uppy/packages/@uppy/companion/src/server/helpers/upload.js

const Uploader = require('../Uploader')
const logger = require('../logger')
const { respondWithError } = require('../provider/error')

async function startDownUpload({ req, res, getSize, download }) {
  try {
    const size = await getSize()
    const { clientSocketConnectTimeout } = req.companion.options

    logger.debug('Instantiating uploader.', null, req.id)
    const uploader = new Uploader(Uploader.reqToOptions(req, size))

    logger.debug('Starting download stream.', null, req.id)
    const stream = await download()

      // "Forking" off the upload operation to background, so we can return the http request:
      ; (async () => {
        // wait till the client has connected to the socket, before starting
        // the download, so that the client can receive all download/upload progress.
        logger.debug('Waiting for socket connection before beginning remote download/upload.', null, req.id)
        await uploader.awaitReady(clientSocketConnectTimeout)
        logger.debug('Socket connection received. Starting remote download/upload.', null, req.id)

        await uploader.tryUploadStream(stream)
      })().catch((err) => logger.error(err))

    // Respond the request
    // NOTE: the Uploader will continue running after the http request is responded
    res.status(200).json({ token: uploader.token })
  } catch (err) {
    if (err.name === 'ValidationError') {
      logger.debug(err.message, 'uploader.validator.fail')
      res.status(400).json({ message: err.message })
      return
    }

    if (respondWithError(err, res)) return

    throw err
  }
}

module.exports = { startDownUpload }