Session validators availability detection (#1759)

packages/api-derive/src/chain/subscribeNewHeads.ts

@@ -4,7 +4,7 @@
 
 import { ApiInterfaceRx } from '@polkadot/api/types';
 
-import { Observable, combineLatest } from 'rxjs';
+import { Observable, combineLatest, of } from 'rxjs';
 import { map } from 'rxjs/operators';
 
 import { HeaderExtended } from '../type';
@@ -27,9 +27,11 @@ export function subscribeNewHeads (api: ApiInterfaceRx): () => Observable<Header
   return memo((): Observable<HeaderExtended> =>
     combineLatest([
       api.rpc.chain.subscribeNewHeads(),
-      api.derive.staking.validators()
+      api.query.session
+        ? api.query.session.validators()
+        : of([])
     ]).pipe(
-      map(([header, { validators }]): HeaderExtended =>
+      map(([header, validators]): HeaderExtended =>
         new HeaderExtended(api.registry, header, validators)
       )
     ));

packages/api-derive/src/staking/validators.ts

@@ -2,13 +2,11 @@
 // This software may be modified and distributed under the terms
 // of the Apache-2.0 license. See the LICENSE file for details.
 
-import { AccountId } from '@polkadot/types/interfaces';
 import { ApiInterfaceRx } from '@polkadot/api/types';
 import { DeriveStakingValidators } from '../types';
 
-import { Observable, of } from 'rxjs';
+import { Observable, combineLatest, of } from 'rxjs';
 import { map } from 'rxjs/operators';
-import { Vec } from '@polkadot/types';
 
 import { memo } from '../util';
 
@@ -17,16 +15,17 @@ import { memo } from '../util';
  */
 export function validators (api: ApiInterfaceRx): () => Observable<DeriveStakingValidators> {
   return memo((): Observable<DeriveStakingValidators> =>
-    (
-      // Sadly the node-template is (for some obscure reason) not comprehensive, so while the derive works
-      // in all actual real-world deployed chains, it does create some confusion for limited template chains
-      api.query.session && api.query.staking
-        ? api.queryMulti<[Vec<AccountId>, Vec<AccountId>]>([
-          api.query.session.validators,
-          api.query.staking.currentElected
-        ])
-        : of([[], []] as [AccountId[], AccountId[]])
-    ).pipe(
+    // Sadly the node-template is (for some obscure reason) not comprehensive, so while the derive works
+    // in all actual real-world deployed chains, it does create some confusion for limited template chains
+    // NOTE: Not doing multi queries here, since we have validators as a single in the derived newHead
+    combineLatest([
+      api.query.session
+        ? api.query.session.validators()
+        : of([]),
+      api.query.staking
+        ? api.query.staking.currentElected()
+        : of([])
+    ]).pipe(
       map(([validators, currentElected]): DeriveStakingValidators => ({
         currentElected, validators
       }))