How to make Firebase Cloud Function wait for a stream to close before terminating

Question

I store an object to Cloud Storage like this:

const bucket = getStorage().bucket();
const fileRef = bucket.file("/foo/bar");
const storageWriteStream = fileRef.createWriteStream();

const myFileStream = someLibraryThatCreatesAStream();

storageWriteStream.on("error", (err) => {
  functions.logger.error(err);
 });

storageWriteStream.on("close", (err) => {
  return await getDownloadURL(fileRef)
});

myFileStream.pipe(storageWriteStream);

But when called on the front-end, the result is always null. Since Functions seems to always has trouble with return as part of an event handler, I think it's because Functions doesn't wait for storageWriteStream's close event.

How do I get around this?

Answer 1

You will need to turn the stream's completion into a promise using new Promise, and then either return or await that promise. For example:

const bucket = getStorage().bucket();
const fileRef = bucket.file("/foo/bar");

try {
  await new Promise((resolve, reject) => {
    const storageWriteStream = fileRef.createWriteStream();
    const myFileStream = someLibraryThatCreatesAStream();
    storageWriteStream.on("error", reject);
    storageWriteStream.on("close", resolve);
    myFileStream.pipe(storageWriteStream);
  })
  return getDownLoadURL(fileRef);
} catch (error) {
  functions.logger.error(err);
}