Npgsql exceptions on simultaneous reading

Issue

I am trying to read PostgreSQL tables from different threads simultaneously. If Thread1 reads Table1, Thread2 reads Table1 and Thread3 reads Table2, I get exceptions (occasionaly, not always).

Steps to reproduce

Simplified example:

var count = ... ; // quite big: each query is long running, queries are likely to overlap with each other  
using (var db = new Db()) // inherits LinqToDB.Data.DataConnection, is generated by t4 template
{
	ThreadPool.QueueUserWorkItem(state => db.Table1.Take(count).ToList());
	ThreadPool.QueueUserWorkItem(state => db.Table1.Take(count).ToList());
	ThreadPool.QueueUserWorkItem(state => db.Table2.Take(count).ToList());
};

Sometimes Npgsql throws “System.InvalidOperationException: An operation is already in progress”:

System.InvalidOperationException: An operation is already in progress.
   in Npgsql.NpgsqlConnector.StartUserAction(ConnectorState newState)
   in Npgsql.NpgsqlCommand.ExecuteDbDataReaderInternal(CommandBehavior behavior)
   in Npgsql.NpgsqlCommand.ExecuteDbDataReader(CommandBehavior behavior)
   in System.Data.Common.DbCommand.System.Data.IDbCommand.ExecuteReader(CommandBehavior behavior)
   in LinqToDB.Data.DataConnection.ExecuteReader(CommandBehavior commandBehavior) in c:\projects\linq2db\Source\Data\DataConnection.cs:line 1038
   in LinqToDB.Data.DataConnection.QueryRunner.ExecuteReader() in c:\projects\linq2db\Source\Data\DataConnection.QueryRunner.cs:line 428
   in LinqToDB.Linq.QueryRunner.<ExecuteQuery>d__2b`1.MoveNext() in c:\projects\linq2db\Source\Linq\QueryRunner.cs:line 327
   in System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   in System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   in TestConsole.Program.<>c__DisplayClass0_1.<Main>b__3(Object state) in C:\Code2\MeterCollector\TestConsole\Program.cs:line 95
   in System.Threading.QueueUserWorkItemCallback.WaitCallback_Context(Object state)
   in System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   in System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   in System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem()
   in System.Threading.ThreadPoolWorkQueue.Dispatch()
   in System.Threading._ThreadPoolWaitCallback.PerformWaitCallback()

I suppose it’s caused by simultaneous access to the same table from one DataConnection. Let’s try to use different DataConnection instances:

var count = ... ; // quite big: each query is long running, queries are likely to overlap with each other  
using (var db1 = new Db())
using (var db2 = new Db()) // inherits LinqToDB.Data.DataConnection, is generated by t4 template
{
	ThreadPool.QueueUserWorkItem(state => db1.Table1.Take(count).ToList());
	ThreadPool.QueueUserWorkItem(state => db2.Table1.Take(count).ToList());
	ThreadPool.QueueUserWorkItem(state => db1.Table2.Take(count).ToList());
};

In this case Npgsql throws (sometimes) another exception:

System.Exception: Internal Npgsql error, please report: acquired both locks and connector is in state Connecting
   in Npgsql.NpgsqlConnector.StartUserAction(ConnectorState newState)
   in Npgsql.NpgsqlCommand.ExecuteDbDataReaderInternal(CommandBehavior behavior)
   in Npgsql.NpgsqlCommand.ExecuteDbDataReader(CommandBehavior behavior)
   in System.Data.Common.DbCommand.System.Data.IDbCommand.ExecuteReader(CommandBehavior behavior)
   in LinqToDB.Data.DataConnection.ExecuteReader(CommandBehavior commandBehavior) in c:\projects\linq2db\Source\Data\DataConnection.cs:line 1038
   in LinqToDB.Data.DataConnection.QueryRunner.ExecuteReader() in c:\projects\linq2db\Source\Data\DataConnection.QueryRunner.cs:line 428
   in LinqToDB.Linq.QueryRunner.<ExecuteQuery>d__2b`1.MoveNext() in c:\projects\linq2db\Source\Linq\QueryRunner.cs:line 327
   in System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   in System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   in TestConsole.Program.<>c__DisplayClass0_1.<Main>b__3(Object state) in C:\Code2\MeterCollector\TestConsole\Program.cs:line 87
   in System.Threading.QueueUserWorkItemCallback.WaitCallback_Context(Object state)
   in System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   in System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   in System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem()
   in System.Threading.ThreadPoolWorkQueue.Dispatch()
   in System.Threading._ThreadPoolWaitCallback.PerformWaitCallback()

Environment details

linq2db version: 1.9.0
Database Server: PostgreSQL 9.6.3
Database Provider: Npgsql 3.1.6
Operating system: Windows 10
Framework version: .NET Framework 4.6.1

[sdanyliv]

Yes, the following code is thread safe. DataConnection is not thread safe object but if you create for each thread new connection - it is ok. We do not have change tracking and entities chache, so blocking is possible only on server side.

[MaceWindu]

Closing answered/non-actionable issues. Feel free to reopen/continue discussion if it wasn’t anwsered.