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https://issues.apache.org/jira/browse/SLING-13261?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Joerg Hoh reassigned SLING-13261:
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Assignee: Joerg Hoh
> Racecondition when resolving servlets
> -------------------------------------
>
> Key: SLING-13261
> URL: https://issues.apache.org/jira/browse/SLING-13261
> Project: Sling
> Issue Type: Task
> Components: Servlets
> Affects Versions: Servlets Resolver 3.0.6
> Reporter: Joerg Hoh
> Assignee: Joerg Hoh
> Priority: Major
>
> h3. Summary
> When two servlets are registered for the same resource type, method and
> extension, the {{SlingServletResolver}} can intermittently resolve and
> execute the _wrong_ servlet. The incorrect servlet is served from the
> {{ResolutionCache}} and continues to be served for that request signature
> until the cache is next flushed. The problem is timing-dependent, so in the
> vast majority of cases the correct servlet is resolved under identical load.
> h3. Root cause
> Both servlets collapse onto a single {{ResolutionCache}} key (the key is
> derived from resource type/super type, extension, method and selectors — not
> from the servlet identity), and the winner is decided by service ranking. The
> cache is populated in {{SlingServletResolver.getServletInternal()}} via a
> non-atomic _get → resolve → put_ sequence with no guard against a concurrent
> {{{}ResolutionCache.flushCache(){}}}.
> When the (higher-ranked) servlet registers, the resource tree immediately
> starts resolving to it and the cache is flushed. If a resolution that already
> read the _old_ winner from the tree performs its {{put}} _after_ that flush,
> it re-inserts the now-stale servlet into the just-flushed cache. That entry
> then survives and is served to every subsequent request for the same key
> until an unrelated flush clears it.
> The window is small (a single resolution's collect-to-put span) and only
> armed while the key is in a miss state around the registration, which is why
> the failure is rare and intermittent. Any later flush (content change,
> script-engine/adapter event, JMX {{{}flushCache{}}}, etc.) heals it
> permanently, because once the correct servlet is the stable tree winner,
> re-population can only ever cache the correct servlet.
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