In qdf mode, don't write extra XRef streams (fixes #386)

fix-qdf assumes there is exactly one XRef stream and that it is at the
end of the file.

ChangeLog

@@ -1,5 +1,12 @@
 2020-01-26  Jay Berkenbilt  <ejb@ql.org>
 
+	* Bug fix: in qdf mode, do not write out any XRef streams that may
+	have appeared in the original file. These are usually
+	unreferenced, but with --preserve-unreferenced, they could be
+	written out, which breaks fix-qdf's assumption that there is at
+	most one XRef stream and that it appears at the end of the file.
+	Fixes #386.
+
 	* Bug fix: when externalizing inline images, a colorspace value
 	that was a lookup key in the page's /Resource -> /ColorSpace
 	dictionary was not properly handled. Fixes #392.

libqpdf/QPDFWriter.cc

@@ -1241,6 +1241,21 @@ QPDFWriter::enqueueObject(QPDFObjectHandle object)
                 " another file.");
         }
 
+        if (this->m->qdf_mode &&
+            object.isStream() && object.getDict().getKey("/Type").isName() &&
+            (object.getDict().getKey("/Type").getName() == "/XRef"))
+        {
+            // As a special case, do not output any extraneous XRef
+            // streams in QDF mode. Doing so will confuse fix-qdf,
+            // which expects to see only one XRef stream at the end of
+            // the file. This case can occur when creating a QDF from
+            // a file with object streams when preserving unreferenced
+            // objects since the old cross reference streams are not
+            // actually referenced by object number.
+            QTC::TC("qpdf", "QPDFWriter ignore XRef in qdf mode");
+            return;
+        }
+
 	QPDFObjGen og = object.getObjGen();
 
 	if (this->m->obj_renumber.count(og) == 0)

qpdf/qpdf.testcov

@@ -448,3 +448,4 @@ QPDFWriter stream in ostream 0
 QPDFObjectHandle duplicate dict key 0
 QPDFWriter no encryption sig contents 0
 QPDFPageObjectHelper colorspace lookup 0
+QPDFWriter ignore XRef in qdf mode 0

qpdf/qtest/qpdf.test

@@ -2779,7 +2779,7 @@ for (my $n = 16; $n <= 19; ++$n)
 show_ntests();
 # ----------
 $td->notify("--- Specific File Tests ---");
-$n_tests += 4;
+$n_tests += 7;
 
 # Special PDF files that caused problems at some point
 
@@ -2800,6 +2800,16 @@ $td->runtest("compress objstm and xref",
 $td->runtest("check output",
 	     {$td->FILE => "a.pdf"},
 	     {$td->FILE => "compress-objstm-xref.pdf"});
+$td->runtest("qdf + preserved-unreferenced + xref streams",
+	     {$td->COMMAND => "qpdf --qdf --preserve-unreferenced" .
+                  " --static-id compress-objstm-xref.pdf a.pdf"},
+             {$td->STRING => "", $td->EXIT_STATUS => 0});
+$td->runtest("check output",
+	     {$td->FILE => "a.pdf"},
+	     {$td->FILE => "compress-objstm-xref-qdf.pdf"});
+$td->runtest("check fix-qdf idempotency",
+	     {$td->COMMAND => "fix-qdf a.pdf"},
+	     {$td->FILE => "a.pdf", $td->EXIT_STATUS => 0});
 
 show_ntests();
 # ----------