[jira] Commented: (JCR-798) ConcurrentModificationException during logout

["Martijn Hendriks (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/JCR-798?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12483090 ] 

Martijn Hendriks commented on JCR-798:
--------------------------------------

It seems to me that the only way of avoiding such concurrent modifications is to have some form of synchronization. For instance, having a read-write lock in the LISM that serializes the calls to dispose and the ItemStateListener methods could prevent the first scenario as sketched above. This, however, does not solve the second scenario. I.e., when an item is to be evicted from a MLRUItemStateCache, it must be certain that nobody iterates over the related ItemStateReferenceMap (via the ItemStateReferenceCache).

Martijn

> ConcurrentModificationException during logout
> ---------------------------------------------
>
>                 Key: JCR-798
>                 URL: https://issues.apache.org/jira/browse/JCR-798
>             Project: Jackrabbit
>          Issue Type: Bug
>          Components: core
>         Environment: Jackrabbit 1.2.1
>            Reporter: Martijn Hendriks
>         Assigned To: Stefan Guggisberg
>             Fix For: 1.3
>
>
> We regularly get the following exception:
> java.util.ConcurrentModificationException
>         at org.apache.commons.collections.map.AbstractReferenceMap$ReferenceEntrySetIterator.checkMod(AbstractReferenceMap.java:761)
>         at org.apache.commons.collections.map.AbstractReferenceMap$ReferenceEntrySetIterator.hasNext(AbstractReferenceMap.java:735)
>         at java.util.Collections$UnmodifiableCollection$1.hasNext(Collections.java:1009)
>         at java.util.Collections$UnmodifiableCollection$1.hasNext(Collections.java:1009)
>         at org.apache.jackrabbit.core.state.LocalItemStateManager.dispose(LocalItemStateManager.java:341)
>         at org.apache.jackrabbit.core.WorkspaceImpl.dispose(WorkspaceImpl.java:170)
>         at org.apache.jackrabbit.core.SessionImpl.logout(SessionImpl.java:1225)
>         at org.apache.jackrabbit.core.XASessionImpl.logout(XASessionImpl.java:379)
> Two causes for this exception have been identified:
>  (Taken from an email to the dev-list from Marcel Reutegger):
> > - session A reads some items I
> > - session B transiently removes items in I
> > - session A logs out and starts to iterate over I in  LocalItemStateManager (LISM)
> > - session B saves changes and removed items are evicted from A's LISM
> > - session A gets concurrent modification exception
> Another scenario is the following:
> - Session A gets the iterator of the values of (the primary cache of) an ItemStateReferenceCache in LocalItemStateManager.dispose.
> - Session B then does something that triggers the CacheManager.
> - The CacheManager then calls resizeAll, and evicts some items from the secondary cache of the ItemStateReferenceCache of which the LocalItemStateManager has a values iterator.
> - The garbage collector then runs and evicts the removed items also from the primary cache, which effectively modifies the set over which is iterated.
> Regards,
> Martijn Hendriks

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