Only upgrade imports once

src/CustomElements/v1/CustomElements.js

@@ -311,9 +311,11 @@ var CustomElementDefinition;
 
     /**
      * @param {NodeList} nodeList
+     * @param {Set<Node>=} visitedNodes
      * @private
      */
-    _addNodes: function(nodeList) {
+    _addNodes: function(nodeList, visitedNodes) {
+      visitedNodes = visitedNodes || new Set();
       for (var i = 0; i < nodeList.length; i++) {
         var root = nodeList[i];
 
@@ -342,15 +344,18 @@ var CustomElementDefinition;
           if (node.shadowRoot) {
             // TODO(justinfagnani): do we need to check that the shadowRoot
             // is observed?
-            this._addNodes(node.shadowRoot.childNodes);
+            this._addNodes(node.shadowRoot.childNodes, visitedNodes);
           }
-          if (node.tagName === 'LINK' && node.rel.toLowerCase() === 'import') {
+          if (node.tagName === 'LINK' &&
+              node.rel.toLowerCase() === 'import' &&
+              !visitedNodes.has(node)) {
+            // visitedNodes.add(node);
             var onLoad = (function() {
               var link = node;
               return function() {
                 link.removeEventListener('load', onLoad);
                 this._observeRoot(link.import);
-                this._addNodes(link.import.childNodes);
+                this._addNodes(link.import.childNodes, visitedNodes);
               }.bind(this);
             }).bind(this)();
             if (node.import) {

tests/CustomElements/v1/html/imported-doc.html

@@ -1 +1,2 @@
+<link rel="import" href="sub-import.html">
 <x-foo></x-foo>

tests/CustomElements/v1/html/imports.html

@@ -33,14 +33,18 @@
     <script src="../../../../dist/HTMLImports.js"></script>
     <script src="../../../../src/CustomElements/v1/CustomElements.js"></script>
     <script src="../../../../../web-component-tester/browser.js"></script>
+    <link rel="import" href="sub-import.html" id="sub-import">
     <link rel="import" href="imported-doc.html" id="import">
-    <link rel="not-import" href="imported-doc.html" id="import">
+    <link rel="not-import" href="imported-doc.html" id="not-import">
     <script>
       customElements.enableFlush = true;
       var elementsCreated = 0;
       var nonImportDocumentAccessed = false;
+      var subImportDocumentAccessCount = 0;
 
-      var nonImportLink = document.querySelector('link[rel="not-import"]');
+      // To tell if CustomElementsRegistry tries to import a non-import link
+      // we fail if the .import accessor is read.
+      var nonImportLink = document.querySelector('#not-import');
       Object.defineProperty(nonImportLink, 'import', {
         get: function() {
           nonImportDocumentAccessed = true;
@@ -48,6 +52,15 @@
         enumberable: true,
       });
 
+      var subImportLink = document.querySelector('#sub-import');
+      Object.defineProperty(subImportLink, 'import', {
+        get: function() {
+          subImportDocumentAccessCount++;
+        },
+        enumberable: true,
+      });
+
+
       class XFoo extends HTMLElement {
         constructor() {
           console.log("XFoo");
@@ -67,6 +80,10 @@
           chai.assert.isFalse(nonImportDocumentAccessed, 'CustomElements is trying to upgrade a non-import link');
         });
 
+        test('CustomElements only upgrades a <link rel=import> once', function() {
+          chai.assert.equal(subImportDocumentAccessCount, 1);
+        });
+
       });
     </script>
   </head>

tests/CustomElements/v1/html/sub-import.html

@@ -0,0 +1 @@
+<div></div>