Optimize processXlinks.

- Only traverse children once.
- Avoid traversing into SegmentTimeline, which is usually large and
  won't contain xlinks.

Issue #1640

Backported to v2.4.x

Change-Id: I8a7a05d580740f9a9953b0a8aec89a06cc7e33f2

lib/dash/mpd_utils.js

@@ -612,7 +612,7 @@ shaka.dash.MpdUtils.processXlinks =
     return handled;
   }
 
-  // Filter out any children that should be nulled.
+  let childOperations = [];
   for (let i = 0; i < element.childNodes.length; i++) {
     let child = element.childNodes[i];
     if (child instanceof Element) {
@@ -622,21 +622,19 @@ shaka.dash.MpdUtils.processXlinks =
         // be removed, as specified by the mpeg-dash rules for xlink.
         element.removeChild(child);
         i -= 1;
+      } else if (child.tagName != 'SegmentTimeline') {
+        // Don't recurse into a SegmentTimeline since xlink attributes aren't
+        // valid in there and looking at each segment can take a long time with
+        // larger manifests.
+
+        // Replace the child with its processed form.
+        childOperations.push(shaka.dash.MpdUtils.processXlinks(
+            /** @type {!Element} */ (child), retryParameters, failGracefully,
+            baseUri, networkingEngine, opt_linkDepth));
       }
     }
   }
 
-  let childOperations = [];
-  for (let i = 0; i < element.childNodes.length; i++) {
-    let child = element.childNodes[i];
-    if (child.nodeType == Node.ELEMENT_NODE) {
-      // Replace the child with its processed form.
-      childOperations.push(shaka.dash.MpdUtils.processXlinks(
-          /** @type {!Element} */ (child), retryParameters, failGracefully,
-          baseUri, networkingEngine, opt_linkDepth));
-    }
-  }
-
   return shaka.util.AbortableOperation.all(childOperations).chain(() => {
     return element;
   });

test/dash/mpd_utils_unit.js

@@ -688,6 +688,15 @@ describe('MpdUtils', function() {
       operation.finally(done);
     });
 
+    it('ignores SegmentTimeline children', (done) => {
+      let baseXMLString = inBaseContainer(
+          '<SegmentTimeline>' +
+          '  <ToReplace xlink:href="https://xlink1" ' +
+          '     xlink:actuate="onRequest" />' +
+          '</SegmentTimeline>');
+      testSucceeds(baseXMLString, baseXMLString, 0, done);
+    });
+
     function testSucceeds(
         baseXMLString, desiredXMLString, desiredNetCalls, done) {
       let desiredXML = parser.parseFromString(desiredXMLString, 'text/xml')