Sync ExecuteReader doesn't stop after calling command.Cancel() when DB connection is lost

Before submitting

Hello! I am stuck with an issue when using the synchronous API in Npgsql in a situation when DB connection is lost. The bottom line is that after calling the command interrupt method (NpgsqlCommand.Cancel), the execution of the NpgsqlCommand.ExecuteReader method is not interrupted (it will be executed until the Command Timeout has passed).

Steps to reproduce

For example, I can attach this code:

class Program
    {
        static void Main(string[] args)
        {
            var connectionString =
                "Server=192.168.135.208;Username=npgsql_tests;Password=npgsql_tests;Database=npgsql_tests;Timeout=30;Command Timeout=180";
            using (var conn = new NpgsqlConnection(connectionString))
            {
                conn.Open();

                using (var cmd = new NpgsqlCommand("SELECT pg_sleep(150)", conn))
                {
                    Task.Factory.StartNew(() =>
                    {
                        Thread.Sleep(1000);
                        try
                        {
                            cmd.Cancel();
                        }
                        catch (Exception e)
                        {
                            Console.WriteLine(e);
                        }
                    });
                    
                    try
                    {
                        var reader = cmd.ExecuteReader();
                    }
                    catch (Exception e)
                    {
                        Console.WriteLine(e);
                    }
                }
            }
        }
    }

Here is details steps:

1. Add breakepoint where cmd.Cancel() called
2. Run the program and stop at the breakepoint
3. Emulate loss of DB connection (for example, I use ifdown interface in my virtual machine, or with iptables rule).
4. See what after calling cmd.Cancel()- cmd.ExecuteReader() still continues execution and will only be stopped after reaching Command Timeout.

In ExecuterReaderAsync this case works correctly. In spite of the connection breaks, the method (ExecuterReaderAsync) call terminates.

The issue

I expect that after the connection is broken and the command close method is called, there will be an immediate execution stop. Maybe, correct way is: https://github.com/npgsql/npgsql/blob/main/src/Npgsql/NpgsqlConnector.cs#L1489

        internal void PerformUserCancellation()
        {
            _userCancellationRequested = true;

            if (AttemptPostgresCancellation && SupportsPostgresCancellation)
            {
                var cancellationTimeout = Settings.CancellationTimeout;
                if (PerformPostgresCancellation() && cancellationTimeout >= 0)
                {
                    if (cancellationTimeout > 0)
                    {
                        UserTimeout = cancellationTimeout;
                        ReadBuffer.Timeout = TimeSpan.FromMilliseconds(cancellationTimeout);
                        ReadBuffer.Cts.CancelAfter(cancellationTimeout);
                    }
                        
                    return;
                }
            }

            UserTimeout = -1;
            //ReadBuffer.Timeout = _cancelImmediatelyTimeout;
            _socket.Shutdown(SocketShutdown.Both);
            ReadBuffer.Cts.Cancel();
        }

use _socket.Shutdown here.

Further technical details

Npgsql version: 5.0.1.1 (or 4.1.3.1) PostgreSQL version: 10.7 (not important) Operating system: Windows 10 (or Fedora 32)

[roji]

Duplicate of #3187

[ruslanen]

@vonzshik, OK, thank you.