Handle empty DecodeParams array for (fixes #331)

On read, ignore /DecodeParms when empty list; on write, delete it.
Some files have been found that include an empty list for
/DecodeParms, but this is not technically compliant with the spec, and
the only sensible interpretation is to treat it as if there are no
decode parameters.

5 files changed, 26 insertions, 1 deletions

diff --git a/ChangeLog b/ChangeLog
index 96e7f82f..deece77c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2019-06-09  Jay Berkenbilt  <ejb@ql.org>
+
+	* When /DecodeParms is an empty list, ignore it on read and delete
+	it on write. Fixes #331.
+
 2019-05-18  Jay Berkenbilt  <ejb@ql.org>
 
 	* 8.4.2: release
diff --git a/libqpdf/QPDFWriter.cc b/libqpdf/QPDFWriter.cc
index fee287bc..0cc0547d 100644
--- a/libqpdf/QPDFWriter.cc
+++ b/libqpdf/QPDFWriter.cc
@@ -1604,6 +1604,14 @@ QPDFWriter::unparseObject(QPDFObjectHandle object, int level,
             // Suppress /Length since we will write it manually
             object.removeKey("/Length");
 
+            // If /DecodeParms is an empty list, remove it.
+            if (object.getKey("/DecodeParms").isArray() &&
+                (0 == object.getKey("/DecodeParms").getArrayNItems()))
+            {
+                QTC::TC("qpdf", "QPDFWriter remove empty DecodeParms");
+                object.removeKey("/DecodeParms");
+            }
+
 	    if (flags & f_filtered)
             {
                 // We will supply our own filter and decode
diff --git a/libqpdf/QPDF_Stream.cc b/libqpdf/QPDF_Stream.cc
index 3733940d..3ddb1498 100644
--- a/libqpdf/QPDF_Stream.cc
+++ b/libqpdf/QPDF_Stream.cc
@@ -391,6 +391,10 @@ QPDF_Stream::filterable(std::vector<std::string>& filters,
 
     QPDFObjectHandle decode_obj = this->stream_dict.getKey("/DecodeParms");
     std::vector<QPDFObjectHandle> decode_parms;
+    if (decode_obj.isArray() && (decode_obj.getArrayNItems() == 0))
+    {
+        decode_obj = QPDFObjectHandle::newNull();
+    }
     if (decode_obj.isArray())
     {
         for (int i = 0; i < decode_obj.getArrayNItems(); ++i)
diff --git a/qpdf/qpdf.testcov b/qpdf/qpdf.testcov
index b62dc663..f6a1cedf 100644
--- a/qpdf/qpdf.testcov
+++ b/qpdf/qpdf.testcov
@@ -439,3 +439,4 @@ QPDFPageObjectHelper externalize inline image 0
 QPDFPageObjectHelper keep inline image 0
 qpdf image optimize colorspace 0
 qpdf image optimize bits per component 0
+QPDFWriter remove empty DecodeParms 0
diff --git a/qpdf/qtest/qpdf.test b/qpdf/qtest/qpdf.test
index e95c22bc..fa2b5113 100644
--- a/qpdf/qtest/qpdf.test
+++ b/qpdf/qtest/qpdf.test
@@ -847,7 +847,7 @@ $td->runtest("get stream data",
 show_ntests();
 # ----------
 $td->notify("--- Decode parameter problems ---");
-$n_tests += 4;
+$n_tests += 6;
 
 # Make sure we ignore decode parameters that we don't understand
 $td->runtest("unknown decode parameters",
@@ -870,6 +870,13 @@ $td->runtest("check file",
              {$td->FILE => "a.pdf"},
              {$td->FILE => "indirect-decode-parms-out.pdf"});
 
+$td->runtest("decode parameters empty list",
+             {$td->COMMAND => "qpdf --static-id empty-decode-parms.pdf a.pdf"},
+             {$td->STRING => "", $td->EXIT_STATUS => 0});
+$td->runtest("check file",
+             {$td->FILE => "a.pdf"},
+             {$td->FILE => "empty-decode-parms-out.pdf"});
+
 show_ntests();
 # ----------
 $td->notify("--- Cross reference streams ---");