fix: chain interop/safe subscriber unsubscriptions correctly

spec/Subscriber-spec.ts

@@ -1,6 +1,7 @@
 import { expect } from 'chai';
 import { SafeSubscriber } from 'rxjs/internal/Subscriber';
-import { Subscriber } from 'rxjs';
+import { Subscriber, Observable } from 'rxjs';
+import { asInteropSubscriber } from './helpers/interop-helper';
 
 /** @test {Subscriber} */
 describe('Subscriber', () => {
@@ -97,4 +98,22 @@ describe('Subscriber', () => {
 
     expect(argument).to.have.lengthOf(0);
   });
+
+  it('should chain interop unsubscriptions', () => {
+    let observableUnsubscribed = false;
+    let subscriberUnsubscribed = false;
+    let subscriptionUnsubscribed = false;
+
+    const subscriber = new Subscriber<void>();
+    subscriber.add(() => subscriberUnsubscribed = true);
+
+    const source = new Observable<void>(() => () => observableUnsubscribed = true);
+    const subscription = source.subscribe(asInteropSubscriber(subscriber));
+    subscription.add(() => subscriptionUnsubscribed = true);
+    subscriber.unsubscribe();
+
+    expect(observableUnsubscribed).to.be.true;
+    expect(subscriberUnsubscribed).to.be.true;
+    expect(subscriptionUnsubscribed).to.be.true;
+  });
 });

src/internal/Subscriber.ts

@@ -1,7 +1,7 @@
 import { isFunction } from './util/isFunction';
 import { empty as emptyObserver } from './Observer';
 import { Observer, PartialObserver } from './types';
-import { Subscription } from './Subscription';
+import { Subscription, isSubscription } from './Subscription';
 import { rxSubscriber as rxSubscriberSymbol } from '../internal/symbol/rxSubscriber';
 import { config } from './config';
 import { hostReportError } from './util/hostReportError';
@@ -187,8 +187,8 @@ export class SafeSubscriber<T> extends Subscriber<T> {
       complete = (<PartialObserver<T>> observerOrNext).complete;
       if (observerOrNext !== emptyObserver) {
         context = Object.create(observerOrNext);
-        if (isFunction(context.unsubscribe)) {
-          this.add(<() => void> context.unsubscribe.bind(context));
+        if (isSubscription(observerOrNext)) {
+          observerOrNext.add(this.unsubscribe.bind(this));
         }
         context.unsubscribe = this.unsubscribe.bind(this);
       }

src/internal/Subscription.ts

@@ -206,6 +206,12 @@ export class Subscription implements SubscriptionLike {
   }
 }
 
+export function isSubscription(value: any): value is Subscription {
+  return value &&
+    typeof value.add === 'function' &&
+    typeof value.unsubscribe === 'function';
+}
+
 function flattenUnsubscriptionErrors(errors: any[]) {
  return errors.reduce((errs, err) => errs.concat((err instanceof UnsubscriptionError) ? err.errors : err), []);
 }

src/internal/operators/finalize.ts

@@ -2,7 +2,6 @@ import { Operator } from '../Operator';
 import { Subscriber } from '../Subscriber';
 import { Observable } from '../Observable';
 import { MonoTypeOperatorFunction, TeardownLogic } from '../types';
-import { subscribeWith } from '../util/subscribeWith';
 
 /**
  * Returns an Observable that mirrors the source Observable, but will call a specified function when
@@ -68,7 +67,7 @@ class FinallyOperator<T> implements Operator<T, T> {
   }
 
   call(subscriber: Subscriber<T>, source: any): TeardownLogic {
-    const subscription = subscribeWith(source, subscriber);
+    const subscription = source.subscribe(subscriber);
     subscription.add(this.callback);
     return subscription;
   }

src/internal/util/subscribeToObservable.ts

@@ -1,6 +1,5 @@
 import { Subscriber } from '../Subscriber';
 import { observable as Symbol_observable } from '../symbol/observable';
-import { subscribeWith } from './subscribeWith';
 
 /**
  * Subscribes to an object that implements Symbol.observable with the given
@@ -13,6 +12,6 @@ export const subscribeToObservable = <T>(obj: any) => (subscriber: Subscriber<T>
     // Should be caught by observable subscribe function error handling.
     throw new TypeError('Provided object does not correctly implement Symbol.observable');
   } else {
-    return subscribeWith(obs, subscriber);
+    return obs.subscribe(subscriber);
   }
 };