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.
--
This message is automatically generated by JIRA.
-
You can reply to this email to add a comment to the issue online.
