runaway zselect

runaway zselect

[Atom Smasher]

i would expect zselect to return an error and stop the while loop when "w" 
stops producing output. instead i'm seeing an infinite loop of blank 
lines, after "w" finishes:

while zselect -r 0
do
   read -r foo
   print -r -- $foo
done < <( w )

same thing reading from a coproc with "done <& p"

the weird thing is that i have it working as desired (reading from a 
coproc) in a script, but i can't reproduce it outside of the script... the 
weirder thing is that i can get this working when i test it:
 	sysread -i 0 -o 1 -t 200 <& p
  but in the script mentioned above, it fails:
 	coprocess: bad file descriptor

any ideas? thanks...


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Re: runaway zselect

[Atom Smasher]

On Sun, 17 May 2009, Atom Smasher wrote:

> in a script, but i can't reproduce it outside of the script... the 
> weirder thing is that i can get this working when i test it:
> 	sysread -i 0 -o 1 -t 200 <& p
> but in the script mentioned above, it fails:
===========

doh!!

the script needs zsh/system loaded!


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Re: runaway zselect

[Bart Schaefer]

On May 17,  4:43pm, Atom Smasher wrote:
}
} i would expect zselect to return an error and stop the while loop when
} "w" stops producing output.

That's not how the select() system call works.  The descriptor is still
open on the reading end of the pipe even though the descriptor on the
writing end of the pipe has been closed.  That means from select()'s
point of view, the descriptor is available for reading, even though the
only thing that can be read from it is the end-of-file condition.

If this were not the case, zselect would fail as soon as the writer
exits, even if all output from the writer had not yet been consumed.
(Remember that there is OS-level buffering involved, so the writing
end may be closed before the pipe is "empty".)

What you need to fix this is to test the result of "read" as well.
Either:

    while zselect -r 0 && read -r foo
    do
	print -r -- $foo
    done <<( w )

Or equivalently:

    while zselect -r 0
    do
	read -r foo || break
	print -r -- $foo
    done <<( w )

Re: runaway zselect

[Atom Smasher]

cool... thanks!


On Sun, 17 May 2009, Bart Schaefer wrote:

> On May 17,  4:43pm, Atom Smasher wrote:
> }
> } i would expect zselect to return an error and stop the while loop when
> } "w" stops producing output.
>
> That's not how the select() system call works.  The descriptor is still 
> open on the reading end of the pipe even though the descriptor on the 
> writing end of the pipe has been closed.  That means from select()'s 
> point of view, the descriptor is available for reading, even though the 
> only thing that can be read from it is the end-of-file condition.
>
> If this were not the case, zselect would fail as soon as the writer 
> exits, even if all output from the writer had not yet been consumed. 
> (Remember that there is OS-level buffering involved, so the writing end 
> may be closed before the pipe is "empty".)
>
> What you need to fix this is to test the result of "read" as well. 
> Either:
>
>    while zselect -r 0 && read -r foo
>    do
> 	print -r -- $foo
>    done <<( w )
>
> Or equivalently:
>
>    while zselect -r 0
>    do
> 	read -r foo || break
> 	print -r -- $foo
>    done <<( w )
>
>


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