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.

4 files changed, 47 insertions, 39 deletions

diff --git a/qpdf/qtest/qpdf/test14-in.pdf b/qpdf/qtest/qpdf/test14-in.pdf
index 4d761020..d0cd490a 100644
--- a/qpdf/qtest/qpdf/test14-in.pdf
+++ b/qpdf/qtest/qpdf/test14-in.pdf
@@ -65,6 +65,7 @@ endobj
     612
     792
   ]
+  /OrigPage 2
   /Parent 4 0 R
   /Resources <<
     /Font <<
@@ -86,6 +87,7 @@ endobj
     612
     792
   ]
+  /OrigPage 3
   /Parent 4 0 R
   /Resources <<
     /Font <<
@@ -237,21 +239,21 @@ xref
 0000000140 00000 n 
 0000000252 00000 n 
 0000000456 00000 n 
-0000000661 00000 n 
-0000000866 00000 n 
-0000001084 00000 n 
-0000001186 00000 n 
-0000001206 00000 n 
-0000001325 00000 n 
-0000001384 00000 n 
-0000001487 00000 n 
-0000001507 00000 n 
-0000001566 00000 n 
-0000001669 00000 n 
-0000001689 00000 n 
-0000001748 00000 n 
-0000001851 00000 n 
-0000001871 00000 n 
+0000000675 00000 n 
+0000000894 00000 n 
+0000001112 00000 n 
+0000001214 00000 n 
+0000001234 00000 n 
+0000001353 00000 n 
+0000001412 00000 n 
+0000001515 00000 n 
+0000001535 00000 n 
+0000001594 00000 n 
+0000001697 00000 n 
+0000001717 00000 n 
+0000001776 00000 n 
+0000001879 00000 n 
+0000001899 00000 n 
 trailer <<
   /QArray 2 0 R
   /QDict 3 0 R
@@ -260,5 +262,5 @@ trailer <<
   /ID [<20eb74876a3e8212c1b4fd43153860b0><1bb7a926da191c58f675435d77997d21>]
 >>
 startxref
-1907
+1935
 %%EOF
diff --git a/qpdf/qtest/qpdf/test14-out.pdf b/qpdf/qtest/qpdf/test14-out.pdf
index 315d174d..96c25fbd 100644
--- a/qpdf/qtest/qpdf/test14-out.pdf
+++ b/qpdf/qtest/qpdf/test14-out.pdf
@@ -16,10 +16,10 @@ endobj
 << /Contents 9 0 R /MediaBox [ 0 0 612 792 ] /Parent 4 0 R /Resources << /Font << /F1 10 0 R >> /ProcSet 11 0 R >> /Type /Page >>
 endobj
 6 0 obj
-<< /Contents 12 0 R /MediaBox [ 0 0 612 792 ] /Parent 4 0 R /Resources << /Font << /F1 10 0 R >> /ProcSet 13 0 R >> /Type /Page >>
+<< /Contents 12 0 R /MediaBox [ 0 0 612 792 ] /OrigPage 3 /Parent 4 0 R /Resources << /Font << /F1 10 0 R >> /ProcSet 13 0 R >> /Type /Page >>
 endobj
 7 0 obj
-<< /Contents 14 0 R /MediaBox [ 0 0 612 792 ] /Parent 4 0 R /Resources << /Font << /F1 10 0 R >> /ProcSet 15 0 R >> /Type /Page >>
+<< /Contents 14 0 R /MediaBox [ 0 0 612 792 ] /OrigPage 2 /Parent 4 0 R /Resources << /Font << /F1 10 0 R >> /ProcSet 15 0 R >> /Type /Page >>
 endobj
 8 0 obj
 << /Contents 16 0 R /MediaBox [ 0 0 612 792 ] /Parent 4 0 R /Resources << /Font << /F1 10 0 R >> /ProcSet 17 0 R >> /Type /Page >>
@@ -88,18 +88,18 @@ xref
 0000000122 00000 n 
 0000000199 00000 n 
 0000000344 00000 n 
-0000000490 00000 n 
-0000000636 00000 n 
-0000000782 00000 n 
-0000000877 00000 n 
-0000000985 00000 n 
-0000001016 00000 n 
-0000001112 00000 n 
-0000001143 00000 n 
-0000001239 00000 n 
-0000001270 00000 n 
-0000001366 00000 n 
+0000000502 00000 n 
+0000000660 00000 n 
+0000000806 00000 n 
+0000000901 00000 n 
+0000001009 00000 n 
+0000001040 00000 n 
+0000001136 00000 n 
+0000001167 00000 n 
+0000001263 00000 n 
+0000001294 00000 n 
+0000001390 00000 n 
 trailer << /QArray 2 0 R /QDict 3 0 R /Root 1 0 R /Size 18 /ID [<20eb74876a3e8212c1b4fd43153860b0><31415926535897932384626433832795>] >>
 startxref
-1397
+1421
 %%EOF
diff --git a/qpdf/qtest/qpdf/test14.out b/qpdf/qtest/qpdf/test14.out
index 65b640b9..b6bce056 100644
--- a/qpdf/qtest/qpdf/test14.out
+++ b/qpdf/qtest/qpdf/test14.out
@@ -1,6 +1,6 @@
 caught logic error as expected
-old dict: 1
-old dict: 1
+old dict: 2
+swapped array: /Array
 new dict: 2
 swapped array: /Array
 array and dictionary contents are correct
diff --git a/qpdf/test_driver.cc b/qpdf/test_driver.cc
index 0ab25fb3..614d9025 100644
--- a/qpdf/test_driver.cc
+++ b/qpdf/test_driver.cc
@@ -740,7 +740,13 @@ void runtest(int n, char const* filename1, char const* arg2)
 				   " not called 4-page file");
 	}
 	// Swap pages 2 and 3
-	pdf.swapObjects(pages.at(1).getObjGen(), pages.at(2).getObjGen());
+        auto orig_page2 = pages.at(1);
+        auto orig_page3 = pages.at(2);
+        assert(orig_page2.getKey("/OrigPage").getIntValue() == 2);
+        assert(orig_page3.getKey("/OrigPage").getIntValue() == 3);
+	pdf.swapObjects(orig_page2.getObjGen(), orig_page3.getObjGen());
+        assert(orig_page2.getKey("/OrigPage").getIntValue() == 3);
+        assert(orig_page3.getKey("/OrigPage").getIntValue() == 2);
 	// Replace object and swap objects
 	QPDFObjectHandle trailer = pdf.getTrailer();
 	QPDFObjectHandle qdict = trailer.getKey("/QDict");
@@ -759,18 +765,18 @@ void runtest(int n, char const* filename1, char const* arg2)
 	    std::cout << "caught logic error as expected" << std::endl;
 	}
 	pdf.replaceObject(qdict.getObjGen(), new_dict);
-	// Now qdict still points to the old dictionary
-	std::cout << "old dict: " << qdict.getKey("/Dict").getIntValue()
+	// Now qdict points to the new dictionary
+	std::cout << "old dict: " << qdict.getKey("/NewDict").getIntValue()
 		  << std::endl;
 	// Swap dict and array
 	pdf.swapObjects(qdict.getObjGen(), qarray.getObjGen());
-	// Now qarray will resolve to new object but qdict is still
-	// the old object
-	std::cout << "old dict: " << qdict.getKey("/Dict").getIntValue()
+	// Now qarray will resolve to new object and qdict resolves to
+        // the array
+	std::cout << "swapped array: " << qdict.getArrayItem(0).getName()
 		  << std::endl;
 	std::cout << "new dict: " << qarray.getKey("/NewDict").getIntValue()
 		  << std::endl;
-	// Reread qdict, now pointing to an array
+	// Reread qdict, still pointing to an array
 	qdict = pdf.getObjectByObjGen(qdict.getObjGen());
 	std::cout << "swapped array: " << qdict.getArrayItem(0).getName()
 		  << std::endl;