Fallback and try a *previous* generation if all else fails in XRef.indexObjects (issue 15577)

When we fail to find a usable PDF document `trailer` *and* there were errors during parsing, try and fallback to a *previous* generation as a last resort during fetching of uncompressed references.
*Please note:* This will not affect "normal" PDF documents, with valid /XRef data, and even most *corrupt* documents should be completely unaffected by these changes.

src/core/xref.js

@@ -556,8 +556,15 @@ class XRef {
       this.readXRef(/* recoveryMode */ true);
     }
     // finding main trailer
-    let trailerDict;
-    for (const trailer of trailers) {
+    let trailerDict, trailerError;
+    for (const trailer of [...trailers, "generationFallback", ...trailers]) {
+      if (trailer === "generationFallback") {
+        if (!trailerError) {
+          break; // No need to fallback if there were no validation errors.
+        }
+        this._generationFallback = true;
+        continue;
+      }
       stream.pos = trailer;
       const parser = new Parser({
         lexer: new Lexer(stream),
@@ -590,6 +597,7 @@ class XRef {
         }
         // The top-level /Pages dictionary isn't obviously corrupt.
       } catch (ex) {
+        trailerError = ex;
         continue;
       }
       // taking the first one with 'ID'
@@ -780,7 +788,17 @@ class XRef {
     const gen = ref.gen;
     let num = ref.num;
     if (xrefEntry.gen !== gen) {
-      throw new XRefEntryException(`Inconsistent generation in XRef: ${ref}`);
+      const msg = `Inconsistent generation in XRef: ${ref}`;
+      // Try falling back to a *previous* generation (fixes issue15577.pdf).
+      if (this._generationFallback && xrefEntry.gen < gen) {
+        warn(msg);
+        return this.fetchUncompressed(
+          Ref.get(num, xrefEntry.gen),
+          xrefEntry,
+          suppressEncryption
+        );
+      }
+      throw new XRefEntryException(msg);
     }
     const stream = this.stream.makeSubStream(
       xrefEntry.offset + this.stream.start

test/pdfs/issue15577.pdf.link

@@ -0,0 +1 @@
+https://github.com/mozilla/pdf.js/files/9813960/hang-080214-152005-90.pdf

test/test_manifest.json

@@ -661,6 +661,14 @@
        "lastPage": 1,
        "type": "eq"
     },
+    {  "id": "issue15577",
+       "file": "pdfs/issue15577.pdf",
+       "md5": "6939244cf44b7b31ff960b58ed7e4004",
+       "link": true,
+       "rounds": 1,
+       "lastPage": 1,
+       "type": "eq"
+    },
     {  "id": "hmm-pdf",
        "file": "pdfs/hmm.pdf",
        "md5": "e08467e60101ee5f4a59716e86db6dc9",