Correctly handle reuse of xref stream (fixes #809)

2 files changed, 60 insertions, 10 deletions

diff --git a/libqpdf/QPDF.cc b/libqpdf/QPDF.cc
index 262f61e6..8e9a52f8 100644
--- a/libqpdf/QPDF.cc
+++ b/libqpdf/QPDF.cc
@@ -999,7 +999,12 @@ QPDF::read_xrefStream(qpdf_offset_t xref_offset)
         QPDFObjectHandle xref_obj;
         try {
             xref_obj = readObjectAtOffset(
-                false, xref_offset, "xref stream", QPDFObjGen(0, 0), x_og);
+                false,
+                xref_offset,
+                "xref stream",
+                QPDFObjGen(0, 0),
+                x_og,
+                true);
         } catch (QPDFExc&) {
             // ignore -- report error below
         }
@@ -1638,7 +1643,8 @@ QPDF::readObjectAtOffset(
     qpdf_offset_t offset,
     std::string const& description,
     QPDFObjGen const& exp_og,
-    QPDFObjGen& og)
+    QPDFObjGen& og,
+    bool skip_cache_if_in_xref)
 {
     bool check_og = true;
     if (exp_og.getObj() == 0) {
@@ -1718,7 +1724,7 @@ QPDF::readObjectAtOffset(
                 qpdf_offset_t new_offset =
                     this->m->xref_table[exp_og].getOffset();
                 QPDFObjectHandle result = readObjectAtOffset(
-                    false, new_offset, description, exp_og, og);
+                    false, new_offset, description, exp_og, og, false);
                 QTC::TC("qpdf", "QPDF recovered in readObjectAtOffset");
                 return result;
             } else {
@@ -1770,11 +1776,50 @@ QPDF::readObjectAtOffset(
             }
         }
         qpdf_offset_t end_after_space = this->m->file->tell();
-        updateCache(
-            og,
-            QPDFObjectHandle::ObjAccessor::getObject(oh),
-            end_before_space,
-            end_after_space);
+        if (skip_cache_if_in_xref && this->m->xref_table.count(og)) {
+            // Ordinarily, an object gets read here when resolved
+            // through xref table or stream. In the special case of
+            // the xref stream and linearization hint tables, the
+            // offset comes from another source. For the specific case
+            // of xref streams, the xref stream is read and loaded
+            // into the object cache very early in parsing.
+            // Ordinarily, when a file is updated by appending, items
+            // inserted into the xref table in later updates take
+            // precedence over earlier items. In the special case of
+            // reusing the object number previously used as the xref
+            // stream, we have the following order of events:
+            //
+            // * reused object gets loaded into the xref table
+            // * old object is read here while reading xref streams
+            // * original xref entry is ignored (since already in xref table)
+            //
+            // It is the second step that causes a problem. Even
+            // though the xref table is correct in this case, the old
+            // object is already in the cache and so effectively
+            // prevails over the reused object. To work around this
+            // issue, we have a special case for the xref stream (via
+            // the skip_cache_if_in_xref): if the object is already in
+            // the xref stream, don't cache what we read here.
+            //
+            // It is likely that the same bug may exist for
+            // linearization hint tables, but the existing code uses
+            // end_before_space and end_after_space from the cache, so
+            // fixing that would require more significant rework. The
+            // chances of a linearization hint stream being reused
+            // seems smaller because the xref stream is probably the
+            // highest object in the file and the linearization hint
+            // stream would be some random place in the middle, so I'm
+            // leaving that bug unfixed for now. If the bug were to be
+            // fixed, we could use !check_og in place of
+            // skip_cache_if_in_xref.
+            QTC::TC("qpdf", "QPDF skipping cache for known unchecked object");
+        } else {
+            updateCache(
+                og,
+                QPDFObjectHandle::ObjAccessor::getObject(oh),
+                end_before_space,
+                end_after_space);
+        }
     }
 
     return oh;
@@ -1809,7 +1854,7 @@ QPDF::resolve(QPDFObjGen const& og)
                     // Object stored in cache by readObjectAtOffset
                     QPDFObjGen a_og;
                     QPDFObjectHandle oh =
-                        readObjectAtOffset(true, offset, "", og, a_og);
+                        readObjectAtOffset(true, offset, "", og, a_og, false);
                 }
                 break;
 
diff --git a/libqpdf/QPDF_linearization.cc b/libqpdf/QPDF_linearization.cc
index 888b4262..ac7e3efe 100644
--- a/libqpdf/QPDF_linearization.cc
+++ b/libqpdf/QPDF_linearization.cc
@@ -303,7 +303,12 @@ QPDF::readHintStream(Pipeline& pl, qpdf_offset_t offset, size_t length)
 {
     QPDFObjGen og;
     QPDFObjectHandle H = readObjectAtOffset(
-        false, offset, "linearization hint stream", QPDFObjGen(0, 0), og);
+        false,
+        offset,
+        "linearization hint stream",
+        QPDFObjGen(0, 0),
+        og,
+        false);
     ObjCache& oc = this->m->obj_cache[og];
     qpdf_offset_t min_end_offset = oc.end_before_space;
     qpdf_offset_t max_end_offset = oc.end_after_space;