In Firebase storage security rules (not realtime database), is there any way to perform a check if another object exists at path, or another object's metadata exists?
Some background
Currently my storage security rules are set up so that users only have read access, and not write access to their /users/{userId}/ paths.
I have an admin cloud function that saves a file to /users/{userId}/necessary-file.pdf. And I don't want users to be able to modify or write this file and only cloud functions to have the right to do. To achieve this I think I can match for the filename like :
match /users/{userId}/{fileName} {
allow write: if !fileName.matches("necessary-file.pdf")
}
Question
Is there any way for me to only allow users to write some-other-file.pdf if they already have a necessary-file.pdf at the same path (or even somewhere else if that works better). All while still disallowing them to write necessary-file.pdf.
So is there any way for me to do something like this pseudo-code? :
match /users/{userId}/{fileName} {
allow read: if request.auth.uid == userId;
allow write: if (!fileName.matches("necessary-file.pdf")) && ("necessary-file.pdf".exists())
}
As an alternative, I can have my cloud functions write a metadata to necessary-file.pdf and check for that too. is there any way I can perform something like this pseudo-code? :
allow write: if "necessary-file.pdf".metadata['canUserWrite'] == 'yesUserCan'
Finally
What's really cool about this is that, if this is in any way remotely possible, it can be used to communicate between firebase database and firebase storage rules in a not-so-realtime way. (referring to this question here) A cloud function can listen for changes in the intended field in realtime database, and write a file to firebase storage, which firebase storage can check for.
Firebase's Cloud Storage security rules can only access information about the current request and file. They don't have access to the full storage system, so can't check whether another file is present.
The reason for this is that the rules are evaluated in memory for every request. Providing access to Cloud Storage for other objects would slow the performance down, making the system unscalable. That same reason explains why you can't access the Firebase Database from the security rules.
If you want some control like this, you'll want to look in Cloud Functions for Firebase. If you have your users upload their files into a "staging" area, you can have a Cloud Function validate whether they met all prerequisites and only then move the file into the actual location (making it available for further processing or for clients to see).