Re: xargs -p operations out of order!

[Eric Blake]

On 07/31/2013 04:00 PM, Bernhard Voelker wrote:
> Hi Eric,
> 
> On 07/31/2013 07:10 PM, Eric Blake wrote:
>> On 07/31/2013 11:01 AM, address@hidden wrote:
>>> $ seq 111|xargs -n 5 -p
> 
>> Sorry, but this is not xargs' fault.  You asked xargs to spawn multiple
>> processed in parallel, [...]
> 
> I'm not sure:
> 
> the OP used lower-case -p, so xargs is spawning only _one_ process:

Oh, I totally misread the report, and thought it was complaining about
upper-case -P.

> 
>   $ xargs --help
>   [...]
>     -n, --max-args=MAX-ARGS      use at most MAX-ARGS arguments per command 
> line
>     -P, --max-procs=MAX-PROCS    run at most MAX-PROCS processes at a time
>     -p, --interactive            prompt before running commands
> 
> But the effect is the same: with today's implementation, the 'echo'
> process is in parallel to the 'GNU xargs' process asking if it
> should spawn the next 'echo' process.

so the real race here is between xargs prompting about the next command
to be run, vs. the execution of the echo command.

> 
> Hmm, POSIX says:
> http://pubs.opengroup.org/onlinepubs/9699919799/utilities/xargs.html
> 
>   The xargs utility shall then invoke the constructed command line
>   and wait for its completion. This sequence shall be repeated until
>   one of the following occurs: [...]

Indeed, I think you have a real bug after all - xargs should not be
prompting for the fourth command until after it reaps the echo process
spawned by the affirmative answer to the third prompt.

> 
> Do I read it correct?

Yes, I think your reading of POSIX shows we have a bug in xargs.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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