[Pvfs2-developers] state machine ending

[Walter B. Ligon III]

Sam Lang wrote:
> 
> On Jul 24, 2006, at 2:07 PM, Pete Wyckoff wrote:
> 
>> walt at clemson.edu wrote on Mon, 24 Jul 2006 14:47 -0400:
>>
>>> 3) the state machine language has a keyword that lets you state  that 
>>> the
>>> next state is "terminate" which could be used to finish the state
>>> machine, but right now the code has logic that detects this and  
>>> prints a
>>> warning that you should NOT use this.  I don't know why we created  this
>>> method and then made it illegal.  Go figure!
>>
>>
>> I prefer #3 by a large margin over the other options.  It's
>> explicit, not racy, doesn't over-use the return value for a
>> conceptually different event, and doesn't rely on modifying
>> parent state.  Wonder why it was disabled.
>>
> I think terminate ended up being more of a placeholder...we still use  
> it in quite a few of our state machines, but the last state action is  
> expected to call server_state_machine_complete which prevents the SM  
> from transitioning to the 'terminate' state.  I've thought of it as  an 
> assert:  if you get here you're forgetting something.
> 
> It sounds like option 3 is best.  My only concern would be using the  
> state machine definition to define completion makes me forget about  
> cleaning up in the last action (or actions).  I wouldn't mind seeing  
> option 3 and option 4 being required:  check the return code and make  
> sure the next state is 'terminate'.  If one of those fails there's a  
> bug in the code.  I think that might help (at least me) verify that  
> state actions that are meant to complete need to do cleanup first.

I like that idea.  I tend to agree that there is an issue making sure 
the state action matches up to the state machine, and that seems like a 
reasonable way to do it.

So let's assume for the moment we'll take that approach, there is a 
related issue with the "cleaning up."  In the server, the last state 
action is expected to call server_state_machine_complete as you pointed 
out and return the return code it provides (which could easily be 
changed to be SM_ACTION_TERMINATE).  The client has no such call, but 
instead uses code in the "test" and "testsome" loops.  I'd like to see 
the two of them working the same - I could change the client so there is 
a similar cleanup function each SM is expected to call, or alternatively 
both the client and server could be required to provide a cleanup 
callback function that would be called automatically on terminate.

Of course, if cleanup were called automatically on terminate it might 
obviate the need for the SM_ACTION_TERMINATE return code.  :-)

Of course this is all pretty much style issues, but it isn't often 
someone is up to the elbows in this to go ahead and make such changes.

Walt
> 
> -sam
> 
> 
>> Regarding names, STATE_COMPLETE and STATE_MACHINE_TERMINATED seem
>> like find candidates as you have sort of suggested.
>>
>> My pocket change.
>>
>>         -- Pete
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-- 
Dr. Walter B. Ligon III
Associate Professor
ECE Department
Clemson University