use a semi-shallo copy of the message to add own stuff without changing the original struct message msg.

src/pEpMIME.cc

@@ -84,7 +84,61 @@ namespace pEpMIME
             return o;
         }
 
-    }
+        // a shallow copy of pEpEngine's struct message with own opt_fields
+        class MessageCopy
+        {
+        public:
+            MessageCopy(const message* _msg_orig)
+            : msg_orig(_msg_orig)
+            , msg_copy(*msg_orig)
+            {
+                if(msg_orig->opt_fields)
+                {
+                    msg_copy.opt_fields = stringpair_list_dup(msg_orig->opt_fields);
+                }
+            }
+            
+            void add_header(sv key, sv value)
+            {
+                stringpair_t* sp = new_stringpair(key.data(), value.data());
+                if(sp==nullptr)
+                {
+                    throw std::runtime_error("Out of memory. new_stringpair() in add_header() fails.");
+                }
+                
+                auto success = stringpair_list_add(msg_copy.opt_fields, sp);
+                if(!success)
+                {
+                    free_stringpair(sp);
+                    throw std::runtime_error("Out of memory. stringpair_list_add() in add_header() fails.");
+                }
+                
+                // there were no opt_fields, yet?  So set them to the "last" (and only) element:
+                if(msg_copy.opt_fields == nullptr)
+                {
+                    msg_copy.opt_fields = success;
+                }
+            }
+            
+            // Neither copy nor move!
+            MessageCopy(const MessageCopy&) = delete;
+            MessageCopy(MessageCopy&&) = delete;
+            
+            ~MessageCopy()
+            {
+                free_stringpair_list(msg_copy.opt_fields);
+                msg_copy.opt_fields = nullptr;
+            }
+            
+            operator const message*  () const { return &msg_copy; }
+            const message* operator->() const { return &msg_copy; }
+        
+        private:
+            const message* msg_orig;
+            message msg_copy;
+        };
+
+    } // end of anonymous namespace
 
 
 template<class LineEnding>
@@ -153,12 +207,18 @@ message* parse_message(const char* mime_text, size_t length, bool* raise_attachm
 }
 
 
-char* generate_message(const message* msg, bool omit_fields, bool has_pEp_msg_attachment)
+char* generate_message(const message* msg_orig, bool omit_fields, bool has_pEp_msg_attachment)
 {
-    if(msg == nullptr)
+    if(msg_orig == nullptr)
         return nullptr;
     
     LOG << "GEN_MSG omit=" << omit_fields << ", has_pEp_msg_att=" << has_pEp_msg_attachment << std::endl;
+    MessageCopy msg{msg_orig};
+    
+    if(has_pEp_msg_attachment)
+    {
+        msg.add_header(Pseudo_Header_Forwarded, "no");
+    }
     
     const auto attachments = parse_attachments(msg->attachments);
     const unsigned inline_attachments = std::count_if(
@@ -206,7 +266,7 @@ char* generate_message(const message* msg, bool omit_fields, bool has_pEp_msg_at
         default:
             throw std::logic_error("Determinant ouf of range 0...15: det=" + std::to_string(det) );
     }
-
+    
     return new_string( smsg.data(), smsg.size() ); // make a C-compatible copy allocated by the "right" allocator. *sigh*
 }