BeginTransaction throws 'Timeout expired', only in specific place, only with MARS enabled

Setup:

• dotnet core 2.1
• aspnet host, so we mostly IServiceCollection stuff up
• docker, linux image microsoft/dotnet:2.1-aspnetcore-runtime
• sql server on host, listening 0.0.0.0:1433
• connection string : data source=tcp:host.docker.internal,1433;initial catalog=ServiceBus;user id=servicebus;password=<123>;application name=sbtest;MultipleActiveResultSets=True

When I have following code (the parallel bit) in Main, then the actual ReBus transaction creation code (essentially https://github.com/rebus-org/Rebus.SqlServer/blob/master/Rebus.SqlServer/SqlServer/DbConnectionProvider.cs#L42 ) will not blow up with exception below.

public static void Main(string[] args) {
  // will not timeout if this runs
  var res = Parallel.For(0, 8, (num) => {
    var cs = <connectionstring>;
    using (var c = new SqlConnection(cs)) {
      c.Open();          
      var t = c.BeginTransaction(IsolationLevel.ReadCommitted);
      // Dapper
      c.Query("select 2; waitfor delay '00:00:01'", transaction: t);
      t.Commit();
      t.Dispose();
      System.Console.WriteLine($"transtest {num} done");
  }});

  Main<Startup>(args);
}

Exception:

    System.Data.SqlClient.SqlException (0x80131904): Timeout expired.  The timeout period elapsed prior to completion of the operation or the server is not responding. ---> System.ComponentModel.Win32Exception (258): Unknown error 258
   at System.Data.SqlClient.SqlConnection.OnError(SqlException exception, Boolean breakConnection, Action`1 wrapCloseInAction)
   at System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj, Boolean callerHasConnectionLock, Boolean asyncClose)
   at System.Data.SqlClient.TdsParser.TryRun(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj, Boolean& dataReady)
   at System.Data.SqlClient.TdsParser.Run(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj)
   at System.Data.SqlClient.TdsParser.TdsExecuteTransactionManagerRequest(Byte[] buffer, TransactionManagerRequestType request, String transactionName, TransactionManagerIsolationLevel isoLevel, Int32 timeout, SqlInternalTransaction transaction, TdsParserStateObject stateObj, Boolean isDelegateControlRequest)
   at System.Data.SqlClient.SqlInternalConnectionTds.ExecuteTransactionYukon(TransactionRequest transactionRequest, String transactionName, IsolationLevel iso, SqlInternalTransaction internalTransaction, Boolean isDelegateControlRequest)
   at System.Data.SqlClient.SqlInternalConnection.BeginSqlTransaction(IsolationLevel iso, String transactionName, Boolean shouldReconnect)
   at System.Data.SqlClient.SqlConnection.BeginTransaction(IsolationLevel iso, String transactionName)
   at Company.Configuration.ServiceBus.ServiceBusConnectionProvider.<GetConnection>b__13_0()

• If I modify Parallel to run 0..2, it will also timeout.
• If I change connection string to not have MARS, it will not timeout
• In all cases the queries and BEGIN TRANSACTION are visible in SQL Server Profiler and no errors are on MSSQL log
• The connection string that ReBus ends up creating is 1:1 that the "workaround" uses
• Rebus ends up being first that uses SQL connection, but also does (successfull) queries to it's own queues/subs tables

My guess is that ReBus might be doing something to environment which somehow affects the transaction creation, but as there is a lot happening, I can't pinpoint what. Kinda guessing on TransactionScope being used and somehow involved with this.

edit: Transaction.Current is null when exception is raised so I suspect TransactionScope is not involved

My guess is that ReBus might be doing something to environment which somehow affects the transaction creation, but as there is a lot happening, I can't pinpoint what. Kinda guessing on TransactionScope being used and somehow involved with this.

Rebus doesn't use TransactionScope(*), so that's not it.

I'm sorry, but I didn't understand where Rebus fit into this. There's no Rebus bits showing in the code you posted, and there's no Rebus things involved in the stack trace, so it's a little bit unclear how this is connected to Rebus.

(*) You can hook Rebus' transaction up with System.Transactions.Transaction.Current by including the Rebus.TransactionScopes package and calling the scope.EnlistRebus() extension method, but it doesn't seem like anything you're doing.