Re: [PATCH] docs: show how to spawn qemu-storage-daemon with fd passing

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, Mar 01, 2021 at 04:50:14PM +0000, Stefan Hajnoczi wrote:
> On Mon, Mar 01, 2021 at 03:44:42PM +0000, Daniel P. Berrangé wrote:
> > On Mon, Mar 01, 2021 at 03:39:06PM +0000, Richard W.M. Jones wrote:
> > > On Mon, Mar 01, 2021 at 03:31:59PM +0000, Stefan Hajnoczi wrote:
> > > > The QMP monitor, NBD server, and vhost-user-blk export all support file
> > > > descriptor passing. This is a useful technique because it allows the
> > > > parent process to spawn and wait for qemu-storage-daemon without busy
> > > > waiting, which may delay startup due to arbitrary sleep() calls.
> > > > 
> > > > This Python example is inspired by the test case written for libnbd by
> > > > Richard W.M. Jones <rjones@redhat.com>:
> > > > https://gitlab.com/nbdkit/libnbd/-/commit/89113f484effb0e6c322314ba75c1cbe07a04543
> > > > 
> > > > Thanks to Daniel P. Berrangé <berrange@redhat.com> for suggestions on
> > > > how to get this working. Now let's document it!
> > > > 
> > > > Reported-by: Richard W.M. Jones <rjones@redhat.com>
> > > > Cc: Kevin Wolf <kwolf@redhat.com>
> > > > Cc: Daniel P. Berrangé <berrange@redhat.com>
> > > > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > > > ---
> > > >  docs/tools/qemu-storage-daemon.rst | 38 ++++++++++++++++++++++++++++--
> > > >  1 file changed, 36 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/docs/tools/qemu-storage-daemon.rst b/docs/tools/qemu-storage-daemon.rst
> > > > index f63627eaf6..45854c131e 100644
> > > > --- a/docs/tools/qemu-storage-daemon.rst
> > > > +++ b/docs/tools/qemu-storage-daemon.rst
> > > > @@ -101,10 +101,12 @@ Standard options:
> > > >  
> > > >  .. option:: --nbd-server addr.type=inet,addr.host=<host>,addr.port=<port>[,tls-creds=<id>][,tls-authz=<id>][,max-connections=<n>]
> > > >    --nbd-server addr.type=unix,addr.path=<path>[,tls-creds=<id>][,tls-authz=<id>][,max-connections=<n>]
> > > > +  --nbd-server addr.type=fd,addr.str=<fd>[,tls-creds=<id>][,tls-authz=<id>][,max-connections=<n>]
> > > >  
> > > >    is a server for NBD exports. Both TCP and UNIX domain sockets are supported.
> > > > -  TLS encryption can be configured using ``--object`` tls-creds-* and authz-*
> > > > -  secrets (see below).
> > > > +  A listen socket can be provided via file descriptor passing (see Examples
> > > > +  below). TLS encryption can be configured using ``--object`` tls-creds-* and
> > > > +  authz-* secrets (see below).
> > > >  
> > > >    To configure an NBD server on UNIX domain socket path ``/tmp/nbd.sock``::
> > > >  
> > > > @@ -127,6 +129,38 @@ QMP commands::
> > > >        --chardev socket,path=qmp.sock,server,nowait,id=char1 \
> > > >        --monitor chardev=char1
> > > >  
> > > > +Launch the daemon from Python with a QMP monitor socket using file descriptor
> > > > +passing so there is no need to busy wait for the QMP monitor to become
> > > > +available::
> > > > +
> > > > +  #!/usr/bin/env python3
> > > > +  import os
> > > > +  import subprocess
> > > > +  import socket
> > > > +
> > > > +  sock_path = '/tmp/qmp-{}.sock'.format(os.getpid())
> > > 
> > > Not sure how much you worry about the insecure / easily guessable tmp
> > > path here.  I notice that there's already one in the surrounding
> > > documentation (/tmp/nbd.sock) so maybe it's not a problem :-)
> > > 
> > > > +  with socket.socket(socket.AF_UNIX, socket.SOCK_STREAM) as listen_sock:
> > > > +      listen_sock.bind(sock_path)
> > > > +      listen_sock.listen()
> > > > +
> > > > +      fd = listen_sock.fileno()
> > > > +
> > > > +      subprocess.Popen(
> > > > +          ['qemu-storage-daemon',
> > > > +           '--chardev', f'socket,fd={fd},server=on,id=char1',
> > > > +           '--monitor', 'chardev=char1'],
> > > > +          pass_fds=[fd],
> > > > +      )
> > > > +
> > > > +  qmp_sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
> > > > +  qmp_sock.connect(sock_path)
> > > 
> > > A note that the order of opening the sockets is slightly different
> > > from how I did it in the interop test.  But I believe it makes no
> > > difference, as long as you don't connect to the socket until it's in
> > > the listening state, which is what you're doing here.  So it should be
> > > fine.
> > 
> > Nothing here is closing listen_sock in the parent though.
> > 
> > The trick of passing the listener FD into the child relies on the
> > listener being closed in the parent, so that the parent can get
> > a socket error if the child exits abnormally during startup. Keeping
> > the listen socket open means the parent will wait forever for an
> > accept() that never comes.
> 
> The listen socket is closed by the context manager at the end of the
> 'with' statement. This is the modern Python approach for resource
> acquisition that also handles exceptions automatically. It's like RAII
> in C++.

Hmm, yes, I didn't remember that at first. I'm not sure that is a good
idea as an example code, because people mapping this example into other
languages are likely to miss that critical detail.


Regards,
Daniel
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