PlatformFile.source() not available in WasmJS?

[galex]

Hello,

As I am working with large video files on a CMP app with Android / iOS / Desktop JVM / WasmJS targets, I wonder why isn't PlatformFile.source() available on WasmJS?

If it's not possible to buffer files on web, how am I supposed to stream a file into a multipart request so that I do not load the whole file in memory by having to use readBytes ?

Thank you in advance 🙏

[vinceglb]

Hi @galex, thanks for opening this issue.

I investigated PlatformFile.source() for WASM / JS and I would like your opinion before we decide the scope.

Why adding this feature is tricky:

• Browser File APIs are asynchronous.
• RawSource reads are synchronous once the source is created.
• So a true streaming source() on browser is not straightforward with the current shape.

kotlinx-io doesn't support async API right now. In a dedicated issue, there is a comment that explains why this is blocking for "reading large files in JS/WASM in the browser": Kotlin/kotlinx-io#163 (comment)

For now, in order to use Sink/Source interfaces I have to read the whole file into memory which has obvious downsides.

Given this, I don't see a good solution right now to support PlatformFile.source() on WASM / JS with the same behavior as other platforms.

We could technically add an implementation that buffers the whole file in memory, but for large files like in your situation, this is not ideal and could be misleading.

My preference is to keep source() unsupported on web targets for now, and revisit this when kotlinx-io provides async-compatible APIs (or another reliable streaming approach).

I'm really interested in hearing your thoughts here. What do you think?