Fix: Use period ID instead of start time

lib/dash/dash_parser.js

@@ -99,13 +99,13 @@ shaka.dash.DashParser = class {
      * Largest period start time seen.
      * @private {?number}
      */
-    this.largestPeriodStartTime_ = null;
+    this.largestPeriodId_ = null;
 
     /**
      * Period start times seen in previous manifest.
      * @private {!Array.<number>}
      */
-    this.seenPeriodStartTime_ = [];
+    this.seenPeriodIds = [];
 
     /**
      * The minimum of the availabilityTimeOffset values among the adaptation
@@ -542,7 +542,7 @@ shaka.dash.DashParser = class {
         mpd, 'mediaPresentationDuration', XmlUtils.parseDuration);
 
     const periods = [];
-    const periodStartTime = [];
+    const periodIds = [];
     let prevEnd = 0;
     const periodNodes = XmlUtils.findChildren(mpd, 'Period');
     // This uses a for-loop rather than a for-of loop because this needs to look
@@ -551,6 +551,7 @@ shaka.dash.DashParser = class {
     for (const {i, item: elem, next} of enumerate(periodNodes)) {
       const start = /** @type {number} */ (
         XmlUtils.parseAttr(elem, 'start', XmlUtils.parseDuration, prevEnd));
+      const periodId = Number(elem.id);
       const givenDuration =
           XmlUtils.parseAttr(elem, 'duration', XmlUtils.parseDuration);
 
@@ -585,9 +586,11 @@ shaka.dash.DashParser = class {
       }
 
       /**
-       * This is to improve the robustness of the player
-       * when it received bad content/manifest, while still supporting
-       * modification of previous periods.
+       * This is to improve robustness when the player observes manifest with
+       * past periods that are inconsistent to previous ones.
+       *
+       * This may happen when a CDN or proxy server switches its upstream from
+       * one encoder to another redundant encoder.
        *
        * Skip periods that match all of the following criteria:
        * - Start time is earlier than latest period start time ever seen
@@ -597,26 +600,26 @@ shaka.dash.DashParser = class {
        * Periods that meet the aforementioned criteria are considered invalid
        * and should be safe to discard.
        */
-      if (this.largestPeriodStartTime_ !== null && start !== null) {
-        if (start < this.largestPeriodStartTime_ &&
-          !this.seenPeriodStartTime_.includes(start) &&
+      if (this.largestPeriodId_ !== null && periodId !== null) {
+        if (periodId < this.largestPeriodId_ &&
+          !this.seenPeriodIds.includes(periodId) &&
           i + 1 != periodNodes.length) {
           shaka.log.debug(
-              `Skipping Period ${i+1} as its start time is smaller than ` +
-              'the largest period start time that has been seen, and start ' +
-              'time is unseen before');
+              `Skipping Period ${i+1} as its period ID is smaller than ` +
+              'the largest period ID that has been seen, and ID ' +
+              'is unseen before');
           continue;
         } else {
-          periodStartTime.push(start);
+          periodIds.push(periodId);
         }
       }
 
 
       // Save maximum period start time if it is the last period
-      if (start !== null &&
-          (this.largestPeriodStartTime_ === null ||
-          start > this.largestPeriodStartTime_)) {
-        this.largestPeriodStartTime_ = start;
+      if (periodId !== null &&
+          (this.largestPeriodId_ === null ||
+            periodId > this.largestPeriodId_)) {
+        this.largestPeriodId_ = periodId;
       }
 
       // Parse child nodes.
@@ -651,7 +654,7 @@ shaka.dash.DashParser = class {
     } // end of period parsing loop
 
     // Replace previous seen start time with the current one.
-    this.seenPeriodStartTime_ = periodStartTime;
+    this.seenPeriodIds = periodIds;
 
     if (presentationDuration != null) {
       if (prevEnd != presentationDuration) {

test/dash/dash_parser_manifest_unit.js

@@ -2155,7 +2155,7 @@ describe('DashParser Manifest', () => {
      * @param {!Array.<number>} periods Start time of multiple periods
      * @return {string}
      */
-  function buildManifestWithPeriodStartTime(periods) {
+  function buildManifestWithPeriodId(periods) {
     const mpdTemplate = [
       `<MPD type="dynamic"`,
       'availabilityStartTime="1970-01-01T00:00:00Z"',
@@ -2179,7 +2179,7 @@ describe('DashParser Manifest', () => {
     };
     const periodXmls = periods.map((period, i) => {
       const duration = i+1 === periods.length ? 10 : periods[i+1] - period;
-      return periodTemplate(i+1, period, duration);
+      return periodTemplate(period, period, duration);
     });
     return sprintf(mpdTemplate, {
       periods: periodXmls.join('\n'),
@@ -2193,13 +2193,11 @@ describe('DashParser Manifest', () => {
   // redundant servers. The period start time might become out of sync
   // during the switch-over/recovery.
 
-  // Solution: Ignore old DASH periods that are older than the latest one.
-
   it('skip periods that are earlier than max period start time', async () => {
     const sources = [
-      buildManifestWithPeriodStartTime([5, 15]),
-      buildManifestWithPeriodStartTime([6, 15]), // simulate out-of-sync of -1s
-      buildManifestWithPeriodStartTime([4, 15]), // simulate out-of-sync of +1s
+      buildManifestWithPeriodId([5, 15]),
+      buildManifestWithPeriodId([6, 15]), // simulate out-of-sync of -1s
+      buildManifestWithPeriodId([4, 15]), // simulate out-of-sync of +1s
     ];
     const segments = [];