[python-win32] Technique to limit number of instances of our application under Terminal Server

[Mark Hammond]

It sounds like you are after a named semaphore in the "Global\\" 
namespace.  The semaphore count should work fine when your process 
terminates abnormally - but will not if it simply hangs - so something 
will need to terminate a hung process before the semaphore becomes 
available.

HTH,

Mark

On 11/03/2010 8:16 AM, python at bdurham.com wrote:
> Hi Tim,
>
>> It's not quite clear whether you want something which you can build into the application itself
>
> Yes, since I control the source code, this is a feature I would like to
> build into my applications.
>
>> ... in which case, the answer's probably Semaphores:
>> http://msdn.microsoft.com/en-us/library/ms685129%28VS.85%29.aspx
>
> Thanks for that link. My understanding is that semaphores only to apply
> to threads within a single running application?
>
> On the other hand, perhaps the term 'threads' applies to applications as
> well? I say that because the term thread is used to describe mutex's
> which I've used on a program vs. thread basis.
>
> Microsoft's description of Mutex's
> http://msdn.microsoft.com/en-us/library/ms684266%28VS.85%29.aspx
>
> My research on semaphors also leads me to believe that if an application
> incremented a semaphor and crashed without decrementing the semaphore,
> then my semaphore count would be incorrect. Since the code in question
> is being run in a test environment, the possibility of abnormal
> terminations is higher than one would normally expect. Does this sound
> accurate to you?
>
>> Obviously there are other approaches: you could write transient pid files in the Unix fashion, you could use Job objects to group your processes together in one job:
> http://msdn.microsoft.com/en-us/library/ms684161%28VS.85%29.aspx
>
> What are your thoughts on using a pre-assigned list of mutexes. An
> application would walk a list of named mutex's trying to lock one for
> itself. If an application iterated through a list of mutex's without
> securing one for itself, it would exit.
>
> The advantage of mutex's over semaphores would be that applications that
> terminate abnormally would have their mutex released, while applications
> using semaphors that terminated abnormally would leave their semaphore
> with an incorrect count?
>
> The disadvantage of using mutex's vs. semaphores is that the time to
> find a free mutex might be much slower than the time to simply increment
> or decrement a semaphor.
>
> On the other hand, one of my colleagues claims that neither semphores or
> mutuxes will be visible to other Terminal Service users. In other words,
> all I will be able to do with semaphores and mutex's is to make sure a
> SPECIFIC user doesn't run an application more than once.  Put another
> way, there may be no such thing as Global (that are visible across user
> sessions) semaphores and mutex's.
>
> So I'm back to thinking about your PID file idea :)
>
> Regards,
> Malcolm
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