[Libguestfs] [nbdkit PATCH 1/2] server: Propagate unexpected nbdkit failure with --run

[Eric Blake]

On 9/30/19 12:15 PM, Eric Blake wrote:
> When running captive, we were blindly calling kill(pid) of the captive
> nbdkit child process, even if that process has already exited
> unexpectedly (most likely, from an assertion failure) and another
> opened in its place (pid recycling is rare, but not impossible).

Actually, pid recycling is NOT possible in this case, because the nbdkit 
process is in zombie state until we either wait() on it (which we 
didn't) or exit (in which case init(1) reaps it instead).  Still, the 
fact that we did not check whether kill() failed due to ESRCH was not 
helping the fact that we need the wait to detect early failure.

(Pid recycling is more of an issue when dealing with pids that are not 
your direct children, or if you ignore SIGCHLD)

I'm tempted to send a v2 of this that unconditionally tries kill() 
(which is safe after all), and which waits until the nbdkit process is 
gone (possibly sending SIGKILL if too much time elapses) rather than 
hoping that SIGTERM was sufficient (it's not nice to strand a wedged 
nbdkit child process), and which improves the commit message.

>  We
> need to check that the child process still exists, and if it
> unexpectedly died, ensure that our exit status reflects that fact.

This is true regardless of how we handle a zombie process.

> Note that nbdkit normally should exit with status 0 (even when it is
> sent a signal like SIGHUP or SIGINT) or die from a signal.
> 
> While at it, fix the fact that system() can fail with a value that is
> not appropriate to hand to WIFEXITED() if the child was not even
> spawned, but cannot fail with WIFSTOPPED.  Also, reflect death from
> signal to a status > 128 rather than 1 (we could be even fancier and
> also re-raise the signal so that we die from the same thing, but it's
> not obvious we need that much work...).
> 
> Signed-off-by: Eric Blake <eblake at redhat.com>
> ---
>   server/captive.c | 43 +++++++++++++++++++++++++++++++++----------
>   1 file changed, 33 insertions(+), 10 deletions(-)
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org