PH40768: MQ Z/OS: MQGET WITH MQMO_MATCH_MSG_TOKEN SOMETIMES RETURNS A DUPLICATE MESSAGE TOKEN

Obtain the fix for this APAR.

APAR status

• Closed as program error.

Error description

• The circumstances for the problem as initially reported:
  We are receiving duplicate message tokens when browsing a queue
  on an MQ for z/OS queue manager with the MQ for Java API with
  read ahead turned on. The message payload is around 100,000
  bytes of size.  There is only one message on the queue, but the
  message is received twice when the application does a repeated
  GET with MQOO_BROWSE. The expected behavior would be the first
  GET to succeed and the second to fail with
  MQRC_NO_MSG_AVAILABLE.
  
  The issue occurs for messages of a size of around 100000 bytes
  and browsing with read ahead turned on browsing z/OS. It was
  not noticed for messages significantly smaller (e.g. 50000
  bytes) or bigger (e.g. 150000 bytes) on that platform. It also
  was not noticed on distributed platforms.
  
  
  As initially reported, the problem did not occur with read
  ahead turned off (MQOO_NO_READ_AHEAD), but subsequent testing
  found that the problem does not depend on whether read-ahead is
  used.
  
  The Java application issues MQDestination gets against the
  target queue. These calls resolve to multiple MQGET calls to
  the queue manager. The first call is made with
  MQGMO_BROWSE_NEXT and a small buffer length which fails with
  MQRC_TRUNCATED_MSG_FAILED. A subsequent MQGET with
  MQGMO_BROWSE_NEXT and match option MQMO_MATCH_MSG_TOKEN is then
  made with the correct buffer length to again browse the
  previously truncated message.
  
  CSECT CSQIMGEK is called to get the message with the specified
  MsgToken, and it is here where the problem lies. As part of
  validating the supplied MsgToken, the routine does a check
  against the browse cursor priority, but unfortunately the check
  is done against an uninitialized stack variable. The result of
  the check is used to decide whether the message matches the
  user-supplied MsgToken.
  
  It follows that many subtle differences which change the call
  stack leading up to the CSQIMGEK call are susceptible to this
  problem. The problem here is not related to read ahead
  processing.  The problem could also affect MQGETs using
  selectors.
  
  The problem was also reliably recreated with a C bindings
  connection without the use of read ahead.
  

Local fix

• N/A
  

Problem summary

• ****************************************************************
  * USERS AFFECTED: All users of IBM MQ for z/OS Version 9       *
  *                 Release 1 Modification 0, Release 2          *
  *                 Modification 0 and Release 3 Modification 0. *
  ****************************************************************
  * PROBLEM DESCRIPTION: When browsing a large message with      *
  *                      the MQMO_MATCH_MSG_TOKEN and            *
  *                      MQGMO_BROWSE_NEXT options set,          *
  *                      the same message can be retrieved       *
  *                      twice.                                  *
  ****************************************************************
  The code that checks whether a message token has already been
  matched with a message was not checking the put time
  sufficiently which allowed for the same message to be retrieved
  twice in this scenario.
  

Problem conclusion

• The code has been updated with improved checks to ensure we
  don't get the same message twice. As a result of this,
  duplicate messages are no longer returned when getting a large
  message with the MQMO_MATCH_MSG_TOKEN and
  MQGMO_BROWSE_NEXT options set.
  

Temporary fix

Comments

APAR Information

• APAR is sysrouted FROM one or more of the following:

• APAR is sysrouted TO one or more of the following:

  UI83189 UI83190 UI83191

Modules/Macros

• CSQIMGEK
  

Fix information

• Fixed component name

  IBM MQ Z/OS V9

• Fixed component ID

  5655MQ900

Applicable component levels

• R100 PSY UI83191

     UP22/11/29 P F211

• R200 PSY UI83190

     UP22/11/29 P F211

• R300 PSY UI83189

     UP22/11/29 P F211

Fix is available

• Select the PTF appropriate for your component level. You will be required to sign in. Distribution on physical media is not available in all countries.