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https://issues.apache.org/jira/browse/RIVER-142?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13051625#comment-13051625
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Hudson commented on RIVER-142:
------------------------------
Integrated in River-trunk #493 (See
[https://builds.apache.org/job/River-trunk/493/])
River-142 Slightly different to the original patch, this commit fixes
delayed garbage collection synchronization issues by processing expired leases
immediately, without locking the entire object table. Lease has been changed
to be responsible for expiry, notification and processing (on the garbage
collection thread), synchronized internally. A Lease in the object table must
now be replaced once it expires and cannot be renewed, it is removed from the
table after it is marked expired, to prevent garbage collection of potentially
active leases. Internal classes have been separated from ObjectTable and
BasicExportTable to encapsulate or simplify synchronization and locking.
Target is now more faithful to Exporter.unexport's documented behaviour and
interrupts dispatched method calls when force is true when possible.
I wasn't able to create a test to simulate the original failure condition, to
do so requires a large number of leases to be processed (to create a time
window to process garbage collection of leases after releasing the table lock)
and proper timing of dirty calls, garbage collection and clean calls. The new
code processes the lease immediately and isn't subject to the time window.
> concurrency problem in DGC lease expiration handling
> ----------------------------------------------------
>
> Key: RIVER-142
> URL: https://issues.apache.org/jira/browse/RIVER-142
> Project: River
> Issue Type: Bug
> Components: net_jini_jeri
> Affects Versions: jtsk_2.0
> Reporter: Peter Jones
> Assignee: Peter Firmstone
> Priority: Minor
> Attachments: River-142.patch
>
>
> Bugtraq ID
> [4848840|http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4848840]
> In the server-side DGC implementation's thread that check's for lease
> expirations
> ({{com.sun.jini.jeri.internal.runtime.ObjectTable.LeaseChecker.run}}), it
> checks for them while synchronized on the overall lease table, but it delays
> notifying the expired leases' individual registered {{Targets}} about the
> expirations until after it has released the lease table lock. This approach
> was taken from the JRMP implementation, which is that way because of the fix
> for 4118056 (a previous deadlock bug-- but now, I'm thinking that the JRMP
> implementation has this bug too).
> The problem seems to be that after releasing the lease table lock, it is
> possible for another lease renewal/request to come in (from the same DGC
> client and for the same remote object) that would then be invalidated by the
> subsequent {{Target}} notification made by the lease expiration check
> thread-- and thus the client's lease renewal (for that remote object) will be
> forgotten. It would appear that the synchronization approach here needs to
> be reconsidered.
> h4. ( Comments note: )
> In addition to the basic problem of the expired-then-renewed client being
> removed from the referenced set, there is also the problem of the sequence
> table entry being forgotten-- which prevents detection of a "late clean call".
> Normally, late clean calls are not a problem because sequence numbers are
> retained while the client is in the referenced set (and there is no such
> thing as a "strong dirty"). But in this case, with the following order of
> events on the server side:
> # dirty, seqNo=2
> # (lease expiration)
> # clean, seqNo=1
> The primary bug here is that the first two events will leave the client
> missing from the referenced set. But the secondary bug is that even if
> that's fixed, with the sequence number forgotten, the third event (the "late
> clean call") will still cause the client to be removed from the referenced
> set.
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