Print a useful error explaining why Runtime Config doesn't work (#930)

* Print a useful error explaining why Runtime Config doesn't work

* PR feedback

Author: inlined

spec/v1/config.spec.ts

@@ -47,6 +47,23 @@ describe('config()', () => {
     (config as any).firebaseConfigCache = null;
     delete process.env.FIREBASE_CONFIG;
     delete process.env.CLOUD_RUNTIME_CONFIG;
+    delete process.env.K_CONFIGURATION;
+  });
+
+  it('will never load in GCFv2', () => {
+    const json = JSON.stringify({
+      foo: 'bar',
+      firebase: {},
+    });
+    readFileSync
+      .withArgs('/srv/.runtimeconfig.json')
+      .returns(Buffer.from(json));
+
+    process.env.K_CONFIGURATION = 'my-service';
+    expect(() => config.config()).to.throw(
+      Error,
+      /transition to using environment variables/
+    );
   });
 
   it('loads config values from .runtimeconfig.json', () => {

src/v1/config.ts

@@ -26,6 +26,14 @@ import * as path from 'path';
 import * as firebase from 'firebase-admin';
 
 export function config(): config.Config {
+  // K_CONFIGURATION is only set in GCFv2
+  if (process.env.K_CONFIGURATION) {
+    throw new Error(
+      'functions.config() is no longer available in Cloud Functions for ' +
+        'Firebase v2. Please see the latest documentation for information ' +
+        'on how to transition to using environment variables'
+    );
+  }
   if (typeof config.singleton === 'undefined') {
     init();
   }