fix(ffmpeg): backport upstream load, proxy, writeFile and Node path fixes

[Project516]

Backports from upstream ffmpegwasm/ffmpeg.wasm, fitted to this fork's code.

Changed

• A second load() on a loaded instance no longer creates a second core and leaks the first. It waits for any load in flight, keeps the loaded core, and resolves false (the existing "first" meaning of IsFirst, so the return type is unchanged). A failed load clears the state so a retry works. Both worker.ts and worker-node-entry.mts. From #972.
• When the wasm response is not WebAssembly (V8's "magic word" error), load() throws an error naming the wasmURL. Also from Fix double load(), name a non-wasm wasmURL, TS < 4.4 types, internal setTimeout ffmpegwasm/ffmpeg.wasm#972. The TS<4.4 typing change is skipped.
• mount() checks Object.hasOwn on FS.filesystems, so __proto__ and constructor return false. From #958, that part only.
• on and off are arrow-function class fields, so they work through a Proxy such as Vue's reactive() (upstream issue Using vite + vue: TypeError: Cannot read from private field ffmpegwasm/ffmpeg.wasm#687). writeFile(path, data, { transfer: false }) copies the array instead of transferring it; the default is unchanged. Docs and the load() doc comment updated. From #979, without its unload-after-trap part (PR fix(core): end the argv array with NULL #70 covers that area).
• /* turbopackIgnore: true */ next to @vite-ignore on the dynamic core import. From #994. classWorkerURL resolution is untouched.
• Node worker accepts filesystem paths (including Windows paths) and file: URLs for coreURL and wasmURL, converting paths with pathToFileURL. fetchFile(Blob) uses blob.arrayBuffer(), so it works where FileReader is undefined. Remainder of #956.

CHANGELOG has an Unreleased section for these.

Tests

Added to tests/ffmpeg-node.test.mjs: Blob read without FileReader, on/off through a Proxy, second load() keeps the core, mount("__proto__") and mount("constructor") return false, writeFile transfer vs transfer: false, path coreURL/wasmURL, and a non-wasm wasmURL error.

Verified locally

pnpm install --frozen-lockfile, pnpm build, pnpm lint, pnpm run test:node:helpers:args, and the Blob, Proxy and fetchFile cases of ffmpeg-node.test.mjs (they need no core).

Not run locally

• The core-dependent cases in ffmpeg-node.test.mjs (second load, mount names, transfer, path URLs, non-wasm error), in st and mt. No built core on the Pi.
• Browser suites.
• The non-wasm error relies on matching V8's "magic word" message from the core's wasm instantiate; CI is the first real check of that.

Noticed, not fixed

• Concurrent load() calls where the first fails: the waiting calls reject with the same error rather than retrying.

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 36bf577 and a768a9d.

📒 Files selected for processing (8)

• CHANGELOG.md
• apps/website/docs/getting-started/usage.md
• packages/ffmpeg/src/classes.ts
• packages/ffmpeg/src/errors.ts
• packages/ffmpeg/src/worker-node-entry.mts
• packages/ffmpeg/src/worker.ts
• packages/util/src/index.ts
• tests/ffmpeg-node.test.mjs

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[project516-review-bot]

This PR backports several upstream fixes: (1) second load() waits for in-flight load and returns false without creating a duplicate core; failed loads clear state for retry. (2) load() errors name the wasmURL when the response isn't WebAssembly. (3) mount() uses Object.hasOwn to reject proto/constructor. (4) on/off are arrow functions so they work through Proxies. (5) writeFile adds {transfer:false} to copy instead of transfer. (6) Node worker accepts filesystem paths and file: URLs for coreURL/wasmURL via pathToFileURL. (7) fetchFile(Blob) uses blob.arrayBuffer() instead of FileReader. (8) Dynamic import carries turbopackIgnore. Tests cover all new behavior.

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_{review-bot, model nvidia/nemotron-3-ultra-550b-a55b:free, verdict approve}