Throw a specific exception for deadlocks

[oliverhanappi]

The method SqlApplicationLock.ParseExitCode throws an InvalidOperationException when a deadlock is detected. It would be very useful if a specific exception would be thrown.

[madelson]

Hi @oliverhanappi thank you for your interest in the library.

I'm curious whether you've ever seen the deadlock exit code returned. I remember trying to force this a while back and not seeing it.

Provided we can create a test case where this is actually returned, this suggestion makes sense and would be an easy enhancement. For backwards compat, the new exception should extend InvalidOperationException.

[oliverhanappi]

It is actually pretty easy. You just need to acquire multiple locks within the same connection or transaction such that two connections/transactions are waiting on each other. Here is a simple repro:

using System;
using System.Data.SqlClient;
using System.Threading;
using System.Threading.Tasks;
using Medallion.Threading.Sql;

namespace SqlDistributedLockDeadlock
{
  public static class Program
  {
    public static void Main ()
    {
      using (var barrier = new Barrier (2))
      {
        var task1 = Task.Run (() => WithTransaction((connection, transaction) =>
        {
          new SqlDistributedLock ("test1", transaction).Acquire();
          barrier.SignalAndWait();
          new SqlDistributedLock ("test2", transaction).Acquire();
        }));
        
        var task2 = Task.Run (() => WithTransaction((connection, transaction) =>
        {
          new SqlDistributedLock ("test2", transaction).Acquire();
          barrier.SignalAndWait();
          new SqlDistributedLock ("test1", transaction).Acquire();
        }));

        Task.WaitAll (task1, task2);
      }
    }

    private static void WithTransaction (Action<SqlConnection, SqlTransaction> action)
    {
      const string connectionString = "Server=localhost;Database=Playground;Integrated Security=True";

      using (var connection = new SqlConnection (connectionString))
      {
        connection.Open();

        using (var transaction = connection.BeginTransaction())
          action.Invoke (connection, transaction);

        connection.Close();
      }
    }
  }
}

[madelson]

This is addressed as of version 1.4