NullReferenceException on Npgsql.NpgsqlConnection.Close (trunk)

[monty241]

During debugging another project, the Close of NpgsqlConnection raised a NullReferenceException.

Debugger halted at the line with // *** since Connector variable was null:

else  // Non-pooled connection
{
    if (EnlistedTransaction == null)
        Connector.Close();
    // If a non-pooled connection is being closed but is enlisted in an ongoing
    // TransactionScope, simply detach the connector from the connection and leave
    // it open. It will be closed when the TransactionScope is disposed.
    Connector.Connection = null; // *** CONNECTOR == NULL
    EnlistedTransaction = null;
}

Suggested change:

else  // Non-pooled connection
{
    if (Connector != null)
    {
        if (EnlistedTransaction == null)
            Connector.Close();
        // If a non-pooled connection is being closed but is enlisted in an ongoing
        // TransactionScope, simply detach the connector from the connection and leave
        // it open. It will be closed when the TransactionScope is disposed.
        Connector.Connection = null; // *** CONNECTOR == NULL
    }
    EnlistedTransaction = null;
}

Details:

  HResult=0x80004003
  Message=Object reference not set to an instance of an object.
  Source=Npgsql
  StackTrace:
   at Npgsql.NpgsqlConnection.Close(Boolean wasBroken) 
     in C:\projects\npgsql\src\Npgsql\NpgsqlConnection.cs:line 644

[YohDeadfall]

How to reproduce the error?

[monty241]

No known reproduction scenario. On leaving app using npgsql it just jumped into the exception.

[roji]

This is likely some sort of concurrency race condition when closing connections (see #1642). It would definitely be useful to get some sort of repro.

[monty241]

Problem occurred once more after massive loading data using bulk insert. Yet no reproduction scenario.

[austindrenski]

@monty241 Can you put together a console app and simulate a rapid increase in traffic/load?

[monty241]

No the two cases in which it appeared were both the same case running but interactively during many hours ( background: we have our own sql implementation which in this case is used to massively load bank statements from a cloud app exact online into a payment history database on PostgreSQL but the sql statements used are being debugged and functionally changed, run, thread aborted, again adapted and that many times over the hours. So it is probably something with thread abort but not yet reproducible or isolatable)

[austindrenski]

@roji Did you mean to point to #1642?

[marcosgerene]

Exception thrown: 'System.NullReferenceException' in Npgsql.dll
'GSis.exe' (CLR v4.0.30319: GSis.exe): Loaded 'C:\WINDOWS\Microsoft.Net\assembly\GAC_MSIL\System.Transactions.resources\v4.0_4.0.0.0_pt-BR_b77a5c561934e089\System.Transactions.resources.dll'. Module was built without symbols.
System.Transactions Critical: 0 : <TraceRecord xmlns="http://schemas.microsoft.com/2004/10/E2ETraceEvent/TraceRecord" Severity="Critical"><TraceIdentifier>http://msdn.microsoft.com/TraceCodes/System/ActivityTracing/2004/07/Reliability/Exception/Unhandled</TraceIdentifier><Description>Exceção não tratada</Description><AppDomain>GSis.exe</AppDomain><Exception><ExceptionType>System.NullReferenceException, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</ExceptionType><Message>Referência de objeto não definida para uma instância de um objeto.</Message><StackTrace>   em Npgsql.NpgsqlConnection.Close(Boolean wasBroken)
   em Npgsql.NpgsqlConnection.Close()
   em GereneSI.Core.DAL.Conexao.Disconnect()
   em GereneSI.Core.DAL.Conexao.Finalize()</StackTrace><ExceptionString>System.NullReferenceException: Referência de objeto não definida para uma instância de um objeto.
   em Npgsql.NpgsqlConnection.Close(Boolean wasBroken)
   em Npgsql.NpgsqlConnection.Close()
   em GereneSI.Core.DAL.Conexao.Disconnect()
   em GereneSI.Core.DAL.Conexao.Finalize()</ExceptionString></Exception></TraceRecord>
An unhandled exception of type 'System.NullReferenceException' occurred in Npgsql.dll
Referência de objeto não definida para uma instância de um objeto.

The scenario:

Another task is executing (loading a List< MyObject > in memory) and user close the application (WinForms).

[roji]

Sorry, the issue I wanted to refer to was #1127

[roji]

Npgsql connections are not threadsafe: you cannot perform concurrent operations on them from different threads at the same time - this causes undefined behavior. #1127 exists for handling this better, possibly even automatic cancellation of running command upon close, but this isn't implemented yet.

In the meantime it's the application's responsibility to make sure Close() isn't called while a command is in progress (e.g. via locks at the application level).

If someone thinks these exceptions can happen without any concurrent use, please put together a repro so we can investigate: if indeed concurrency isn't involved, that should be quite simple.

[monty241]

No reproduction yet, but also occurs in non-concurrent execution, see picture of stacks when this error occurred today on npgsql 4.0.7.

Alternative suggested code change:

else  // Non-pooled connection
{
    var temp = Connector;
    if (temp != null)
    {
        if (EnlistedTransaction == null)
            temp.Close();
        // If a non-pooled connection is being closed but is enlisted in an ongoing
        // TransactionScope, simply detach the connector from the connection and leave
        // it open. It will be closed when the TransactionScope is disposed.
        temp.Connection = null; // *** CONNECTOR == NULL
    }
    EnlistedTransaction = null;
}