test: Add missing test case for SegmentIterator

Based on discussion of PR #3419

Change-Id: I02bfc615a169b54b94cdc624ace77d9ffefd624e

test/media/segment_index_unit.js

@@ -567,6 +567,12 @@ describe('SegmentIndex', /** @suppress {accessControls} */ () => {
       makeReference(uri(20.30), 20, 30),
     ];
 
+    const additionalRefs = [
+      makeReference(uri(30.40), 30, 40),
+      makeReference(uri(40.50), 40, 50),
+      makeReference(uri(50.60), 50, 60),
+    ];
+
     const partialRefs1 = [
       makeReference(uri(10.15), 10, 15),
       makeReference(uri(15.20), 15, 20),
@@ -626,6 +632,45 @@ describe('SegmentIndex', /** @suppress {accessControls} */ () => {
       expect(Array.from(index)).toEqual(inputRefs.slice(1));
     });
 
+    it('resumes iteration after new references are added', () => {
+      const refs = inputRefs.slice();
+      const index = new shaka.media.SegmentIndex(refs);
+
+      // This simulates the pattern of calls in StreamingEngine when we buffer
+      // to the edge of a live stream.
+      const iterator = index[Symbol.iterator]();
+      iterator.next();
+      expect(iterator.current()).toBe(inputRefs[0]);
+
+      iterator.next();
+      expect(iterator.current()).toBe(inputRefs[1]);
+
+      iterator.next();
+      expect(iterator.current()).toBe(inputRefs[2]);
+
+      iterator.next();
+      expect(iterator.current()).toBe(null);
+      // After we reach the end of the iteration, we should still get null back
+      // on subsequent requests.
+      expect(iterator.current()).toBe(null);
+      expect(iterator.current()).toBe(null);
+
+      // Although we have reached the end, we should still be able to add new
+      // references and iterate to them without starting over the iteration.
+      refs.push(...additionalRefs);
+
+      expect(iterator.current()).toBe(additionalRefs[0]);
+
+      iterator.next();
+      expect(iterator.current()).toBe(additionalRefs[1]);
+
+      iterator.next();
+      expect(iterator.current()).toBe(additionalRefs[2]);
+
+      iterator.next();
+      expect(iterator.current()).toBe(null);
+    });
+
     describe('seek', () => {
       it('returns the matching segment', () => {
         const index = new shaka.media.SegmentIndex(inputRefs);