Sv: Sv: Sv: Named pipes and multiple writers

[Kristian Ivarsson via Cygwin]

>On 3/26/2020 7:19 PM, Ken Brown via Cygwin wrote:
>> On 3/26/2020 6:39 PM, Ken Brown via Cygwin wrote:
>>> On 3/26/2020 6:01 PM, sten.kristian.ivars...@gmail.com wrote:
>>>> The ENIXIO occurs when parallel child-processes simultaneously using 
>>>> O_NONBLOCK opening the descriptor.
>>>
>>> This is consistent with my guess that the error is generated by 
>>> fhandler_fifo::wait.  I have a feeling that read_ready should have 
>>> been created as a manual-reset event, and that more care is needed to 
>>> make sure it's set when it should be.
>>>
>>>> I could provide a code-snippet
>>>> to reproduce it if wanted ?
>>>
>>> Yes, please!
>> 
>> That might not be necessary.  If you're able to build the git repo 
>> master branch, please try the attached patch.

>Here's a better patch.


I finally succeeded to build latest master (make is not my favourite tool)
and added the patch, but still no success in my little test-program (see
attachment) when creating a write-file-descriptor with O_NONBLOCK

 
>Ken

#include <cstring>

#include <fcntl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>

#include <iostream>
#include <string>

namespace
{
   auto error(const int error)
   {
      std::cerr << getpid() << "\terror:\t" << error << '\t' << 
std::strerror(error) << std::endl;
      return error;
   }
}

int main()
{
   const auto name{"/tmp/my_name"};

   const auto result = mkfifo(name, 0666);

   if(result) return error(errno);

   constexpr auto writers{5};
   constexpr auto messages{4};

   for(auto idx = 0; idx < writers; ++idx)
   {
      const auto pid = fork();

      if(pid < 0) return error(errno);

      if(pid == 0)
      {
         std::cout << "child " << getpid() << std::endl;
         for(auto idx = 0; idx < messages; ++idx)
         {
            const auto wfd = open(name, O_WRONLY | O_NONBLOCK);
            if(wfd < 0) return error(errno);
            const auto msg{std::to_string(getpid())};
            if(write(wfd, msg.data(), msg.size() + 1) < 0) error(errno);
            close(wfd);
         }
         return 0;
      }
   }

   {
      std::cout << "parent" << std::endl;

      const auto rfd = open(name, O_RDONLY);
      const auto wfd = open(name, O_WRONLY);
      if(rfd < 0) return error(errno);
      for(auto idx = 0; idx < writers * messages; ++idx)
      {
         std::string buffer;
         buffer.resize(80);
         if(read(rfd, &buffer[0], buffer.size()) < 0) error(errno);
         std::cout << buffer << std::endl;
      }
      close(wfd);
      close(rfd);
   }

   if(unlink(name) < 0) return error(errno);

   return 0;
}
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