Q216860: PRB: Chat Server Hangs When Extension Filter Is Installed

Article: Q216860
Product(s): Microsoft Exchange
Version(s): winnt:5.5
Operating System(s): 
Keyword(s): kbExchange550 kbMsg kbGrpDSMsg kbDSupport
Last Modified: 18-JUN-2000

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The information in this article applies to:

- Microsoft Exchange Server, version 5.5 
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SYMPTOMS
========

After you add an Extension Filter to Microsoft Chat Server version 5.5 and
later, the server may stop responding when commands are issued to the Extension
Filter.

CAUSE
=====

You may have created the Extension Filter as a Single Threaded Apartment (STA)
COM object. The Extension Filter registers various callback routines with the
Chat Server. If a callback routine invokes methods on the COM objects that the
routine is given as parameters, a second callback may be generated. If the
second callback routine attempts to reference any of the COM objects that it is
passed (by getting properties, invoking methods, and so on), it can cause the
server to lock.

Because all attempts to access STA objects are serialized by COM, the same thread
is used to handle all callback routines. The first callback routine waits for
the return of the method that triggered the second callback routine. The second
callback cannot return until it finishes manipulating the COM object, which is
waiting for the first callback routine to release the synchronization lock.

Both callback routines are blocked on the same synchronization object, which
results in a deadlock condition. Because the same thread is used for all access
to the Extension Filter object, any subsequent requests also block, which causes
the server to appear to stop responding (hang).

RESOLUTION
==========

There are two ways to avoid this problem:

- Do not create the Extension Filter as a STA COM object.

-or-

- Do not generate a second callback.

  A callback is not typically generated when you get and set properties to the
  passed COM objects. If you invoke methods on the COM objects, however, a
  callback can be generated.

  For example, if the Close method is invoked on the IChatUser object during a
  callback, the OnCloseUser callback routine (if registered) is generated. This
  may lead to a deadlock if, during the OnCloseUser callback, one of the passed
  COM objects is referenced.

Microsoft strongly recommends that you do not create Chat Server Extension
Filters as STA COM objects. You run the risk of deadlock as described above, and
you may incur a severe performance penalty because all requests are serialized
through a single COM object, which negates any benefit of concurrency by having
a multithreaded server.

STATUS
======

This behavior is by design.

MORE INFORMATION
================

Steps to Reproduce Behavior
---------------------------

The following code sample, if it is a part of an Extension Filter that is created
as a STA COM object, can lead to a deadlock condition:

  STDMETHODIMP CMyExtension::OnConfigureExtension(IChatUser *pUser, 
                                                  BSTR bstrCommand) 
  {
     pUser->Close(NULL, L"Bye, bye");
  }

  STDMETHODIMP CMyExtension::OnCloseUser(IChatUser* pUser)
  {
     BSTR  szName = NULL;

     pUser->get_Nick(&szName);

     // Do something with the nickname....
  }

In the code above, during the OnConfigureExtension callback, a second callback,
OnCloseUser, is generated by explicitly closing the session for the IChatUser
object. During the second callback, the extension attempts to retrieve the
nickname of the IChatUser object.

The first callback routine has already obtained the synchronization object to the
IChatUser object and is waiting for the second callback to return. The second
callback routine, however, is waiting for the synchronization object to be
released, so a deadlock condition is produced.

REFERENCES
==========

- Microsoft Exchange Chat Server Extensibility Software Development Kit (SDK).

- Microsoft Exchange Chat Server Online Documentation.

Additional query words: Chat server

======================================================================
Keywords          : kbExchange550 kbMsg kbGrpDSMsg kbDSupport 
Technology        : kbExchangeSearch kbExchange550 kbZNotKeyword2
Version           : winnt:5.5
Issue type        : kbprb

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