Fix: Discard only invalid periods

Discard only invalid periods instead of all obsolete periods to support modification of previous periods

lib/dash/dash_parser.js

@@ -102,10 +102,10 @@ shaka.dash.DashParser = class {
     this.largestPeriodStartTime_ = null;
 
     /**
-     * Period IDs that have seen before.
+     * Period start times seen in previous manifest.
      * @private {!Array.<number>}
      */
-    this.seenPeriodIds_ = [];
+    this.seenPeriodStartTime_ = [];
 
     /**
      * The minimum of the availabilityTimeOffset values among the adaptation
@@ -542,6 +542,7 @@ shaka.dash.DashParser = class {
         mpd, 'mediaPresentationDuration', XmlUtils.parseDuration);
 
     const periods = [];
+    const periodStartTime = [];
     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
@@ -583,21 +584,34 @@ shaka.dash.DashParser = class {
         periodDuration = givenDuration;
       }
 
-      // Skip all periods with start time < maximum period start time, excepts
-      // the last period in manifest
-      if (this.largestPeriodStartTime_ !== null && start !== null &&
-          start < this.largestPeriodStartTime_ &&
-          !this.seenPeriodIds_.includes(start) &&
+      /**
+       * This is to improve the robustness of the player
+       * when it received bad content/manifest, while still supporting
+       * modification of previous periods.
+       *
+       * Skip periods that match all of the following criteria:
+       * - Start time is earlier than latest period start time ever seen
+       * - Start time is never seen in the previous manifest
+       * - Not the last period in the manifest
+       *
+       * 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) &&
           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');
-        continue;
-      } else if (!this.seenPeriodIds_.includes(start)) {
-        this.seenPeriodIds_.push(start);
+          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');
+          continue;
+        } else {
+          periodStartTime.push(start);
+        }
       }
 
+
       // Save maximum period start time if it is the last period
       if (start !== null &&
           (this.largestPeriodStartTime_ === null ||
@@ -636,6 +650,9 @@ shaka.dash.DashParser = class {
       prevEnd = start + periodDuration;
     } // end of period parsing loop
 
+    // Replace previous seen start time with the current one.
+    this.seenPeriodStartTime_ = periodStartTime;
+
     if (presentationDuration != null) {
       if (prevEnd != presentationDuration) {
         shaka.log.warning(