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



Richard W.M. Jones wrote:
> 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.

Actually adding the above would help in *some* cases.
It's not as if --help and --version being robust is a big deal,
but try this:

    $ ./virt-cat.pl --version > /dev/full && echo oops                     :
    oops

    $ ./virt-cat.pl --help > /dev/full
    $

Note that that always exits with status of 1 (aka EXIT_FAILURE),
while the convention (at least for most GNU tools) is that PROG --help
exits with status 0, unless it fails, as above.

    $ cat --help > /dev/full                                                :
    cat: write error: No space left on device
    [Exit 1]


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