Apply fallback on non-sub storage (e.g. at) (#1004)

* Apply fallback on non-sub storage (e.g. at)

* Fallback test in rpc-code (getStorage)

packages/api/test/e2e/promise-queries.spec.ts

@@ -40,6 +40,19 @@ describe.skip('Promise e2e queries', () => {
     expect(api.derive).toBeDefined();
   });
 
+  it('allows retrieval of an fallback entry (once-off query)', async () => {
+    const nonce = await api.query.system.accountNonce('5DSo5RVtfrtgHoz2c7jK7Tca7FgJgpCzFnxoRVDeYUQcKPng');
+
+    expect(nonce.toHex()).toEqual('0x0000000000000000');
+  });
+
+  it('allows retrieval of fallback when at query is made', async () => {
+    const header = await api.rpc.chain.getHeader() as Header;
+    const nonce = await api.query.system.accountNonce.at(header.hash, '5DSo5RVtfrtgHoz2c7jK7Tca7FgJgpCzFnxoRVDeYUQcKPng');
+
+    expect(nonce.toHex()).toEqual('0x0000000000000000');
+  });
+
   it('queries state for a balance', async () => {
     return (
       api.query.balances.freeBalance(keyring.alice.address, (balance) => {

packages/rpc-core/src/formatting.spec.ts

@@ -15,6 +15,10 @@ import Api from '.';
 
 const ADDR_ONE = '5DkQbYAExs3M2sZgT1Ec3mKfZnAQCL4Dt9beTCknkCUn5jzo';
 const ADDR_TWO = '5C62W7ELLAAfix9LYrcx5smtcffbhvThkM5x7xfMeYXCtGwF';
+const BALANCE_KEYS = [
+  /* v3- */ '0x4af2c53fce3ec33c6ccccf22e926f1a7',
+  /* v4+ */ '0xec8f96437274a883afcac82d01a9defeb68209cd4f2c084632813692aa5e65ad'
+];
 
 function formattingTests (version: string, storage: Storage, encodedValues: [String, String]) {
   const [ENC_ONE, ENC_TWO] = encodedValues;
@@ -25,8 +29,12 @@ function formattingTests (version: string, storage: Storage, encodedValues: [Str
 
     beforeEach(() => {
       provider = {
-        send: jest.fn((method, params) =>
-          Promise.resolve('0x01020000000000000000000000000000')
+        send: jest.fn((method, [key]) =>
+          Promise.resolve(
+            BALANCE_KEYS.includes(key)
+              ? '0x01020000000000000000000000000000'
+              : null
+          )
         ),
         subscribe: jest.fn((type, method, params, subscription) =>
           subscription(null, {
@@ -55,6 +63,14 @@ function formattingTests (version: string, storage: Storage, encodedValues: [Str
         });
     });
 
+    it('returns the fallback result on not-found values', () => {
+      return api.state
+        .getStorage([storage.system.accountNonce, ADDR_ONE])
+        .then((value) => {
+          expect(value.toHex()).toEqual('0x0000000000000000');
+        });
+    });
+
     it('encodes multiple keys, decoding multiple results', (done) => {
       return api.state.subscribeStorage(
         [

packages/rpc-core/src/index.ts

@@ -242,7 +242,7 @@ export default class Rpc implements RpcInterface {
       );
     }
 
-    return createType(type, base, true);
+    return createType(type, isNull ? meta.fallback : base, true);
   }
 
   private formatStorageSet (key: StorageKey, base: StorageChangeSet): Codec | undefined {