Remove the {BaseViewer, PDFThumbnailViewer}._pagesRequests caches

In the `BaseViewer` this cache is mostly relevant in the `disableAutoFetch = true` mode, since the pages are being initialized *lazily* in that case.
In the `PDFThumbnailViewer` this cache is mostly used for thumbnails that are actually being rendered, as opposed to those created directly from the "regular" pages.

Please note that I'm not suggesting that we remove these caches because they're only used in some situations, but rather because they're for all intents and purposes actually *redundant*. In the API itself, we're already caching both the page-promises and the actual pages themselves on the `WorkerTransport`-instance.
Hence these viewer-caches aren't really necessary in practice, and adds what to me mostly seems like an unnecessary level of indirection.[1]

Given that the viewer now relies on caching in the API itself, this patch also adds a new unit-test to ensure that page-caching works (and keep working) as expected.

---
[1] In the `WorkerTransport.getPage`-method the parameter is being validated on every call, but that's hardly enough code to warrant keeping the "duplicate" caches in the viewer in my opinion.

test/unit/api_spec.js

@@ -660,6 +660,20 @@ describe("api", function () {
       await loadingTask.destroy();
     });
 
+    it("gets page multiple time, with working caches", async function () {
+      const promiseA = pdfDocument.getPage(1);
+      const promiseB = pdfDocument.getPage(1);
+
+      expect(promiseA instanceof Promise).toEqual(true);
+      expect(promiseA).toBe(promiseB);
+
+      const pageA = await promiseA;
+      const pageB = await promiseB;
+
+      expect(pageA instanceof PDFPageProxy).toEqual(true);
+      expect(pageA).toBe(pageB);
+    });
+
     it("gets page index", async function () {
       const ref = { num: 17, gen: 0 }; // Reference to second page.
       const pageIndex = await pdfDocument.getPageIndex(ref);

web/base_viewer.js

@@ -601,7 +601,7 @@ class BaseViewer {
         }
         // Set the first `pdfPage` immediately, since it's already loaded,
         // rather than having to repeat the `PDFDocumentProxy.getPage` call in
-        // the `this._ensurePdfPageLoaded` method before rendering can start.
+        // the `this.#ensurePdfPageLoaded` method before rendering can start.
         const firstPageView = this._pages[0];
         if (firstPageView) {
           firstPageView.setPdfPage(firstPdfPage);
@@ -708,7 +708,6 @@ class BaseViewer {
     this._location = null;
     this._pagesRotation = 0;
     this._optionalContentConfigPromise = null;
-    this._pagesRequests = new WeakMap();
     this._firstPageCapability = createPromiseCapability();
     this._onePageRenderedCapability = createPromiseCapability();
     this._pagesCapability = createPromiseCapability();
@@ -1356,32 +1355,22 @@ class BaseViewer {
 
   /**
    * @param {PDFPageView} pageView
-   * @returns {Promise} Returns a promise containing a {PDFPageProxy} object.
-   * @private
+   * @returns {Promise<PDFPageProxy | null>}
    */
-  _ensurePdfPageLoaded(pageView) {
+  async #ensurePdfPageLoaded(pageView) {
     if (pageView.pdfPage) {
-      return Promise.resolve(pageView.pdfPage);
+      return pageView.pdfPage;
     }
-    if (this._pagesRequests.has(pageView)) {
-      return this._pagesRequests.get(pageView);
+    try {
+      const pdfPage = await this.pdfDocument.getPage(pageView.id);
+      if (!pageView.pdfPage) {
+        pageView.setPdfPage(pdfPage);
+      }
+      return pdfPage;
+    } catch (reason) {
+      console.error("Unable to get page for page view", reason);
+      return null; // Page error -- there is nothing that can be done.
     }
-    const promise = this.pdfDocument
-      .getPage(pageView.id)
-      .then(pdfPage => {
-        if (!pageView.pdfPage) {
-          pageView.setPdfPage(pdfPage);
-        }
-        this._pagesRequests.delete(pageView);
-        return pdfPage;
-      })
-      .catch(reason => {
-        console.error("Unable to get page for page view", reason);
-        // Page error -- there is nothing that can be done.
-        this._pagesRequests.delete(pageView);
-      });
-    this._pagesRequests.set(pageView, promise);
-    return promise;
   }
 
   #getScrollAhead(visible) {
@@ -1432,7 +1421,7 @@ class BaseViewer {
     this.#toggleLoadingIconSpinner(visiblePages.ids);
 
     if (pageView) {
-      this._ensurePdfPageLoaded(pageView).then(() => {
+      this.#ensurePdfPageLoaded(pageView).then(() => {
         this.renderingQueue.renderView(pageView);
       });
       return true;

web/pdf_thumbnail_viewer.js

@@ -166,7 +166,6 @@ class PDFThumbnailViewer {
     this._pageLabels = null;
     this._pagesRotation = 0;
     this._optionalContentConfigPromise = null;
-    this._pagesRequests = new WeakMap();
     this._setImageDisabled = false;
 
     // Remove the thumbnails from the DOM.
@@ -211,7 +210,7 @@ class PDFThumbnailViewer {
         }
         // Set the first `pdfPage` immediately, since it's already loaded,
         // rather than having to repeat the `PDFDocumentProxy.getPage` call in
-        // the `this._ensurePdfPageLoaded` method before rendering can start.
+        // the `this.#ensurePdfPageLoaded` method before rendering can start.
         const firstThumbnailView = this._thumbnails[0];
         if (firstThumbnailView) {
           firstThumbnailView.setPdfPage(firstPdfPage);
@@ -262,32 +261,22 @@ class PDFThumbnailViewer {
 
   /**
    * @param {PDFThumbnailView} thumbView
-   * @returns {PDFPage}
-   * @private
+   * @returns {Promise<PDFPageProxy | null>}
    */
-  _ensurePdfPageLoaded(thumbView) {
+  async #ensurePdfPageLoaded(thumbView) {
     if (thumbView.pdfPage) {
-      return Promise.resolve(thumbView.pdfPage);
+      return thumbView.pdfPage;
     }
-    if (this._pagesRequests.has(thumbView)) {
-      return this._pagesRequests.get(thumbView);
+    try {
+      const pdfPage = await this.pdfDocument.getPage(thumbView.id);
+      if (!thumbView.pdfPage) {
+        thumbView.setPdfPage(pdfPage);
+      }
+      return pdfPage;
+    } catch (reason) {
+      console.error("Unable to get page for thumb view", reason);
+      return null; // Page error -- there is nothing that can be done.
     }
-    const promise = this.pdfDocument
-      .getPage(thumbView.id)
-      .then(pdfPage => {
-        if (!thumbView.pdfPage) {
-          thumbView.setPdfPage(pdfPage);
-        }
-        this._pagesRequests.delete(thumbView);
-        return pdfPage;
-      })
-      .catch(reason => {
-        console.error("Unable to get page for thumb view", reason);
-        // Page error -- there is nothing that can be done.
-        this._pagesRequests.delete(thumbView);
-      });
-    this._pagesRequests.set(thumbView, promise);
-    return promise;
   }
 
   #getScrollAhead(visible) {
@@ -308,7 +297,7 @@ class PDFThumbnailViewer {
       scrollAhead
     );
     if (thumbView) {
-      this._ensurePdfPageLoaded(thumbView).then(() => {
+      this.#ensurePdfPageLoaded(thumbView).then(() => {
         this.renderingQueue.renderView(thumbView);
       });
       return true;