3 files changed, 41 insertions, 7 deletions

diff --git a/core/fpdfapi/edit/cpdf_creator.cpp b/core/fpdfapi/edit/cpdf_creator.cpp
index 38abb85822..590229bf4f 100644
--- a/core/fpdfapi/edit/cpdf_creator.cpp
+++ b/core/fpdfapi/edit/cpdf_creator.cpp
@@ -624,7 +624,7 @@ int32_t CPDF_Creator::WriteDoc_Stage4() {
   }
 
   if (m_pParser) {
-    CPDF_Dictionary* p = m_pParser->GetTrailer();
+    std::unique_ptr<CPDF_Dictionary> p = m_pParser->GetCombinedTrailer();
     for (const auto& it : *p) {
       const ByteString& key = it.first;
       CPDF_Object* pValue = it.second.get();
diff --git a/core/fpdfapi/parser/cpdf_parser.cpp b/core/fpdfapi/parser/cpdf_parser.cpp
index d02989000c..5b57b949ba 100644
--- a/core/fpdfapi/parser/cpdf_parser.cpp
+++ b/core/fpdfapi/parser/cpdf_parser.cpp
@@ -61,6 +61,20 @@ class CPDF_Parser::TrailerData {
 
   CPDF_Dictionary* GetMainTrailer() const { return main_trailer_.get(); }
 
+  std::unique_ptr<CPDF_Dictionary> GetCombinedTrailer() const {
+    std::unique_ptr<CPDF_Dictionary> result =
+        ToDictionary(main_trailer_->Clone());
+
+    // Info is optional.
+    uint32_t info_obj_num = GetInfoObjNum();
+    if (info_obj_num > 0)
+      result->SetNewFor<CPDF_Reference>("Info", nullptr, GetInfoObjNum());
+
+    // Root is required.
+    result->SetNewFor<CPDF_Reference>("Root", nullptr, GetRootObjNum());
+    return result;
+  }
+
   void SetMainTrailer(std::unique_ptr<CPDF_Dictionary> trailer) {
     ASSERT(trailer);
     main_trailer_ = std::move(trailer);
@@ -75,6 +89,7 @@ class CPDF_Parser::TrailerData {
   void Clear() {
     main_trailer_.reset();
     last_info_obj_num_ = 0;
+    last_root_obj_num_ = 0;
   }
 
   uint32_t GetInfoObjNum() const {
@@ -83,6 +98,12 @@ class CPDF_Parser::TrailerData {
     return pRef ? pRef->GetRefObjNum() : last_info_obj_num_;
   }
 
+  uint32_t GetRootObjNum() const {
+    const CPDF_Reference* pRef = ToReference(
+        GetMainTrailer() ? GetMainTrailer()->GetObjectFor("Root") : nullptr);
+    return pRef ? pRef->GetRefObjNum() : last_root_obj_num_;
+  }
+
  private:
   void ApplyTrailer(const CPDF_Dictionary* dict) {
     // The most recent Info object number contained in last added trailer.
@@ -90,10 +111,15 @@ class CPDF_Parser::TrailerData {
     const auto* pRef = ToReference(dict->GetObjectFor("Info"));
     if (pRef)
       last_info_obj_num_ = pRef->GetRefObjNum();
+
+    const auto* pRoot = ToReference(dict->GetObjectFor("Root"));
+    if (pRoot)
+      last_root_obj_num_ = pRoot->GetRefObjNum();
   }
 
   std::unique_ptr<CPDF_Dictionary> main_trailer_;
   uint32_t last_info_obj_num_ = 0;
+  uint32_t last_root_obj_num_ = 0;
 };
 
 CPDF_Parser::CPDF_Parser()
@@ -1145,20 +1171,22 @@ const CPDF_Array* CPDF_Parser::GetIDArray() const {
   return GetTrailer() ? GetTrailer()->GetArrayFor("ID") : nullptr;
 }
 
-uint32_t CPDF_Parser::GetRootObjNum() {
-  CPDF_Reference* pRef =
-      ToReference(GetTrailer() ? GetTrailer()->GetObjectFor("Root") : nullptr);
-  return pRef ? pRef->GetRefObjNum() : 0;
-}
-
 CPDF_Dictionary* CPDF_Parser::GetTrailer() const {
   return m_TrailerData->GetMainTrailer();
 }
 
+std::unique_ptr<CPDF_Dictionary> CPDF_Parser::GetCombinedTrailer() const {
+  return m_TrailerData->GetCombinedTrailer();
+}
+
 uint32_t CPDF_Parser::GetInfoObjNum() {
   return m_TrailerData->GetInfoObjNum();
 }
 
+uint32_t CPDF_Parser::GetRootObjNum() {
+  return m_TrailerData->GetRootObjNum();
+}
+
 std::unique_ptr<CPDF_Object> CPDF_Parser::ParseIndirectObject(
     CPDF_IndirectObjectHolder* pObjList,
     uint32_t objnum) {
diff --git a/core/fpdfapi/parser/cpdf_parser.h b/core/fpdfapi/parser/cpdf_parser.h
index 375bc01621..45b8cb9f2e 100644
--- a/core/fpdfapi/parser/cpdf_parser.h
+++ b/core/fpdfapi/parser/cpdf_parser.h
@@ -57,7 +57,13 @@ class CPDF_Parser {
 
   void SetPassword(const char* password) { m_Password = password; }
   ByteString GetPassword() { return m_Password; }
+
   CPDF_Dictionary* GetTrailer() const;
+
+  // Returns a new trailer which combines the last read trailer with the /Root
+  // and /Info from previous ones.
+  std::unique_ptr<CPDF_Dictionary> GetCombinedTrailer() const;
+
   FX_FILESIZE GetLastXRefOffset() const { return m_LastXRefOffset; }
 
   uint32_t GetPermissions() const;