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.

Answer 2

*use this function:

function mediaLinkToDownloadableUrl(object) {
        var firstPartUrl = object.mediaLink.split("?")[0] // 'https://www.googleapis.com/download/storage/v1/b/abcbucket.appspot.com/o/songs%2Fsong1.mp3.mp3'
        var secondPartUrl = object.mediaLink.split("?")[1] // 'generation=123445678912345&alt=media'

        firstPartUrl = firstPartUrl.replace("https://www.googleapis.com/download/storage", "https://firebasestorage.googleapis.com")
        firstPartUrl = firstPartUrl.replace("v1", "v0")

        firstPartUrl += "?" + secondPartUrl.split("&")[1]; // 'alt=media'
        firstPartUrl += "&token=" + object.metadata.firebaseStorageDownloadTokens

        return firstPartUrl
    }

this is how your code might look like:

export const onAddSong = functions.storage.object().onFinalize((object) => {

    console.log("object: ", object);

    var url = mediaLinkToDownloadableUrl(object);

    //do anything with url, like send via email or save it in your database in playlist table 
    //in my case I'm saving it in mongodb database

    return new playlistModel({
        name: storyName,
        mp3Url: url,
        ownerEmail: ownerEmail
    })
    .save() // I'm doing nothing on save complete
    .catch(e => {
        console.log(e) // log if error occur in database write
    })

})

*I have tested this method on mp3 files, I'm sure it will work on all type of files but incase if it doesnt work for you simply go to firebase storage dashboard open any file and copy download url, and try to generate the same url in your code, and edit this answer too if possible