Accessing req.body.tee results in attempting to access detached ArrayBuffer

[radu-matei]

Repro is a simple function that tees the request body stream; one is forwarded to an upstream (upstream is /api/upstream, needs to be allow-listed in spin.toml), the other stream is logged:

import { AutoRouter } from 'itty-router';

let router = AutoRouter();

router
    .post('/api/tee', async (req) => {
        console.log('[tee]: received request to tee');
        const [forwardStream, logStream] = req.body!.tee();

        const forwardRequest = new Request('http://localhost:3000/api/upstream', { body: forwardStream, method: req.method });

        const response = await fetch(forwardRequest);

        await logRequestBody(logStream);

        return response;
    })

    .post('/api/upstream', async (req) => {
        console.log(`[forwarded]: ${await req.text()}`)

        return new Response('Successfully forwarded', { status: 200 })
    })

async function logRequestBody(stream: any) {
    const reader = stream.getReader();
    const chunks = [];
    while (true) {
        const { done, value } = await reader.read();
        if (done) break;
        chunks.push(value);
    }
    const fullArray = concatChunks(chunks);
    // Decode the Uint8Array into a UTF-8 string.
    const text = new TextDecoder("utf-8").decode(fullArray);
    console.log(`[tee]: Logged request body: ${text}`);
}

function concatChunks(chunks: any) {
    const totalLength = chunks.reduce((sum: any, chunk: any) => sum + chunk.length, 0);
    const result = new Uint8Array(totalLength);
    let offset = 0;
    for (const chunk of chunks) {
        result.set(chunk, offset);
        offset += chunk.length;
    }
    return result;
}

//@ts-ignore
addEventListener('fetch', async (event: FetchEvent) => { event.respondWith(router.fetch(event.request)) });

This results in the following error sent to the client:

* upload completely sent off: 36 bytes
< HTTP/1.1 500 Internal Server Error
< content-length: 66
< content-type: application/json; charset=utf-8
< date: Fri, 07 Feb 2025 20:05:02 GMT
<
* Connection #0 to host localhost left intact
{"status":500,"error":"attempting to access detached ArrayBuffer"}%

[radu-matei]

ref #330

[karthik2804]

I believe there is a fix in progress upstream for this in StarlingMonkey. cc: @tschneidereit

[tschneidereit]

Correct, yes: this should be fixed by bytecodealliance/StarlingMonkey#210, for which I added a regression test based on the code here.