firebase realtime database reads data that doesnt exist

问题

we came across what might be a major bug in the firebase database, pls see code below. the code below tries to set a value to a child "EXAMPLE" which doesnt have a read or write permission. the write operation doesn't write anything to the database and throws an error " setValue at /EXAMPLE/VALUE failed: DatabaseError: Permission denied" in the log, which is a good thing.

however a major issue is with the code that comes after which tries to read the value of child "EXAMPLE", the code actually goes into the ondatachange method and reads the value as "ONE" instead of going into the onCancelled method to throw a permission error, the data doesnt even exist in the database and there is no read or write permission for the child "EXAMPLE" so how can fireabase claim to read a value that is not even there.

myReftwo.child("EXAMPLE").child("VALUE").setValue("ONE");

myReftwo.child("EXAMPLE").child("VALUE").addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot)
    {

        Log.d("print", dataSnapshot.getValue().toString()  );


    }
    @Override
    public void onCancelled(DatabaseError databaseError)
    {
        Log.d("print",  databaseError.getMessage() );
    }
});

回答1:

In the snippet as you share it, most likely the listener is getting the value from the local cache before that cache has been updated because of the rejection from the server.

When you add a listener, Firebase tries to give you the value it expects the node to have immediately. And since you call addListenerForSingleValueEvent, it them immediately stops listening for the value. So you end up seeing only stale value from the local cache, and never see the actual value (or lack thereof) from the server.

For this reason you should not use both disk persistence and addListenerForSingleValueEvent in your app. Using addValueEventListener in the same scenario would lead to two calls to onDataChange: the first one with the value from the local cache, and the second one with the correct snapshot from the server.

For a longer answer on how these work, and why they don't result in the behavior you'd like, see: Firebase Offline Capabilities and addListenerForSingleValueEvent

来源：https://stackoverflow.com/questions/63751187/firebase-realtime-database-reads-data-that-doesnt-exist