Hanging promise in TransformStream impl

[lucacasonato]

Reproduction:

async function collect(o) {
    const buffer = [];
    const reader = o.getReader();
    while(true){
        const { value , done  } = await reader.read();
        if (done) {
            return buffer;
        }
        buffer.push(value);
    }
}
function concat(...os) {
    const { writable , readable  } = new TransformStream();
    (async function() {
        for (const o of os){
            await o.pipeTo(writable, {
                preventClose: true
            });
        }
        writable.getWriter().close();
    })();
    return readable;
}
const EOF = Symbol();
function external() {
    let next;
    const observable = new ReadableStream({
        start (controller) {
            next = (v)=>{
                if (v === EOF) {
                    return controller.close();
                }
                controller.enqueue(v);
            };
        }
    });
    return {
        observable,
        next: next
    };
}
function range(start, end) {
    const { observable , next  } = external();
    const len = Math.abs(end - start);
    const dir = Math.sign(end - start);
    for(let i = 0; i <= len; i++){
        next(start + i * dir);
    }
    next(EOF);
    return observable;
}
async function main() {
    const list = collect(concat(range(1, 2)));
    console.log(await list, [
        1,
        2,
        3,
        4
    ]);
}
await main();

The isolate crashes with error: Module evaluation is still pending but there are no pending ops or dynamic imports. This situation is often caused by unresolved promise. at o.pipeTo call. Looking at the runtime implementation, a deferred promise is created (and returned), that is not resolved (line 1881 in op_crates/fetch/11_stream.js). This results in a hanging promise, breaking the isolate.

cc @kitsonk