Flutter: implement any authentication in Firebase storage with google authentication fails

Question

my app is using this code in flutter to upload images:

final StorageReference imageRef = FirebaseStorage.instance.ref().child('postimages');
    final StorageUploadTask uploadTask = imageRef.child('/' + currentUser.id + '/post_$postid.jpg').putFile(_image);
    var upurl = await (await uploadTask.onComplete).ref.getDownloadURL();
    url = upurl.toString();
    debugPrint('url:$url');

which is working fine if i grant full rewrite access by these unrecommended rules:

rules_version = '2';
service firebase.storage {
  match /b/{bucket}/o {
    match /{allPaths=**} {
      allow read, write: if true;
    }
  }
}

but if i try to implement any security for example with this rule:

rules_version = '2';
service firebase.storage {
  match /b/{bucket}/o {
    match /{allPaths=**} {      
      allow read, write: if request.auth != null;

    }
  }
}

i end up getting the following exception: [VERBOSE-2:ui_dart_state.cc(177)] Unhandled Exception: PlatformException(Error -13021, FIRStorageErrorDomain, User does not have permission to access gs://picturegramm.appspot.com/postimages/118190432651181005229/post_b050df01-7b58-4016-8d3c-28c7f3ace630.jpg., null)

The app is using firebase authentication with google as sign-in provider. Reading the docs it looks like the auth info is always sent automatically. The signing seems to work perfectly, since i am getting the current user with the userid. So why do the requests fail?

Are you sure the user has signed in with google. If the user is signed out this will not work. They must be signed in. — Gabe
Any idea how i verify this before uploading? The login seems to be working since i am getting e.g. email, id and profile pic from it. — Sebastian
Check if the user exists by final user = FirebaseAuth.instance.currentUser; user will be null if the user isn't logged in. — Gabe
ty! it is null. now i have to find out why that is the case after a signing has been completed — Sebastian
Now at least you have narrowed down the problem. I'm guessing you have some error somewhere in your login logic. — Gabe

Sebastian Sebastian · Accepted Answer · 2020-10-28T15:28:39

thanks to the comments from Gabe i found out the code is indeed valid, but my authentication was incomplete, since i was not passing the login to firebase. for future reference this the complete login code that is needed to make it work:

  final GoogleSignInAccount googleUser = await GoogleSignIn().signIn();

  // Obtain the auth details from the request
  final GoogleSignInAuthentication googleAuth = await googleUser.authentication;

  // Create a new credential
  final GoogleAuthCredential credential = GoogleAuthProvider.credential(
    accessToken: googleAuth.accessToken,
    idToken: googleAuth.idToken,
  );

  // Once signed in, return the UserCredential
  UserCredential creds = await  FirebaseAuth.instance.signInWithCredential(credential);
  debugPrint('creds:$creds');

Flutter: implement any authentication in Firebase storage with google authentication fails

1 Answers