Add DOMParser-based XML reader

This adds a `DOMParser`-based XML reader which emits events like a SAX XML
reader for compatibility with the feed processor.

resource/feeds/FeedProcessor.js

@@ -3,6 +3,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 /* eslint-disable quote-props */
+/* globals SAXXMLReader */
 
 "use strict";
 
@@ -10,8 +11,6 @@ function LOG(str) {
 	Zotero.debug("Feed Processor: " + str);
 }
 
-const SAX_CONTRACTID = "@mozilla.org/saxparser/xmlreader;1";
-
 const XMLNS = "http://www.w3.org/XML/1998/namespace";
 const RSS090NS = "http://my.netscape.com/rdf/simple/0.9/";
 
@@ -956,7 +955,7 @@ function WrapperElementInfo(fieldName) {
 // Implements nsIFeedProcessor, nsISAXContentHandler, nsISAXErrorHandler,
 //            nsIStreamListener, nsIRequestObserver
 function FeedProcessor() {
-	this._reader = Cc[SAX_CONTRACTID].createInstance(Ci.nsISAXXMLReader);
+	this._reader = new SAXXMLReader();
 	this._buf = "";
 	this._feed = {};
 	this._handlerStack = [];

resource/feeds/SAXXMLReader.js

@@ -0,0 +1,140 @@
+/*
+    ***** BEGIN LICENSE BLOCK *****
+
+    Copyright © 2021 Corporation for Digital Scholarship
+                     Vienna, Virginia, USA
+                     https://www.zotero.org
+
+    This file is part of Zotero.
+
+    Zotero is free software: you can redistribute it and/or modify
+    it under the terms of the GNU Affero General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    Zotero is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU Affero General Public License for more details.
+
+    You should have received a copy of the GNU Affero General Public License
+    along with Zotero.  If not, see <http://www.gnu.org/licenses/>.
+
+    ***** END LICENSE BLOCK *****
+*/
+
+"use strict";
+
+/**
+ * This implements `nsISAXXMLReader` using content-accessible APIs, such as `DOMParser` and
+ * `TreeWalker`. It should be usable in any web platform environment that supports those standard
+ * APIs.
+ * 
+ * Note that while this class implements a SAX-style API (which usually implies streaming style
+ * parsing for documents of any length), this class actually uses whole document parsing internally.
+ * Instead, `DOMParser` reads the entire document and this walks the resulting DOM. Thus, this class
+ * is mainly useful only for smaller documents where it's useful to conform to SAX-style API to
+ * support existing code.
+ * 
+ * Higher-level components are notified of XML content via the `nsISAXContentHandler` and
+ * `nsISAXErrorHandler` interfaces as this reader walks through the XML content.
+ */
+class SAXXMLReader {
+	constructor() {
+		this.contentHandler = null;
+		this.errorHandler = null;
+		this.baseURI = null;
+		this._data = null;
+		this._walker = null;
+	}
+	
+	// nsISAXXMLReader
+	
+	parseAsync(requestObserver) {
+		if (requestObserver) {
+			throw new Error("requestObserver argument parseAsync is not currently supported");
+		}
+	}
+	
+	// Fetch API
+
+	async onResponseAvailable(response) {
+		if (!response.ok) {
+			throw new Error("Unable to fetch data");
+		}
+		this._data = await response.text();
+		this._parseAndNotify();
+	}
+	
+	// Parsing and notification
+	
+	_parseAndNotify() {
+		if (!this.contentHandler) {
+			return;
+		}
+		
+		const doc = new DOMParser().parseFromString(this._data, "text/xml");
+		this._walker = doc.createTreeWalker(doc.documentElement);
+		
+		this.contentHandler.startDocument();
+		this._walk();
+		this.contentHandler.endDocument();
+		
+		this._data = null;
+		this._walker = null;
+	}
+	
+	_walk() {
+		const node = this._walker.currentNode;
+		
+		switch (node.nodeType) {
+			// ELEMENT_NODE
+			case 1: {
+				this.contentHandler.startElement(
+					node.namespaceURI,
+					node.localName,
+					"", // qualifed names are not used
+					node.attributes,
+				);
+				
+				// Try to move down
+				if (this._walker.firstChild()) {
+					this._walk();
+					// Move up
+					this._walker.parentNode();
+				}
+				
+				this.contentHandler.endElement(
+					node.namespaceURI,
+					node.localName,
+					"", // qualifed names are not used
+				);
+				break;
+			}
+			// TEXT_NODE
+			case 3: {
+				this.contentHandler.characters(node.data);
+				break;
+			}
+			// CDATA_SECTION_NODE
+			case 4: {
+				this.contentHandler.characters(node.data);
+				break;
+			}
+			// PROCESSING_INSTRUCTION_NODE
+			case 7: {
+				this.contentHandler.processingInstruction(node.target, node.data);
+				break;
+			}
+		}
+
+		// Try to move across
+		if (this._walker.nextSibling()) {
+			this._walk();
+		}
+	}
+}
+
+if (typeof module == "object") {
+	module.exports = SAXXMLReader;
+}