Re: [Xen-devel] [PATCH V2] tools/xenstore-watch: Add new timeout parameter

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 03/17/2016 06:39 PM, Ian Jackson wrote:
> Razvan Cojocaru writes ("[PATCH V2] tools/xenstore-watch: Add new timeout 
> parameter"):
>> This patch allows xenstore-watch to exit even if no changes to its
>> XenStore key have occured in a specified interval (in seconds), via
>> a new -T parameter.
> 
> I realise I come a bit late with this, but:
> 
> I'm not opposed to making it possible to wait with a timeout for a
> suitable situation in xenstore.
> 
> However, xenstore-watch needs to be run in a loop because of the
> possibility of spurious watch triggers.  And this means that to use
> your new option and get the timeout right, the caller would need to
> write shell code to query the time, do arithmetic to calculate the
> appropriate timeout, and so on.
> 
> Or to put it another way, xenstore-watch is a fine debugging and
> monitoring tool but not very good for scripting.
> 
> For scripting what you probably want is something like:
> 
>   "wait up to T seconds for xenstore node /X/Y/Z
>    to contain something matching PAT; bail immediately if
>    /X/Y/Z contains something matching QAT".
> 
> How are you using this new option ?

I've actually ended up doing something very similar to what you're
describing, so this is no longer needed here, but I thought that it
might benefit someone in the community so I've carried on.

I basically wanted to be able to shut a program down via writing to a
XenStore key - the program acknowledges that it shut down by removing
the key, but it could also crash before it does that. So in that worst
case scenario, I didn't want xenstore-watch to wait forever. If you
don't feel that's a valid use for it I don't mind dropping the patch,
but I think it might benefit someone.


Thanks,
Razvan

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