toBlobURL / downloadWithProgress method don't handle compressed files

[trzeciak]

Hey,
in short: I have a problem with handling progress in toBlobURL method, when downloading a compressed wasm module.

Some observations:

• if we pass a progress block, the downloadWithProgress implementation is used under-the-hood,
• the display of the progress block is based on the Content-Length header
• the Content-Length header is related to Content-Encoding (which is controlled by the client, e.g.: Accept-Encoding: gzip, deflate, br, zstd)
• the fetch method downloads the compressed module, but returns the uncompressed one,

This causes this exception to occur in this downloadWithProgress method:

        if (total != -1 && total !== received)
          throw ERROR_INCOMPLETED_DOWNLOAD;

Then in the catch block, we call buf = await resp.arrayBuffer(), and this causes a second exception on the already downloaded file:

ffmpegwasm Failed to execute 'arrayBuffer' on 'Response': body stream already read…

Which effectively makes it impossible to download this wasm module.

Wasm files compress very well, for example, the ffmpeg-core.wasm module compresses from ~30MB to ~7MB using brotli.

Personally, I don't know how to fix this properly, that the downloadWithProgress method handles compressed files.