Add INVARIANT on the expected number of bytes being correctly written

This could end up mismatching for sufficently large messages exceding
the length of the pipe's buffer. For the moment it is better to halt if
this potential issue is encountered rather than risk introducing
communication problems. If this problem is encountered later then it may
be possible to work around by splitting the message into multiple writes
to the pipe.

Author: thomasspriggs

src/util/piped_process.cpp

@@ -347,11 +347,16 @@ piped_processt::send_responset piped_processt::send(const std::string &message)
   }
 #ifdef _WIN32
   const auto message_size = narrow<DWORD>(message.size());
-  if(!WriteFile(child_std_IN_Wr, message.c_str(), message_size, NULL, NULL))
+  DWORD bytes_written = 0;
+  if(!WriteFile(
+       child_std_IN_Wr, message.c_str(), message_size, &bytes_written, NULL))
   {
     // Error handling with GetLastError ?
     return send_responset::FAILED;
   }
+  INVARIANT(
+    message_size == bytes_written,
+    "Number of bytes written to sub process must match message size.");
 #else
   // send message to solver process
   int send_status = fputs(message.c_str(), command_stream);