[jbossts-issues] [JBoss JIRA] (JBTM-2394) Transaction Reaper "abuses" in-use connection, leading to RPC protocol corruption

[Tom Jenkinson (JIRA)]

    [ https://issues.jboss.org/browse/JBTM-2394?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13066504#comment-13066504 ] 

Tom Jenkinson commented on JBTM-2394:
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In that scenario the reaper would attempt to rollback the XAR a number of times (detecting the wedged resource) and finally it would give up trying to call rollback on the XAR and just call setRollbackOnly on the TX. I can say that the transaction reaper has been working in this manner successfully with many resource managers over the years. 

The reason we added JBTM-2342 is to assist with JPA which specifically is not thread-safe and the app server integration is required to call EntityManager.close() during afterCompletion(). You mentioned that you cannot use reaper thread names - this is entirely true and was considered during discussions on our forum regarding JBTM-2342. It was determined that this would not work in all scenarios as it is possible that a timed out transaction in a remote parent coordinator _may_ fire before the reaper anyway and in that case the rollback would come via (for example) Jacorb RequestProcessors and would be undetectable as a concurrent call. Hence JBTM-2342 provides an SPI to assist further.

Alternatively, it would be possible for an XAResource implementation to detect concurrent access and use multiple (internal) connections to the database if that is a requirement of the implementation. 

I honestly feel that you will get better advice by discussing this on the forum. The transaction manager component (so Narayana) is not intended to be aware of connections. A caller invokes TX::enlist(XAResource). Callers do not provide us with "connections" or factories (I quote the word to signify it draw out how these can be JDBC/JMS/etc) to attempt to obtain an XAR from. 

Finally, as you point out the recovery manager is able to obtain XAR from helpers but the recovery manager has a very specific purpose and is not intended for this use case. Not that I would advocate a change in this area but if you were considering such a thing, determining which of the XAR registered with the recovery manager should be used to issue the rollback would be impossible as it stands.

> Transaction Reaper "abuses" in-use connection, leading to RPC protocol corruption
> ---------------------------------------------------------------------------------
>
>                 Key: JBTM-2394
>                 URL: https://issues.jboss.org/browse/JBTM-2394
>             Project: JBoss Transaction Manager
>          Issue Type: Bug
>            Reporter: Christian von Kutzleben
>            Assignee: Tom Jenkinson
>
> The scenario is as follows:
> An regular JBoss application thread uses the connection associated with an XAResource for it's work with the database backend. Start(xid) had been invoked, which means, the associated connection is "switched" to that xid. After finishing work, the regular JBoss application thread would eventually call end(xid, TMSUCCESS).
> The application executes a query that takes longer than the configured timeout.
> The work with the database is done via database specific RPC invocations, caused by a enterprise bean (which uses the JPA API and the JPA implementation eventually talks to the database via the database protocol).
> The (lower-level) connection is a TCP/IP connection, and
> the RPC protocol if of the form: "send data to server, then wait for server reply"
> At this point it should be clear, that the TCP/IP connection should not be shared by another thread whilst in the middle of an RPC invocation, because neither does the database server expect any data at this moment on this particular TCP/IP connection nor does it work, that then 2 threads wait on the same socket to receive a reply. (E.g. a ClosedByInterruptException is likely, there might be other error though, depending how wrong data is interpreted, e.g. BufferUnderflowException).
> Unfortunately, this is exactly the behavior of the JBoss "Transaction Reaper" thread, that uses the very same XAResource, that had been used by the regular JBoss application thread and is currently associated to a connection, "switched" to the current xid.
> By definition of the XA specification, any other XAResource could be used to terminate that transaction branch, and it would be fine, if the transaction reaper thread would use any XAResource (regular one, or one from our recovery module) to do that.
> This is a conceptual flaw of the transaction reaper implementation, and we can't implement a workaround (except extremely silly things like reaper thread recognition by comparing thread names ...)
> We could synchronize access to the TCP/IP connection, to not allow another thread communication, while another thread is active doing so, however, that kind of defeats the purpose of the reaper thread, as it would be blocked indefinitely.



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