[api-minor] Immediately release the font.data property once the font been attached to the DOM (PR 11777 follow-up)

*This patch implements https://github.com/mozilla/pdf.js/pull/11777#issuecomment-609741348*

This extends the work from PR 11773 and 11777 further, by immediately releasing the `font.data` property once the font been attached to the DOM. By not unnecessarily holding onto this data on the main-thread, we'll thus reduce the memory usage of fonts even further (especially beneficial in longer documents with composite fonts).

The new behaviour is controlled by the recently added `fontExtraProperties` API option (adding a new option just for this patch didn't seem necessary), since there's one edge-case in the SVG renderer where the `font.data` property is necessary (see the `pdf2svg` example).

Note that while the default viewer does run clean-up with an idle timeout, that timeout will be reset whenever rendering occurs *or* when scrolling happens in the viewer. In practice this means that unless the user doesn't interact with the viewer in *any* way during an extended period of time, currently set to 30 seconds, the `PDFDocumentProxy.cleanup` method will never be called and font resources will thus not be cleaned-up.

examples/node/pdf2svg.js

@@ -86,6 +86,7 @@ function writeSvgToFile(svgElement, filePath) {
 // callback.
 var loadingTask = pdfjsLib.getDocument({
   data: data,
+  fontExtraProperties: true,
   // Try to export JPEG images directly if they don't need any further
   // processing.
   nativeImageDecoderSupport: pdfjsLib.NativeImageDecoding.DISPLAY,

src/display/api.js

@@ -2244,20 +2244,22 @@ class WorkerTransport {
             fontRegistry,
           });
 
-          this.fontLoader.bind(font).then(
+          this.fontLoader
-            () => {
+            .bind(font)
+            .catch(reason => {
+              return messageHandler.sendWithPromise("FontFallback", { id });
+            })
+            .finally(() => {
+              if (!params.fontExtraProperties && font.data) {
+                // Immediately release the `font.data` property once the font
+                // has been attached to the DOM, since it's no longer needed,
+                // rather than waiting for a `PDFDocumentProxy.cleanup` call.
+                // Since `font.data` could be very large, e.g. in some cases
+                // multiple megabytes, this will help reduce memory usage.
+                font.data = null;
+              }
               this.commonObjs.resolve(id, font);
-            },
+            });
-            reason => {
-              messageHandler
-                .sendWithPromise("FontFallback", {
-                  id,
-                })
-                .finally(() => {
-                  this.commonObjs.resolve(id, font);
-                });
-            }
-          );
           break;
         case "FontPath":
         case "FontType3Res":

src/display/canvas.js

@@ -1498,7 +1498,7 @@ var CanvasGraphics = (function CanvasGraphicsClosure() {
       var isAddToPathSet = !!(
         textRenderingMode & TextRenderingMode.ADD_TO_PATH_FLAG
       );
-      const patternFill = current.patternFill && font.data;
+      const patternFill = current.patternFill && !font.missingFile;
 
       var addToPath;
       if (font.disableFontFace || isAddToPathSet || patternFill) {

src/display/svg.js

@@ -948,6 +948,12 @@ if (typeof PDFJSDev === "undefined" || PDFJSDev.test("GENERIC")) {
     }
 
     addFontStyle(fontObj) {
+      if (!fontObj.data) {
+        throw new Error(
+          "addFontStyle: No font data available, " +
+            'ensure that the "fontExtraProperties" API parameter is set.'
+        );
+      }
       if (!this.cssStyle) {
         this.cssStyle = this.svgFactory.createElement("svg:style");
         this.cssStyle.setAttributeNS(null, "type", "text/css");
@@ -972,7 +978,7 @@ if (typeof PDFJSDev === "undefined" || PDFJSDev.test("GENERIC")) {
 
       if (
         this.embedFonts &&
-        fontObj.data &&
+        !fontObj.missingFile &&
         !this.embeddedFonts[fontObj.loadedName]
       ) {
         this.addFontStyle(fontObj);