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There is no reason to create another background task when the task that
created it is going to block waiting for it to finish. Instead we will
just execute the logic in the worker thread that is servicing the worker
queue.
Instead of creating a thread to handle the case where a client
is calling job.Wait, we will utilize a timer. This significantly
reduces the number of threads that get created and destroyed while
the service is running.
When a client is doing a wait on a job, any other clients will hang
when trying to do anything with the service. This is caused by
the wait code which was placing the thread that handles
incoming dbus requests to sleep until either the timeout expired or
the job operation completed.
This change creates a thread for the wait request, so that the thread
processing incoming requests can continue to run.
The following operations would hang if lvm was compiled with
'enable-notify-dbus' and the client specified -1 for the timeout:
* LV snapshot merge
* VG move
* LV move
This was caused because the implementation of these three dbus methods is
different. Most of the dbus method calls are executed by gathering information
needed to fulfill it, placing that information on a thread safe queue and
returning. The results later to be returned to the client with callbacks.
With this approach we can process an arbitrary number of commands without any
of them blocking other dbus commands. However, the 3 dbus methods listed
above did not utilize this functionality because they were implemented with a
separate thread that handles the fork & exec of lvm. This is done because these
operations can be very slow to complete. However, because of this the lvm
command that we were waiting on is trying to call back into the dbus service to
notify it that something changed. Because the code was blocking the process
that handles the incoming dbus activity the lvm command blocked. We were stuck
until the client timed-out the connection, which then causes the service to
unblock and continue. If the client did not have a timeout, we would have been
hung indefinitely.
The fix is to always utilize the worker queue on all dbus methods. We need to
ensure that lvm is tested with 'enable-notify-dbus' enabled and disabled.
