Hummus.JS read/write from Google Cloud Storage

Question

Per the issue I reported for Hummus.js: #342, I'm having trouble writing an appended PDF to Google Cloud Storage.

The simplest MWE, using Firebase functions, is:

const getRawBody = require('raw-body')
const admin = require('firebase-admin')

function asBuffer (filename) {
  const bucket = admin.storage().bucket()
  const file = bucket.file(filename)
  return getRawBody(file.createReadStream())
}

function writePdf (buffer, writeStream) {
  const pdfWriter = hummus.createWriter(
    new hummus.PDFStreamForResponse(writeStream))
  }
  const rstream = new hummus.PDFRStreamForBuffer(buffer)
  pdfWriter.appendPDFPagesFromPDF(rstream) //  Memory error / timeout.
}

const buffer = asBuffer('gcs-path.pdf')
const file = bucket.file('new.pdf')
const fileOptions = {}
const writeStream = file.createWriteStream(fileOptions)

writePdf(buffer, writeStream)

Firebase functions consistently fails at appendPDFPagesFromPDF with a memory limit error or timeout.

This workflow appears to be fine for other types of writeable streams (e.g. memorystream and file streams).

This example could be made simpler by hard-coding a simple PDF, but it's worthwhile to note it since the issue at hand seems to be streaming related — and ideally this process could be simplified even further to remove the intermediate Buffer conversion and switch to streams.