Allow zero length streams when parsing.chromium/3130

It's possible to create a stream of length 0 in a PDF document.
Currently the code will early exit and return a nullptr. This causes
issues when you want to print the given PDF as the FPDF_ImportPages code
ends up only generating up to the zero length object.

This CL allows creating streams with length 0 and updates the PDF saving
code to output a blank stream.

Bug: chromium:732380
Change-Id: I44182ba4aaac7c51284b002ba01bbc34b6bcf9e0
Reviewed-on: https://pdfium-review.googlesource.com/6490
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: dsinclair <dsinclair@chromium.org>

2 files changed, 15 insertions, 7 deletions

diff --git a/core/fpdfapi/edit/cpdf_creator.cpp b/core/fpdfapi/edit/cpdf_creator.cpp
index 1c0cd69a4b..858d56cbaf 100644
--- a/core/fpdfapi/edit/cpdf_creator.cpp
+++ b/core/fpdfapi/edit/cpdf_creator.cpp
@@ -171,13 +171,21 @@ bool CPDF_Creator::WriteStream(const CPDF_Object* pStream,
     encoder.GetDict()->SetNewFor<CPDF_Number>(
         "Length", static_cast<int>(encryptor.GetSize()));
   }
+
   if (!WriteDirectObj(objnum, encoder.GetDict(), true) ||
-      !m_Archive->WriteString("stream\r\n") ||
-      !m_Archive->WriteBlock(encryptor.GetData(), encryptor.GetSize()) ||
-      !m_Archive->WriteString("\r\nendstream")) {
+      !m_Archive->WriteString("stream\r\n")) {
+    return false;
+  }
+
+  // Allow for empty streams.
+  if (encryptor.GetSize() > 0 &&
+      !m_Archive->WriteBlock(encryptor.GetData(), encryptor.GetSize())) {
     return false;
   }
 
+  if (!m_Archive->WriteString("\r\nendstream"))
+    return false;
+
   return true;
 }
 
diff --git a/core/fpdfapi/parser/cpdf_syntax_parser.cpp b/core/fpdfapi/parser/cpdf_syntax_parser.cpp
index fe1a197028..9dccbd936e 100644
--- a/core/fpdfapi/parser/cpdf_syntax_parser.cpp
+++ b/core/fpdfapi/parser/cpdf_syntax_parser.cpp
@@ -725,10 +725,11 @@ std::unique_ptr<CPDF_Stream> CPDF_SyntaxParser::ReadStream(
     }
     m_Pos = streamStartPos;
   }
-
-  // Read up to the end of the buffer.
+  // Read up to the end of the buffer. Note, we allow zero length streams as
+  // we need to pass them through when we are importing pages into a new
+  // document.
   len = std::min(len, m_FileLen - m_Pos - m_HeaderOffset);
-  if (len <= 0)
+  if (len < 0)
     return nullptr;
 
   std::unique_ptr<uint8_t, FxFreeDeleter> pData;
@@ -746,7 +747,6 @@ std::unique_ptr<CPDF_Stream> CPDF_SyntaxParser::ReadStream(
       pData = dest_buf.DetachBuffer();
     }
   }
-
   auto pStream =
       pdfium::MakeUnique<CPDF_Stream>(std::move(pData), len, std::move(pDict));
   streamStartPos = m_Pos;