How to get public download link within a firebase storage trigger function: "onFinalize"?

Question

I am writing a firebase cloud function that records the download link of a recentally uploaded file to real-time database:

exports.recordImage = functions.storage.object().onFinalize((object) => {

});

"object" gives me access to two variables "selfLink" and "mediaLink" but both of them when entered in a browser they return the following:

Anonymous caller does not have storage.objects.get access to ... {filename}

So, they are not public links. How can I get the public download link within this trigger function?

Answer 1

You have to use the asynchronous getSignedUrl() method, see the doc of the Cloud Storage Node.js library: https://cloud.google.com/nodejs/docs/reference/storage/2.0.x/File#getSignedUrl.

So the following code should do the trick:

.....
const defaultStorage = admin.storage();
.....

exports.recordImage = functions.storage.object().onFinalize(object => {    
  const bucket = defaultStorage.bucket();
  const file = bucket.file(object.name);

  const options = {
    action: 'read',
    expires: '03-17-2025'
  };

  // Get a signed URL for the file
  return file
    .getSignedUrl(options)
    .then(results => {
      const url = results[0];

      console.log(`The signed url for ${filename} is ${url}.`);
      return true;
    })

});

Note that, in order to use the getSignedUrl() method, you need to initialize the Admin SDK with the credentials for a dedicated service account, see this SO Question & Answer firebase function get download url after successfully save image to firebase cloud storage.