Attempt to find xref streams during recovery (fixes #1103)

ChangeLog

@@ -1,3 +1,9 @@
+2024-01-06  Jay Berkenbilt  <ejb@ql.org>
+
+	* When recovering a file's xref table, attempt to find xref
+	streams if a traditional trailer dictionary is not found. Fixes
+	#1103.
+
 2024-01-05  Jay Berkenbilt  <ejb@ql.org>
 
 	* Add --set-page-labels command-line argument and supporting API.

libqpdf/QPDF.cc

@@ -579,6 +579,38 @@ QPDF::reconstruct_xref(QPDFExc& e)
     }
     m->deleted_objects.clear();
 
+    if (!m->trailer.isInitialized()) {
+        qpdf_offset_t max_offset{0};
+        // If there are any xref streams, take the last one to appear.
+        for (auto const& iter: m->xref_table) {
+            auto entry = iter.second;
+            if (entry.getType() != 1) {
+                continue;
+            }
+            auto oh = getObjectByObjGen(iter.first);
+            try {
+                if (!oh.isStreamOfType("/XRef")) {
+                    continue;
+                }
+            } catch (std::exception&) {
+                continue;
+            }
+            auto offset = entry.getOffset();
+            if (offset > max_offset) {
+                max_offset = offset;
+                setTrailer(oh.getDict());
+            }
+        }
+        if (max_offset > 0) {
+            try {
+                read_xref(max_offset);
+            } catch (std::exception&) {
+                throw damagedPDF("", 0, "error decoding candidate xref stream while recovering damaged file");
+            }
+            QTC::TC("qpdf", "QPDF recover xref stream");
+        }
+    }
+
     if (!m->trailer.isInitialized()) {
         // We could check the last encountered object to see if it was an xref stream.  If so, we
         // could try to get the trailer from there.  This may make it possible to recover files with

manual/release-notes.rst

@@ -67,6 +67,11 @@ Planned changes for future 12.x (subject to change):
 
       - ``QPDFPageLabelDocumentHelper::pageLabelDict``
 
+    - Improve file recovery logic to better handle files with
+      cross-reference streams. This should enable qpdf to recover some
+      files that it would previously have reported "unable to find
+      trailer dictionary."
+
 11.7.0: December 24, 2023
   - Bug fixes:
 

qpdf/qpdf.testcov

@@ -689,3 +689,4 @@ QPDFPageObjectHelper used fallback without copying 0
 QPDF skipping cache for known unchecked object 0
 QPDF fix dangling triggered xref reconstruction 0
 QPDFPageDocumentHelper flatten resources missing or invalid 0
+QPDF recover xref stream 0

qpdf/qtest/object-stream.test

@@ -16,7 +16,7 @@ cleanup();
 
 my $td = new TestDriver('object-stream');
 
-my $n_tests = 3 + (36 * 4) + (12 * 2);
+my $n_tests = 5 + (36 * 4) + (12 * 2);
 my $n_compare_pdfs = 36;
 
 for (my $n = 16; $n <= 19; ++$n)
@@ -87,5 +87,15 @@ $td->runtest("check file",
              {$td->FILE => "gen1.qdf"});
 
 
+# Recover a file with xref streams
+$td->runtest("recover file with xref stream",
+             {$td->COMMAND => "qpdf --static-id --compress-streams=n" .
+                  " recover-xref-stream.pdf a.pdf"},
+             {$td->FILE => "recover-xref-stream.out", $td->EXIT_STATUS => 3},
+             $td->NORMALIZE_NEWLINES);
+$td->runtest("check file",
+             {$td->FILE => "a.pdf"},
+             {$td->FILE => "recover-xref-stream-recovered.pdf"});
+
 cleanup();
 $td->report(calc_ntests($n_tests, $n_compare_pdfs));

qpdf/qtest/qpdf/bad7-recover.out

@@ -1,4 +1,6 @@
 WARNING: bad7.pdf: file is damaged
 WARNING: bad7.pdf (offset 698): expected trailer dictionary
 WARNING: bad7.pdf: Attempting to reconstruct cross-reference table
+WARNING: bad7.pdf (object 2 0, offset 128): expected endobj
+WARNING: bad7.pdf (object 4 0, offset 389): expected endobj
 bad7.pdf: unable to find trailer dictionary while recovering damaged file

qpdf/qtest/qpdf/issue-146.out

@@ -2,4 +2,7 @@ WARNING: issue-146.pdf: file is damaged
 WARNING: issue-146.pdf: can't find startxref
 WARNING: issue-146.pdf: Attempting to reconstruct cross-reference table
 WARNING: issue-146.pdf (trailer, offset 695): ignoring excessively deeply nested data structure
+WARNING: issue-146.pdf (object 1 0, offset 92): expected endobj
+WARNING: issue-146.pdf (object 7 0, offset 146): unknown token while reading object; treating as string
+WARNING: issue-146.pdf (object 7 0, offset 168): expected endobj
 qpdf: issue-146.pdf: unable to find trailer dictionary while recovering damaged file

qpdf/qtest/qpdf/issue-148.out

@@ -7,4 +7,9 @@ WARNING: issue-148.pdf (offset 73): error decoding stream data for object 8 0: s
 WARNING: issue-148.pdf: file is damaged
 WARNING: issue-148.pdf (offset 73): getStreamData called on unfilterable stream
 WARNING: issue-148.pdf: Attempting to reconstruct cross-reference table
-qpdf: issue-148.pdf: unable to find trailer dictionary while recovering damaged file
+WARNING: issue-148.pdf (xref stream: object 8 0, offset 26): stream dictionary lacks /Length key
+WARNING: issue-148.pdf (xref stream: object 8 0, offset 73): attempting to recover stream length
+WARNING: issue-148.pdf (xref stream: object 8 0, offset 73): recovered stream length: 2
+WARNING: issue-148.pdf (xref stream: object 8 0, offset 85): expected endobj
+WARNING: issue-148.pdf (offset 73): error decoding stream data for object 8 0: stream inflate: inflate: data: incorrect header check
+qpdf: issue-148.pdf: error decoding candidate xref stream while recovering damaged file

qpdf/qtest/qpdf/issue-150.out

@@ -2,4 +2,5 @@ WARNING: issue-150.pdf: can't find PDF header
 WARNING: issue-150.pdf: file is damaged
 WARNING: issue-150.pdf: error reading xref: overflow/underflow converting 9900000000000000000 to 64-bit integer
 WARNING: issue-150.pdf: Attempting to reconstruct cross-reference table
+WARNING: issue-150.pdf (object 8 0): object has offset 0
 qpdf: issue-150.pdf: unable to find trailer dictionary while recovering damaged file

qpdf/qtest/qpdf/issue-202.out

@@ -3,4 +3,6 @@ WARNING: issue-202.pdf: file is damaged
 WARNING: issue-202.pdf (offset 54769): expected trailer dictionary
 WARNING: issue-202.pdf: Attempting to reconstruct cross-reference table
 WARNING: issue-202.pdf (trailer, offset 55770): ignoring excessively deeply nested data structure
+WARNING: issue-202.pdf (object 222 0, offset 50101): dictionary has duplicated key /Creator; last occurrence overrides earlier ones
+WARNING: issue-202.pdf (object 222 0, offset 50101): dictionary has duplicated key /Producer; last occurrence overrides earlier ones
 qpdf: issue-202.pdf: unable to find trailer dictionary while recovering damaged file

qpdf/qtest/qpdf/recover-xref-stream.out

@@ -0,0 +1,5 @@
+WARNING: recover-xref-stream.pdf: file is damaged
+WARNING: recover-xref-stream.pdf: can't find startxref
+WARNING: recover-xref-stream.pdf: Attempting to reconstruct cross-reference table
+WARNING: recover-xref-stream.pdf: reported number of objects (14) is not one plus the highest object number (15)
+qpdf: operation succeeded with warnings; resulting file may have some problems