From 3dd27a1dd6c1f05b0931ec47fc430ab60e0a87e7 Mon Sep 17 00:00:00 2001
From: m-holger <m.holger@qpdf.org>
Date: Sun, 7 Sep 2025 14:53:01 +0100
Subject: [PATCH] Detect and warn about outline loops during structure traversal in `--check`. Update tests and adjust exit status accordingly.

---
 libqpdf/QPDFOutlineDocumentHelper.cc         |  6 +++++-
 libqpdf/QPDFOutlineObjectHelper.cc           |  9 +++++++--
 qpdf/qtest/outlines.test                     |  2 +-
 qpdf/qtest/qpdf/outlines-with-loop-check.out |  5 +++--
 qpdf/qtest/qpdf/outlines-with-loop.out       |  2 ++
 5 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/libqpdf/QPDFOutlineDocumentHelper.cc b/libqpdf/QPDFOutlineDocumentHelper.cc
index 55cd9ee..5403f64 100644
--- a/libqpdf/QPDFOutlineDocumentHelper.cc
+++ b/libqpdf/QPDFOutlineDocumentHelper.cc
@@ -53,7 +53,11 @@ QPDFOutlineDocumentHelper::validate(bool repair)
     }
     QPDFObjectHandle cur = outlines.getKey("/First");
     QPDFObjGen::set seen;
-    while (!cur.null() && seen.add(cur)) {
+    while (!cur.null()) {
+        if (!seen.add(cur)) {
+            cur.warn("Loop detected loop in /Outlines tree");
+            return;
+        }
         m->outlines.emplace_back(QPDFOutlineObjectHelper::Accessor::create(cur, *this, 1));
         cur = cur.getKey("/Next");
     }
diff --git a/libqpdf/QPDFOutlineObjectHelper.cc b/libqpdf/QPDFOutlineObjectHelper.cc
index 0ae818e..3a163b6 100644
--- a/libqpdf/QPDFOutlineObjectHelper.cc
+++ b/libqpdf/QPDFOutlineObjectHelper.cc
@@ -20,15 +20,20 @@ QPDFOutlineObjectHelper::QPDFOutlineObjectHelper(
         return;
     }
     if (QPDFOutlineDocumentHelper::Accessor::checkSeen(m->dh, a_oh.getObjGen())) {
+        a_oh.warn("Loop detected loop in /Outlines tree");
         return;
     }
 
     QPDFObjGen::set children;
     QPDFObjectHandle cur = a_oh.getKey("/First");
-    while (!cur.null() && cur.isIndirect() && children.add(cur)) {
+    while (!cur.null() && cur.isIndirect()) {
+        if (!children.add(cur)) {
+            cur.warn("Loop detected loop in /Outlines tree");
+            break;
+        }
         QPDFOutlineObjectHelper new_ooh(cur, dh, 1 + depth);
         new_ooh.m->parent = std::make_shared<QPDFOutlineObjectHelper>(*this);
-        m->kids.push_back(new_ooh);
+        m->kids.emplace_back(new_ooh);
         cur = cur.getKey("/Next");
     }
 }
diff --git a/qpdf/qtest/outlines.test b/qpdf/qtest/outlines.test
index 9c6b453..0e286da 100644
--- a/qpdf/qtest/outlines.test
+++ b/qpdf/qtest/outlines.test
@@ -32,7 +32,7 @@ foreach my $f (@outline_files)
 
 $td->runtest("outlines: outlines-with-loop --check",
              {$td->COMMAND => "qpdf --check outlines-with-loop.pdf"},
-             {$td->FILE => "outlines-with-loop-check.out", $td->EXIT_STATUS => 0},
+             {$td->FILE => "outlines-with-loop-check.out", $td->EXIT_STATUS => 3},
              $td->NORMALIZE_NEWLINES);
 
 cleanup();
diff --git a/qpdf/qtest/qpdf/outlines-with-loop-check.out b/qpdf/qtest/qpdf/outlines-with-loop-check.out
index a38d385..d7648a2 100644
--- a/qpdf/qtest/qpdf/outlines-with-loop-check.out
+++ b/qpdf/qtest/qpdf/outlines-with-loop-check.out
@@ -2,5 +2,6 @@ checking outlines-with-loop.pdf
 PDF Version: 1.3
 File is not encrypted
 File is not linearized
-No syntax or stream encoding errors found; the file may still contain
-errors that qpdf cannot detect
+WARNING: outlines-with-loop.pdf, object 4 0 at offset 637: Loop detected loop in /Outlines tree
+WARNING: outlines-with-loop.pdf, object 5 0 at offset 855: Loop detected loop in /Outlines tree
+qpdf: operation succeeded with warnings
diff --git a/qpdf/qtest/qpdf/outlines-with-loop.out b/qpdf/qtest/qpdf/outlines-with-loop.out
index 9ca99dd..e4f66f8 100644
--- a/qpdf/qtest/qpdf/outlines-with-loop.out
+++ b/qpdf/qtest/qpdf/outlines-with-loop.out
@@ -1,3 +1,5 @@
+WARNING: outlines-with-loop.pdf, object 4 0 at offset 637: Loop detected loop in /Outlines tree
+WARNING: outlines-with-loop.pdf, object 5 0 at offset 855: Loop detected loop in /Outlines tree
 page 5: Potato 1 -> 5: /XYZ null null null -> [ 11 0 R /XYZ null null null ]
 page 5: Potato 1 -> 5: /XYZ null null null -> [ 11 0 R /XYZ null null null ]
 page 11: Mern 1.1 -> 11: /Fit -> [ 17 0 R /Fit ]
--
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