[jboss-jira] [JBoss JIRA] (JGRP-1495) SEQUENCER needs a lock for delivery of messages

[David Hotham (JIRA)]

David Hotham created JGRP-1495:
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             Summary: SEQUENCER needs a lock for delivery of messages
                 Key: JGRP-1495
                 URL: https://issues.jboss.org/browse/JGRP-1495
             Project: JGroups
          Issue Type: Feature Request
    Affects Versions: 3.1
            Reporter: David Hotham
            Assignee: Bela Ban


I first raised this, or something very like it, in JGRP-1468; but it got lost among the other fixes that were made in that issue.

I've just seen a case where after a merge-view the following happens:

{noformat}
          SEQUENCER            GMS            APPLICATION
             |                  |                  |
 B (view)    |                  |                  |
------------>|                  |                  |
             |----------------->|                  |
             |                  |----\             |
 C (view)    |                  |     \            |
------------>|                  |      \           |
             |----------------->|       \          |
             |                  |        \         |
 C (msg)     |                  |        |         |
------------>|                  |        |         |
             |---------------------------|-------->|
             |                  |        |         |
             |                  |        \         |
             |                  |         \------->|
{noformat}

-  There's a merge view, in which B and C were the old coordinators
-  B and C both broadcast the new view.  In fact both know the physical address of the member shown above (or we're using UDP multicast, if you like); so the view is sent to this node twice.
-  B's view arrives on thread Incoming-1, and gets as far as GMS
-  C's view arrives on thread Incoming-2, and gets as far as GMS (where it is dropped, since it is the same view)
-  A message from C, prompted by the change in view, arrives on thread Incoming-2
-  This overtakes the view on thread Incoming-1, and is delivered to the application
-  Only then does thread Incoming-1 get to deliver the view to the application

ie from the application's point of view: C broadcast the view and then the message; whereas at the node shown above the application received the message and then the view.

I think that the fix will simply be to put a lock in SEQUENCER around up_prot.up(evt) in SEQUENCER.deliver().  That way messages will be delivered to the application in the same order as they arrive at SEQUENCER.  

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