I need some help how to use the GridFSBucket stream in the streamFile function.
In this working example functions a file on the disc will be read and returned as a stream:

import { NextRequest, NextResponse } from "next/server";
import { ReadableOptions } from "stream";

function streamFile(path: string, options?: ReadableOptions): ReadableStream<Uint8Array> {
    const downloadStream = fs.createReadStream(path, options); // Replace this

    return new ReadableStream({
        start(controller) {
            downloadStream.on("data", (chunk: Buffer) => controller.enqueue(new Uint8Array(chunk)));
            downloadStream.on("end", () => controller.close());
            downloadStream.on("error", (error: NodeJS.ErrnoException) => controller.error(error));
        },
        cancel() {
            downloadStream.destroy();
        },
    });
}

export async function GET(req: NextRequest): Promise<NextResponse> {
    const file = req.nextUrl.searchParams.get("file");                                
    const stats: Stats = await fs.promises.stat(file);                              
    const data: ReadableStream<Uint8Array> = streamFile(file);                     
    const res = new NextResponse(data, {                                            
        status: 200,                                                                    
        headers: new Headers({                                                          
            "content-type": "video/mp4",                                            
            "content-length": stats.size + "",                                            
        }),
    });

    return res;                           
}

But my data comes from a mongoDB GridFSBucket.
This is how I’m streaming a video file directly from the GridFSBucket:

export const getServerSideProps: GetServerSideProps = async ({
    res,
    query: { hash }
}) => {        
    const database = await mongodb()
    const Videos = database.collection('videos')

    const { fileId } = await Videos.findOne({ uid: hash })

    const Files = new GridFSBucket(database)
    const id = new ObjectId(fileId)

    const file: GridFSFile = await new Promise((resolve, reject) => {
        Files.find({
            _id: id
        }).toArray((err, files) => {
            if (err) reject(err)
            resolve(files[0])
        })
    })
    const { contentType } = file || {}

    res.writeHead('Content-Type', contentType) // contentType is "video/mp4"
    Files.openDownloadStream(id)
    .on('data', (chunk) => {
        res.write(chunk)
    })
    .on('end', () => {
        res.end()
    })
    .on('error', (err) => {
        throw err
    })

    return {
        props: {}
    }
}

So I would think of using the GridFSBucket part instead of createReadStream from the file.
But the res.writeHead and openDownloadStream makes me struggle. How do I get a ReadableStream? Or maybe this isn’t needed?

I need to convert this, because my getServerSideProps is used in an old nextJS application, which uses pages router and I need to migrate to new app router. Therefore I need a server route component.