Have QPDFObjectHandle notice when replaceObject was called

This results in a performance penalty of 1% to 2% when replaceObject
and swapObjects are never called and a somewhat larger penalty if they
are called, but it's worth it to avoid very confusing behavior as
discussed in depth in qpdf#507.

1 files changed, 6 insertions, 0 deletions

diff --git a/libqpdf/QPDFObjectHandle.cc b/libqpdf/QPDFObjectHandle.cc
index c650bdea..6d4f10ce 100644
--- a/libqpdf/QPDFObjectHandle.cc
+++ b/libqpdf/QPDFObjectHandle.cc
@@ -3194,6 +3194,12 @@ QPDFObjectHandle::dereference()
         throw std::logic_error(
             "attempted to dereference an uninitialized QPDFObjectHandle");
     }
+    if (this->obj.getPointer() && this->objid &&
+        QPDF::Resolver::objectChanged(
+            this->qpdf, QPDFObjGen(this->objid, this->generation), this->obj))
+    {
+        this->obj = nullptr;
+    }
     if (this->obj.getPointer() == 0)
     {
         PointerHolder<QPDFObject> obj = QPDF::Resolver::resolve(