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 <rjo...@redhat.com>: > > > > https://gitlab.com/nbdkit/libnbd/-/commit/89113f484effb0e6c322314ba75c1cbe07a04543 > > > > > > > > Thanks to Daniel P. Berrangé <berra...@redhat.com> for suggestions on > > > > how to get this working. Now let's document it! > > > > > > > > Reported-by: Richard W.M. Jones <rjo...@redhat.com> > > > > Cc: Kevin Wolf <kw...@redhat.com> > > > > Cc: Daniel P. Berrangé <berra...@redhat.com> > > > > Signed-off-by: Stefan Hajnoczi <stefa...@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 -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
