fix(mergeAll): add source subscription to composite before actually subscribing (#2479)

Add subscriptions for source Observables to mergeAll composite subscription
before actually subscribing to any of these Observables, so that if
source Observable emits synchronously and consumer of mergeAll unsubscribes
at that moment (for example `take` operator), subscription to source is
unsubscribed as well and Observable stops emitting.

Closes #2476

Author: mpodlasin

spec/operators/mergeAll-spec.ts

@@ -1,5 +1,5 @@
 import { expect } from 'chai';
-import { mergeAll, mergeMap } from 'rxjs/operators';
+import { mergeAll, mergeMap, take } from 'rxjs/operators';
 import { hot, cold, expectObservable, expectSubscriptions } from '../helpers/marble-testing';
 import { throwError, from, of, Observable } from 'rxjs';
 
@@ -413,6 +413,36 @@ describe('mergeAll oeprator', () => {
       () => { done(new Error('should not be called')); });
   });
 
+  it('should finalize generators when merged if the subscription ends', () => {
+    const iterable = {
+      finalized: false,
+      next() {
+        return {value: 'duck', done: false};
+      },
+      return() {
+        this.finalized = true;
+      },
+      [Symbol.iterator]() {
+        return this;
+      }
+    };
+
+    const results: string[] = [];
+
+    const iterableObservable = from<string>(iterable as any);
+    of(iterableObservable).pipe(
+      mergeAll(),
+      take(3)
+    ).subscribe(
+      x => results.push(x),
+      null,
+      () => results.push('GOOSE!')
+    );
+
+    expect(results).to.deep.equal(['duck', 'duck', 'duck', 'GOOSE!']);
+    expect(iterable.finalized).to.be.true;
+  });
+
   type(() => {
     /* tslint:disable:no-unused-variable */
     const source1 = of(1, 2, 3);

src/internal/operators/mergeMap.ts

@@ -139,7 +139,9 @@ export class MergeMapSubscriber<T, R> extends OuterSubscriber<T, R> {
   }
 
   private _innerSub(ish: ObservableInput<R>, value: T, index: number): void {
-    this.add(subscribeToResult<T, R>(this, ish, value, index));
+    const innerSubscriber = new InnerSubscriber(this, undefined, undefined);
+    this.add(innerSubscriber);
+    subscribeToResult<T, R>(this, ish, value, index, innerSubscriber);
   }
 
   protected _complete(): void {

src/internal/util/subscribeToResult.ts

@@ -1,19 +1,26 @@
-
 import { ObservableInput } from '../types';
 import { Subscription } from '../Subscription';
 import { InnerSubscriber } from '../InnerSubscriber';
 import { OuterSubscriber } from '../OuterSubscriber';
+import { Subscriber } from '../Subscriber';
 import { subscribeTo } from './subscribeTo';
 
-export function subscribeToResult<T, R>(outerSubscriber: OuterSubscriber<T, R>,
-                                        result: any,
-                                        outerValue?: T,
-                                        outerIndex?: number): Subscription;
-export function subscribeToResult<T>(outerSubscriber: OuterSubscriber<any, any>,
-                                     result: ObservableInput<T>,
-                                     outerValue?: T,
-                                     outerIndex?: number): Subscription | void {
-  const destination = new InnerSubscriber(outerSubscriber, outerValue, outerIndex);
-
+export function subscribeToResult<T, R>(
+  outerSubscriber: OuterSubscriber<T, R>,
+  result: any,
+  outerValue?: T,
+  outerIndex?: number,
+  destination?: Subscriber<any>
+): Subscription;
+export function subscribeToResult<T, R>(
+  outerSubscriber: OuterSubscriber<T, R>,
+  result: any,
+  outerValue?: T,
+  outerIndex?: number,
+  destination: Subscriber<any> = new InnerSubscriber(outerSubscriber, outerValue, outerIndex)
+): Subscription | void {
+  if (destination.closed) {
+    return;
+  }
   return subscribeTo(result)(destination);
 }