Possible thread sync issue in XmlLayoutBase:Format()

[David Thornley <th...@gmail.com>]

Hi Everyone,

I ran across a problem with XmlLayoutSchemaLog4j today...

I was getting a NullReferenceException in TextWriterAdapter::Write() after
logging several events from asynchronous socket IO (multiple threads).The
exception failed occured because the internal m_writer of TextWriterAdapter
was null.

The only place I could see m_writer being set to null prior to the
m_protectCloseTextWriter was in XmlLayoutBase::Format() where Attach(null)
is called. So I added a simple lock around the main body of the code in
XmlLayoutBase::Format() using the (m_protectCloseTextWriter) as a sync
object and it fixed the problem. (ie. "lock (m_protectCloseTextWriter)")

I noticed there is synchronisation code in FileAppender::Append(), but I
doubt XmlLayoutBase.Format() is actually atomic. I actually have two
appenders with XmlLayoutSchemaLog4j attached, one is a FileAppender, the
other is a UdpAppender (I suspect that is why).

I'm a noob when it comes to log4net. If anyone with in depth knowledge of
the engine could have a look through the code, and see if there is something
that was missed in the thread synchronisation.

Regards,

David.