Don't iterate over all empty slots in the xref entries (bug 1980958)

src/core/xref.js

@@ -681,11 +681,15 @@ class XRef {
     // When no trailer dictionary candidate exists, try picking the first
     // dictionary that contains a /Root entry (fixes issue18986.pdf).
     if (!trailerDicts.length) {
-      for (const [num, entry] of this.entries.entries()) {
-        if (!entry) {
+      // In case, this.entries is a sparse array we don't want to
+      // iterate over empty entries so we use the `in` operator instead of
+      // using for..of on entries() or a for with the array length.
+      for (const num in this.entries) {
+        if (!Object.hasOwn(this.entries, num)) {
           continue;
         }
-        const ref = Ref.get(num, entry.gen);
+        const entry = this.entries[num];
+        const ref = Ref.get(parseInt(num), entry.gen);
         let obj;
 
         try {
@@ -693,6 +697,7 @@ class XRef {
         } catch {
           continue;
         }
+
         if (obj instanceof BaseStream) {
           obj = obj.dict;
         }

test/pdfs/.gitignore

@@ -741,3 +741,4 @@
 !print_protection.pdf
 !tracemonkey_with_annotations.pdf
 !tracemonkey_with_editable_annotations.pdf
+!bug1980958.pdf

test/pdfs/bug1980958.pdf

@@ -0,0 +1,10 @@
+%PDF-1.7
+1 0 obj <</Type /Catalog /Pages 2 0 R>>
+endobj
+2 0 obj <</Type /Pages /Kids [3 0 R] /Count 1>>
+endobj
+3 0 obj <</Type /Page /Parent 2 0 R /MediaBox [0 0 10 10]>>
+endobj
+
+2147483647 0 obj <</Root 1 0 R>>
+endobj

test/unit/api_spec.js

@@ -878,6 +878,20 @@ describe("api", function () {
 
       await loadingTask.destroy();
     });
+
+    it("Doesn't iterate over all empty slots in the xref entries (bug 1980958)", async function () {
+      if (isNodeJS) {
+        pending("Worker is not supported in Node.js.");
+      }
+      const loadingTask = getDocument(buildGetDocumentParams("bug1980958.pdf"));
+      const { promise, resolve } = Promise.withResolvers();
+      setTimeout(() => resolve(null), 1000);
+
+      const pdfDocument = await Promise.race([loadingTask.promise, promise]);
+      expect(pdfDocument?.numPages).toEqual(1);
+
+      loadingTask._worker.destroy();
+    });
   });
 
   describe("PDFWorker", function () {