Support for merging stdout and stderr to a single pipe in v2

[rconde01]

As far as I can tell this is not possible. Specifying the same pipe for stdout and stderr gives an exception. I think perhaps if shell was added you could redirect within the shell, but i'm not sure.

[rconde01]

With @klemens-morgenstern assistance I was able to get it working with:

#include <iostream>

#include <boost/asio.hpp>
#include <boost/process/v2.hpp>

auto main(int argc, char** argv) -> int {
   if (argc == 1) {
      boost::asio::io_context    io_context;
      boost::asio::writable_pipe merged_write_pipe(io_context);
      boost::asio::readable_pipe merged_read_pipe(io_context);

      boost::asio::connect_pipe(merged_read_pipe, merged_write_pipe);

      boost::process::v2::process p(
         io_context,
         argv[0],
         {"nazz"},
         boost::process::v2::process_stdio{
            nullptr, merged_write_pipe, merged_write_pipe});

      char                      buffer[1024];
      boost::system::error_code ec;

      for (;;) {
         boost::asio::deadline_timer timer(
            io_context, boost::posix_time::milliseconds(10));

         timer.async_wait([&](boost::system::error_code const& a_ec) {
            if (!a_ec)
               merged_write_pipe.cancel();
         });

         merged_read_pipe.async_read_some(
            boost::asio::buffer(buffer),
            [&](boost::system::error_code const& a_ec, std::size_t bytes) {
               timer.cancel();
               if (!a_ec && bytes != 0) {
                  std::cout.write(buffer, bytes);
               }
            });

         io_context.run();
      }
   }
   else {
      size_t i{};

      for (;;) {
         std::cout << "Output " << i << std::endl;
         std::cerr << "Error " << i << std::endl;

         ++i;
      }
   }
}

[rconde01]

FYI This, as written above, does not work on linux.