting
internal worker state.

Introduces a shared loadingPromise guard so all concurrent callers share
a single initialization flow. Failed loads reset both ffmpegInstance and
loadingPromise to allow safe retries.

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Related Issue

Closes #1014

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Type of Contribution

• Bug fix
• New feature
• Documentation update
• Refactor
• GSSoC contribution

Participant Info

• GitHub username: @Pranav-IIITM
• Contribution level : Advanced

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Screen Recording

Not applicable — this is a runtime race condition fix with no UI changes.

Recording / Loom link: N/A — no UI changes

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Checklist

• I have read the contribution guidelines
• My changes follow the project structure
• I have tested my changes in Chrome, Firefox, and Safari
• bun run lint passes (no ESLint errors)
• bunx tsc --noEmit passes (no TypeScript errors)
• New interactive elements have aria-label / accessible names
• No console.log statements left in
• This PR is related to a valid issue
• Screen recording attached above (N/A — no UI changes)

[vercel]

@Pranav-IIITM is attempting to deploy a commit to the magic-peach1's projects Team on Vercel.

A member of the Team first needs to authorize it.

[github-actions]

👋 Thanks for your PR, @Pranav-IIITM!

Welcome to Reframe — a browser-based video editor built for everyone 🎬

🟠 GSSoC'26 PR detected — thanks for contributing under GirlScript Summer of Code 2026!

What happens next

1. 🤖 Automated checks — build & TypeScript typecheck will run automatically
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3. 👀 Code review — a maintainer will review your changes
4. 🚀 Merge — once approved, your PR will be merged!

Quick checklist

• PR title follows Conventional Commits (e.g. feat: add dark mode)
• Linked the issue this PR closes (e.g. Closes #123)
• Tested the changes locally (bun run dev)
• Build passes (bun run build)

Useful links

• 📖 CONTRIBUTING.md
• 🐛 Open Issues
• ⭐ Star the repo to support the project!

Happy coding! 🎉

[github-actions]

⚠️ PR Format Issues — @Pranav-IIITM

Please fix the following before your PR can be reviewed:

• ⚠️ Use a conventional PR title. Examples:
  • feat: add dark mode support
  • fix: resolve aria label missing on slider
  • docs: add deployment guide to README

Push new commits after fixing — this comment will update automatically.

📖 CONTRIBUTING.md

[Pranav-IIITM]

Hey @magic-peach

Just wanted to flag something — this issue (#1014) was originally labeled
level:advanced by you, but it appears the bot has since
relabeled it as level:beginner .

Could you please restore the level:advanced label? The fix involves
resolving a WASM runtime race condition with a shared pending-load promise
guard, which is non-trivial and aligns with the advanced classification
you originally assigned.

Thanks!