[libvirt] [PATCH] util: change virFile*Pid functions to return < 0 on failure

[Eric Blake]

On 07/25/2011 12:19 PM, Laine Stump wrote:
> Although most functions in libvirt return 0 on success and<  0 on
> failure, there are a few functions lingering around that return errno
> (a positive value) on failure, and sometimes code calling those
> functions incorrectly assumes the<0 standard. I noticed one of these
> the other day when auditing networkStartDhcpDaemon after Guido Gunther
> found a place where success was improperly returned on failure (that
> patch has been acked and is pending a push). The problem was that it
> expected the return value from virFileReadPid to be<  0 on failure,
> but it was actually positive (it was also neglected to set the return
> code in this case, similar to the bug found by Guido).
>
> This all led to the fact that *all* of the virFile*Pid functions in
> util.c are returning errno on failure. This patch remedies that
> problem by changing them all to return -errno on failure, and makes
> any necessary changes to callers of the functions. (In the meantime, I
> also properly set the return code on failure of virFileReadPid in
> networkStartDhcpDaemon).

> +++ b/src/lxc/lxc_controller.c

> @@ -996,5 +996,5 @@ cleanup:
>           unlink(sockpath);
>       VIR_FREE(sockpath);
>
> -    return rc;
> +    return -rc; /* rc is 0 or negative, but returns from main should be>=0 */

This hunk is wrong.  errno can validly be a multiple of 256, in which 
case this results in the program exiting with status 0 instead of an 
error.  There's no portable way to use errno as a program's exit status. 
  Rather, this should be:

return rc ? EXIT_FAILURE : EXIT_SUCCESS;

so that we are guaranteed that main() exits with a non-zero positive 
value on failure that cannot wrap around.

ACK to the patch with that problem fixed.

-- 
Eric Blake   eblake at redhat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org