Hi all,

To solve ISPN-2808 (avoid blocking JGroups threads in order to allow to
deliver the request responses), I've created another thread pool to move
the possible blocking commands (i.e. the commands that may block until
some state is achieved).

Problem description:

With this solution, the new thread pool should be large in order to be
able to handle the remote commands without deadlocks. The problem is
that all the threads can be block to process the command that may
unblock other commands.

Example: a bunch of commands are blocked waiting for a new topology ID
and the command that will increment the topology ID is in the thread
pool queue.

Solution:

Use a smart command dispatcher, i.e., keep the command in the queue
until we are sure that it will not wait for other commands. I've already
implemented some kind of executor service (ConditionalExecutorService,
in ISPN-2635 and ISPN-2636 branches, Total Order stuff) that only put
the Runnable (more precisely a new interface called ConditionalRunnable)
in the thread pool when it is ready to be processed. Creative guys, it
may need a better name :)

The ConditionalRunnable has a new method (boolean isReady()) that should
return true when the runnable should not block.

Example how to apply this to ISPN-2808:

Most of the commands awaits for a particular topology ID and/or for lock
acquisition.

In this way, the isReady() implementation can be something
like:

isReady()
  return commandTopologyId <= currentTopologyId && (for all keys; do if
!lock(key).tryLock(); return false; done)

With this, I believe we can keep the number of thread low and avoid the
thread deadlocks.

Now, I have two possible implementations:

1) put a reference for StateTransferManager and/or LockManager in the
commands, and invoke the methods directly (a little dirty)

2) added new method in the CommandInterceptor like: boolean
preProcess<command>(Command, InvocationContext). each interceptor will
check if the command will block on it (returning false) or not (invoke
the next interceptor). For example, the StateTransferInterceptor returns
immediately false if the commandToplogyId is higher than the
currentTopologyId and the *LockingIntercerptor will return false if it
cannot acquire some lock.

Any other suggestions? If I was not clear let me know.