pdf2svg.js: Serialize the SVG to a stream

Implement a serialization "generator" for `DOMElement` in domutils.js
that yields the serialization of the SVG element. This method is used by
a newly added `ReadableSVGStream` class, which can be used like any
other readable stream in Node.js.

This reduces the memory requirements. Now, it is not needed to require
the serialization to fully fit in memory.

Note: The implementation of the serializer is a state machine in ES5
since the rest of the file is also in ES5. Its functionality is
equivalent to:

```
function* serializeSVGElement(elem) {
  yield '<' + elem.nodeName;
  if (elem.nodeName === 'svg:svg') {
    yield ' xmlns:xlink="http://www.w3.org/1999/xlink"' +
          ' xmlns:svg="http://www.w3.org/2000/svg"';
  }
  for (let i in elem.attributes) {
    yield ' ' + i + '="' + xmlEncode(elem.attributes[i]) + '"';
  }

  yield '>';

  if (elem.nodeName === 'svg:tspan' || elem.nodeName === 'svg:style') {
    yield xmlEncode(elem.textContent);
  } else {
    for (let childNode of elem.childNodes) {
      yield* serializeSVGElement(childNode);
    }
  }
  yield '</' + elem.nodeName + '>';
}
```

examples/node/pdf2svg.js

@@ -6,6 +6,9 @@
 //
 
 var fs = require('fs');
+var util = require('util');
+var path = require('path');
+var stream = require('stream');
 
 // HACK few hacks to let PDF.js be loaded not as a module in global space.
 require('./domstubs.js').setStubs(global);
@@ -17,32 +20,66 @@ var pdfjsLib = require('pdfjs-dist');
 var pdfPath = process.argv[2] || '../../web/compressed.tracemonkey-pldi-09.pdf';
 var data = new Uint8Array(fs.readFileSync(pdfPath));
 
-// Dumps svg outputs to a folder called svgdump
-function writeToFile(svgdump, pageNum, callback) {
-  var name = getFileNameFromPath(pdfPath);
-  fs.mkdir('./svgdump/', function(err) {
-    if (!err || err.code === 'EEXIST') {
-      fs.writeFile('./svgdump/' + name + "-" + pageNum + '.svg', svgdump,
-        function(err) {
-          if (err) {
-            console.log('Error: ' + err);
-          } else {
-            console.log('Page: ' + pageNum);
-          }
-          callback();
-        });
-    } else {
-      callback();
-    }
-  });
+var outputDirectory = './svgdump';
+
+try {
+  // Note: This creates a directory only one level deep. If you want to create
+  // multiple subdirectories on the fly, use the mkdirp module from npm.
+  fs.mkdirSync(outputDirectory);
+} catch (e) {
+  if (e.code !== 'EEXIST') {
+    throw e;
+  }
 }
 
-// Get filename from the path
+// Dumps svg outputs to a folder called svgdump
+function getFilePathForPage(pageNum) {
+  var name = path.basename(pdfPath, path.extname(pdfPath));
+  return path.join(outputDirectory, name + '-' + pageNum + '.svg');
+}
 
-function getFileNameFromPath(path) {
-  var index = path.lastIndexOf('/');
-  var extIndex = path.lastIndexOf('.');
-  return path.substring(index, extIndex);
+/**
+ * A readable stream which offers a stream representing the serialization of a
+ * given DOM element (as defined by domstubs.js).
+ *
+ * @param {object} options
+ * @param {DOMElement} options.svgElement The element to serialize
+ */
+function ReadableSVGStream(options) {
+  if (!(this instanceof ReadableSVGStream)) {
+    return new ReadableSVGStream(options);
+  }
+  stream.Readable.call(this, options);
+  this.serializer = options.svgElement.getSerializer();
+}
+util.inherits(ReadableSVGStream, stream.Readable);
+// Implements https://nodejs.org/api/stream.html#stream_readable_read_size_1
+ReadableSVGStream.prototype._read = function() {
+  var chunk;
+  while ((chunk = this.serializer.getNext()) !== null) {
+    if (!this.push(chunk)) {
+      return;
+    }
+  }
+  this.push(null);
+};
+
+// Streams the SVG element to the given file path.
+function writeSvgToFile(svgElement, filePath) {
+  var readableSvgStream = new ReadableSVGStream({
+    svgElement: svgElement,
+  });
+  var writableStream = fs.createWriteStream(filePath);
+  return new Promise(function(resolve, reject) {
+    readableSvgStream.once('error', reject);
+    writableStream.once('error', reject);
+    writableStream.once('finish', resolve);
+    readableSvgStream.pipe(writableStream);
+  }).catch(function(err) {
+    readableSvgStream = null; // Explicitly null because of v8 bug 6512.
+    writableStream.end();
+    throw err;
+  });
 }
 
 // Will be using promises to load document, pages and misc data instead of
@@ -69,13 +106,14 @@ pdfjsLib.getDocument({
         var svgGfx = new pdfjsLib.SVGGraphics(page.commonObjs, page.objs);
         svgGfx.embedFonts = true;
         return svgGfx.getSVG(opList, viewport).then(function (svg) {
-          var svgDump = svg.toString();
-          return new Promise(function(resolve) {
-            writeToFile(svgDump, pageNum, resolve);
+          return writeSvgToFile(svg, getFilePathForPage(pageNum)).then(function () {
+            console.log('Page: ' + pageNum);
+          }, function(err) {
+            console.log('Error: ' + err);
           });
         });
       });
-    })
+    });
   };
 
   for (var i = 1; i <= numPages; i++) {