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https://issues.apache.org/jira/browse/DERBY-3208?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12542884
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Knut Anders Hatlen commented on DERBY-3208:
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The easiest solution is probably to extend the javadoc comment for
DaemonService.serviceNow() with the necessary guarantee for this code
to work. My suggestion is:
/**
* Service this subscription ASAP. When this method is called, the subscriber's
* <code>performWork()</code> method is guaranteed to be invoked at some point
* in the future. However, there is no guarantee that a subscriber's
* <code>performWork()</code> is called the same number of times as the
* subscriber calls this method. More precisely, if a subscriber is already
* waiting for this daemon service to invoke its <code>performWork()</code>
* method, a call to this method may or may not be a no-op.
*/
I think the above description exactly matches how it is implemented in
BasicDaemon, except that "may or may not be" in the last sentence can
be replaced with "is".
> Callers of DaemonService.serviceNow() assume behaviour not guaranteed by the
> interface
> --------------------------------------------------------------------------------------
>
> Key: DERBY-3208
> URL: https://issues.apache.org/jira/browse/DERBY-3208
> Project: Derby
> Issue Type: Bug
> Components: Services, Store
> Affects Versions: 10.4.0.0
> Reporter: Knut Anders Hatlen
> Priority: Minor
>
> There are two callers of DaemonService.serviceNow() in the code:
> - Clock.rotateClock()
> - LogToFile.flush()
> They implicitly assume that if they are not currently waiting to be serviced
> by the daemon service, a call to serviceNow() is guaranteed to result in
> their performWork() method being invoked at some point in the future. They
> maintain flags (Clock.cleanerRunning and LogToFile.checkpointDaemonCalled)
> which tell whether serviceNow() has been called, and if they are true,
> serviceNow() is not called again. When performWork() is called, they reset
> the flag, allowing others to call serviceNow() again.
> However, DaemonService.serviceNow() does not guarantee that performWork()
> always gets called in these cases. Its javadoc says the following:
> /**
> Service this subscription ASAP. Does not guarantee that the daemon
> will actually do anything about it.
> @param clientNumber the number that uniquely identify the client
> */
> If one of the calls to serviceNow() does not result in a call to
> performWork(), the flags which prevent new calls to serviceNow() are not
> reset. Since both Clock and LogToFile are subscribed to the daemon service in
> on-demand-only mode, performWork() is only called if the subscriber calls
> serviceNow(). That means there is no way to reset the flags, and
> Clock/LogToFile are never allowed to call serviceNow() again.
> Although BasicDaemon's implementation of the DaemonService interface does in
> fact give the necessary guarantees (without explicitly stating them, though)
> to make this protocol work, Clock and LogToFile should be coded against the
> specification of the interface rather than against the undocumented behaviour
> of an implementation of the interface.
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