Fix incorrect databaseUrl in Functions emulator

[samtstern]

Description

Fixes #2965

Due to the new location support in Realtime Database it is no longer safe to assume that https://<PROJECT-ID>.firebaseio.com is the default RTDB instance location. This PR uses the getAdminSdkConfig REST endpoint to fetch the real database URL and pass it down to the Functions emulator.

In order to make this behavior work offline there are two fallbacks:

• Successful fetches populate a local cache which can be used when offline.
• If the cache is empty we can try and guess the Admin SDK config using the old (and not so good) method.

This fallback behavior is based on what we do with web config in the Hosting emulator.

Scenarios Tested

See the setup in #2965. I created a brand new project and created the defalt RTDB through firebase init.

First add a message using the Cloud Function:

$ http http://localhost:5001/fir-tools-2965/us-central1/addMessage?text="hello"
HTTP/1.1 200 OK
connection: keep-alive
content-length: 57
content-type: application/json; charset=utf-8
date: Mon, 28 Dec 2020 18:14:23 GMT
etag: W/"39-bGCJNRIDTzsIri/GyVcXPXLVrbs"
x-powered-by: Express

{
    "result": "Message with ID: -MPeYivLKiIxoGNp2Eil added."
}

I can see that the data goes to the right place in the Emulator UI:

Finally if I load the hosting emulator I can see the correct logs:

Sample Commands

N/A

[cjam]

@samtstern I recently updated the tools locally and found that I cant get my local database rules to apply to the correct instance anymore. I have my database.rules.json which automatically gets applied to projectId but now the actual database instance id is pulled in from the cloud. How do I configure the local database rules / firebase.json so that the rules apply to this database instance?

[samtstern]

@cjam can you file a new issue about this and explain in a little more detail? Definitely sounds like a bug.

[cjam]

@samtstern I can. I'm a bit busy at the moment to create all of the required details for the bug report but I'll try and get to it in the next few days.