Stop server does not free the accept socket port (on Windows)

[monsieurgustav]

Hi,
I have an unsual use-case where I must stop and restart the FTP server during the app lifetime.
But when I stop it the 1st time, I can observe that the listening port is still busy. (using "netstat -abn -p tcp") The next start works without an error, but no connection is actually accepted.
It happens even if no client connect to the server.
Do you have a clue?
Best regards,
Guillaume

[FlorianReimold]

Thanks for the report! I will take a look and try to reproduce it. I just don't have the time at the moment, so it will take be a little while. Thanks for the patience!

[FlorianReimold]

Hi @monsieurgustav, when you write you "stop" the sever, what exactly are you doing? Killing an application? Deleting the C++ object?
Is it possible for you to post some code that triggers the issue?

[monsieurgustav]

Sure, here it is:

void restartFtpServer(std::unique_ptr<fineftp::FtpServer>& ftpServer, const QString& folder, uint16_t port, size_t threadCount = 1)
    {
        if (ftpServer)
        {
            ftpServer->stop();
            ftpServer.reset();
        }

        ftpServer = std::make_unique<fineftp::FtpServer>(port);
        ftpServer->addUserAnonymous(folder.toUtf8().toStdString(), fineftp::Permission::ReadOnly);
        ftpServer->start(threadCount);
    }

I call this function when the root folder of the FTP server changes. (I don't think there is another way to do it)

If I remember correctly, one may need to connect (anonymously) to the server before it is destroyed/restarted to trigger the issue. I use WinSCP to connect.

[FlorianReimold]

I confirmed the issue with wireshark. Indeed, the socket is not closed properly. I think the reason is the following code:

fineftp-server/fineftp-server/src/server_impl.cpp

Lines 114 to 122 in 6588420

	void FtpServerImpl::stop()
	{
	io_service_.stop();
	for (std::thread& thread : thread_pool_)
	{
	thread.join();
	}
	thread_pool_.clear();
	}

The server stops the io_service, which makes the application to rely on the OS to clean up the sockets. And that may not happen fast enough.

I will check how to fix that bug. For future references: You can also check the existence of that bug in Wireshark, as it will show the improperly terminated connection in red:

My test application looks like follows (Windows only, due to hacky system commands):

#include <chrono>
#include <fineftp/server.h>

#include <string>
#include <thread>

void restartFtpServer(std::unique_ptr<fineftp::FtpServer>& ftpServer, const std::string& folder, uint16_t port, size_t threadCount = 1)
{
  if (ftpServer)
  {
    ftpServer->stop();
    ftpServer.reset();
  }

  ftpServer = std::make_unique<fineftp::FtpServer>(port);
  ftpServer->addUserAnonymous(folder, fineftp::Permission::ReadOnly);
  ftpServer->start(threadCount);
}


int main() {
  std::unique_ptr<fineftp::FtpServer> ftpServer;

  restartFtpServer(ftpServer, "C:\\", 2121, 1);

  system("pause");

  restartFtpServer(ftpServer, "D:\\", 2121, 1);

  system("pause");
}

[monsieurgustav]

Did you try to pause between the stop/start and wait a long time? (30s? 1mn?)
Does the socket/port frees itself at some point?

[FlorianReimold]

Actually, I did not experience any issue. In my test, fineftp was able to re-open the port immediately (code above). I didn't check netstat though, as I already confirmed the bug with Wireshark.

What needs to be done is entirely drop the io_service_.stop();. Without the io_service, sockets will not be able to properly shut down. Stopping the FTP Server must do the following instead:

1. Prevent new sessions from being accepted
2. Stop all existing FTP sessions
3. Join all threads without stopping the io_service. This will wait for the io_service to run out of work on its own.