`Connection already has associated data reader` guard missing

[MaceWindu]

I've been testing handling of multiple open queries per single connection functionality for provider and discovered that provider hangs instead of one of two options:

1. executing second query (I suspect ClickHouse TCP protocol doesn't support it)
2. throwing guard exception if it is not supported by protocol/provider

Repro:

using var cn = new ClickHouseConnection(cs);
cn.Open();

using var cmd1 = cn.CreateCommand();
cmd1.CommandText = "select * from system.tables";
using var rd1 = cmd1.ExecuteReader();

using var cmd2 = cn.CreateCommand();
cmd2.CommandText = "select * from system.tables";
// hangs here in ClickHouseTcpClient.OpenSession() call
using var rd2 = cmd2.ExecuteReader();

System.Private.CoreLib.dll!System.Threading.Monitor.Wait(object obj, int millisecondsTimeout)
System.Private.CoreLib.dll!System.Threading.SemaphoreSlim.WaitUntilCountOrTimeout(int millisecondsTimeout, uint startTime, System.Threading.CancellationToken cancellationToken)
System.Private.CoreLib.dll!System.Threading.SemaphoreSlim.Wait(int millisecondsTimeout, System.Threading.CancellationToken cancellationToken)
Octonica.ClickHouseClient.dll!Octonica.ClickHouseClient.ClickHouseTcpClient.OpenSession(bool async, Octonica.ClickHouseClient.IClickHouseSessionExternalResources externalResources, System.Threading.CancellationToken sessionCancellationToken, System.Threading.CancellationToken cancellationToken)
System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncMethodBuilderCore.Start<Octonica.ClickHouseClient.ClickHouseTcpClient.<OpenSession>d__15>(ref Octonica.ClickHouseClient.ClickHouseTcpClient.<OpenSession>d__15 stateMachine)
Octonica.ClickHouseClient.dll!Octonica.ClickHouseClient.ClickHouseCommand.OpenSession(bool closeConnection, bool async, System.Threading.CancellationToken cancellationToken)
Octonica.ClickHouseClient.dll!Octonica.ClickHouseClient.ClickHouseCommand.ExecuteDbDataReader(System.Data.CommandBehavior behavior, bool ignoreProfileEvents, bool async, System.Threading.CancellationToken cancellationToken)
System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncMethodBuilderCore.Start<Octonica.ClickHouseClient.ClickHouseCommand.<ExecuteDbDataReader>d__83>(ref Octonica.ClickHouseClient.ClickHouseCommand.<ExecuteDbDataReader>d__83 stateMachine)
Octonica.ClickHouseClient.dll!Octonica.ClickHouseClient.ClickHouseCommand.ExecuteReader()

[SergeyMirvoda]

Thank you for the report, we are on vacation)

Will fix this and other issues on next week.

[SergeyMirvoda]

Hm, looks like it is or beloved MARS feature from MS SQL server. I doubt CH supports this, even in npgsql it is still "in progress" https://github.com/npgsql/npgsql/issues/462 @victor-sushko can we throw exception something like "OperationInProgressException: A command is already in progress"

[MaceWindu]

Exception is fine as far as it doesn't hang like it does now.

BTW, npgsql (and also mysql) are actually exceptions here - most of databases we test support multiple queries over one connection.