detecting write failure [Re: [Libguestfs] [PATCH] virt-cat: Remove some unused Perl module includes.

[Richard W.M. Jones]

On Wed, Sep 23, 2009 at 01:18:06PM +0200, Jim Meyering wrote:
> Unrelated to this particular change, you may want to
> add these lines to virt-cat:
> 
>   END { # use File::Coda; # http://meyering.net/code/Coda/
>     defined fileno STDOUT or return;
>     close STDOUT and return;
>     warn "$ME: failed to close standard output: $!\n";
>     $? ||= 1;
>   }
> 
> Otherwise, virt-cat could fail to detect/report write errors.
> 
> Try this:
> 
>     virt-cat /some/file > /dev/full
> 
> It should fail.

It turns out that virt-cat does catch this error already, or more
precisely guestfs_download catches the error in the write to
/dev/stdout and does the right thing:

$ sudo ./cat/run-cat-locally /dev/vg_trick/F11x64 /boot/grub/grub.conf > /dev/full
download: /dev/stdout: write: No space left on device at /home/rjones/d/libguestfs/cat/virt-cat.pl line 162.
$ echo $?
28

I'm not entirely sure where code 28 comes from.

However, this is a good suggestion in the general case and I'll have
to remember it for other perl scripts.

Rich.

-- 
Richard Jones, Emerging Technologies, Red Hat  http://et.redhat.com/~rjones
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