Re: [PATCH] qemu-nbd: Document benefit of --pid-file

[Eric Blake]

On 10/8/19 4:40 AM, Vladimir Sementsov-Ogievskiy wrote:
> 08.10.2019 12:24, Daniel P. Berrangé wrote:
> > On Mon, Oct 07, 2019 at 02:48:40PM -0500, Eric Blake wrote:
> > > One benefit of --pid-file is that it is easier to probe the file
> > > system to see if a pid file has been created than it is to probe if a
> > > socket is available for connection. Document that this is an
> > > intentional feature.
> >
> > I'm not seeing how checking the pid file is better than checking
> > the socket directly ? I think it is probably actually worse.
> >
> > The main problem with the socket is that while we unlink on clean
> > shutdown, it may still exist in disk if the process has exitted
> > abnormally.
> >
> > With the pidfile though we don't ever unlink it, even on clean
> > shutdown, as we don't use the pid files existance as a mutual
> > exclusion check. We instead acquire fcntl locks on it.
> >
> > IOW the pidfile could exist already when qemu-nbd starts up and
> > will still exist when it quits.
>
> Good point.
>
> I was just a bit confused, because pid file is something unrelated to
> socket, and why use one thing to check the existence of another, if we
> can directly try to connect.

Consider the case of writing a testsuite that involves an nbd client, 
where you want to fire up qemu-nbd as the server.  Checking for a pid 
file in shell is easy, and can be done prior to the point of spawning a 
client.  Checking for a successful connect is harder - the shell is not 
the point that would actually connect, so checking if connect works 
involves the shell actually spawning off the child process that attempts 
the connect.  If the client itself has a retry builtin, then you don't 
need to do anything in shell - just spawn the client with retry (at 
which point, the client retrying on the connection is smarter than the 
client retrying on the pid file).  But pid files are immensely useful 
when you have a client that does not have builtin retry, and when 
writing a testing framework where you use shell to learn whether it is 
safe to spawn the client: rather than having to modify the client or 
write a shell loop that respawns child attempts, you merely need a shell 
loop probing for the pid file to exist.

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