firebase-tools: Functions shell/emulator unable to load default credentials

[REQUIRED] Environment info

firebase-tools: 7.12.1

Platform: Debian on WSL on Windows 10

[REQUIRED] Test case

import * as functions from 'firebase-functions';
const admin = require('firebase-admin').initializeApp(functions.config().firebase);

exports.action1 = functions.https.onCall(() => {
  return admin.database().ref('action').push({ 'a': 1 })
});

[REQUIRED] Steps to reproduce

npm run shell with the above code, execute function action({})

[REQUIRED] Expected behavior

The shell environment should be able to locally simulate the function and access the database, according to the documentation: Cloud Firestore and Realtime Database triggers already have sufficient credentials, and do not require additional setup.

[REQUIRED] Actual behavior

I receive the following message:

@firebase/database: FIREBASE WARNING: {“code”:“app/invalid-credential”,“message”:“Credential implementation provided to initializeApp() via the "credential" property failed to fetch a valid Google OAuth2 access token with the following error: "Error fetching access token: Error while making request: getaddrinfo ENOTFOUND metadata.google.internal. Error code: ENOTFOUND".”}

Some digging through the documentation provided me with various different answers, none of which solved my issue. Please note that I would like to locally develop with the same code as the one that I deploy. Some things I found:

1. I tried with and without functions.config().firebase but it’s not clear when and if this is required

2. Setup Admin Credentials using the export command. I changed the default command to:

"shell": "export GOOGLE_APPLICATION_CREDENTIALS=\"my-owner-key.json\" && tsc --watch --preserveWatchOutput & firebase functions:shell",

According to the (closed) issue #595 for some reason a protect_env flag sets this environment variable to an empty string. I don’t see the reasoning for this ‘protection’ as the documentation explicitly explains you need to set this variable if you wish to use the shell in an authenticated fashion. PR #1252 added the functionality to disable protect_env but I couldn’t get the --disable-features flag working or even find any documentation on it anywhere.

1. Manually updating the autopopulated config

serviceAccount = require('./serviceAccount.json');

const adminConfig = JSON.parse(process.env.FIREBASE_CONFIG);
adminConfig.credential = admin.credential.cert(serviceAccount);
admin.initializeApp(adminConfig);

Somehow injecting the json is supposed to work according to the documentation however this code fails to compile with the following error:

error TS2345: Argument of type 'string | undefined' is not assignable to parameter of type 'string'.
  Type 'undefined' is not assignable to type 'string'.

const adminConfig = JSON.parse(process.env.FIREBASE_CONFIG);

It also feels hacky and would not be transferrable to the server as-is as it explicitly depends on a .json credential file being deployed.

Edit: I found a 4 month old issue on this topic that is still regularly receiving reports of users encountering the same problem: #1708