fix: errors thrown from iterables now properly propagated (#5444)

* fix: errors thrown from iterables now properly propagated

* chore: fix lint failures

Co-authored-by: Nicholas Jamieson <nicholas@cartant.com>

spec/operators/mergeMap-spec.ts

@@ -1,14 +1,22 @@
 import { expect } from 'chai';
-import { mergeMap, map } from 'rxjs/operators';
+import { mergeMap, map, delay } from 'rxjs/operators';
 import { asapScheduler, defer, Observable, from, of, timer } from 'rxjs';
 import { hot, cold, expectObservable, expectSubscriptions } from '../helpers/marble-testing';
 import { asInteropObservable } from '../helpers/interop-helper';
+import { TestScheduler } from 'rxjs/testing';
+import { observableMatcher } from '../helpers/observableMatcher';
 
-declare const type: Function;
 declare const asDiagram: Function;
 
 /** @test {mergeMap} */
 describe('mergeMap', () => {
+  let rxTest: TestScheduler;
+
+  // TODO: Convert the rest of these tests to use run mode!
+  beforeEach(() => {
+    rxTest = new TestScheduler(observableMatcher);
+  });
+
   asDiagram('mergeMap(i => 10*i\u2014\u201410*i\u2014\u201410*i\u2014| )')
   ('should map-and-flatten each item to an Observable', () => {
     const e1 =    hot('--1-----3--5-------|');
@@ -821,14 +829,27 @@ describe('mergeMap', () => {
     }, 0);
   });
 
-  type('should support type signatures', () => {
-    let o: Observable<number>;
-
-    /* tslint:disable:no-unused-variable */
-    let a1: Observable<string> = o.pipe(mergeMap(x => x.toString()));
-    let a2: Observable<string> = o.pipe(mergeMap(x => x.toString(), 3));
-    let a3: Observable<{ o: number; i: string; }> = o.pipe(mergeMap(x => x.toString(), (o, i) => ({ o, i })));
-    let a4: Observable<{ o: number; i: string; }> = o.pipe(mergeMap(x => x.toString(), (o, i) => ({ o, i }), 3));
-    /* tslint:enable:no-unused-variable */
+  // NOTE: From https://github.com/ReactiveX/rxjs/issues/5436
+  it('should properly handle errors from iterables that are processed after some async', () => {
+    rxTest.run(({ cold, expectObservable }) => {
+      const noXError = new Error('we do not allow x');
+      const source = cold('-----A------------B-----|', { A: ['o', 'o', 'o'], B: ['o', 'x', 'o']});
+      const expected = '   -----(ooo)--------(o#)';
+      const iterable = function* (data: string[]) {
+        for (let d of data) {
+          if (d === 'x') {
+            throw noXError;
+          }
+          yield d;
+        }
+      };
+      const result = source.pipe(
+        mergeMap(x => of(x).pipe(
+          delay(0),
+          mergeMap(iterable)
+        ))
+      );
+      expectObservable(result).toBe(expected, undefined, noXError);
+    });
   });
 });

spec/util/subscribeToResult-spec.ts

@@ -119,6 +119,28 @@ describe('subscribeToResult', () => {
     expect(expected).to.be.equal(42);
   });
 
+  // NOTE: From https://github.com/ReactiveX/rxjs/issues/5436
+  it('should pass along errors from an iterable', () => {
+    const generator = function* () {
+      yield 1;
+      yield 2;
+      yield 3;
+      throw 'bad';
+    };
+
+    const results: any[] = [];
+    let foundError: any = null;
+
+    const subscriber = new OuterSubscriber({
+      next: x => results.push(x),
+      error: err => foundError = err
+    });
+
+    subscribeToResult(subscriber, generator());
+    expect(results).to.deep.equal([1, 2, 3]);
+    expect(foundError).to.equal('bad');
+  });
+
   it('should subscribe to to an object that implements Symbol.observable', (done) => {
     const observableSymbolObject = { [$$symbolObservable]: () => of(42) };
 

src/internal/util/subscribeToIterable.ts

@@ -2,9 +2,16 @@ import { Subscriber } from '../Subscriber';
 import { iterator as Symbol_iterator } from '../symbol/iterator';
 
 export const subscribeToIterable = <T>(iterable: Iterable<T>) => (subscriber: Subscriber<T>) => {
-  const iterator = iterable[Symbol_iterator]();
+  const iterator = (iterable as any)[Symbol_iterator]();
+
   do {
-    const item = iterator.next();
+    let item: IteratorResult<T>;
+    try {
+      item = iterator.next();
+    } catch (err) {
+      subscriber.error(err);
+      return;
+    }
     if (item.done) {
       subscriber.complete();
       break;