Re: [PATCH 26/33] io/channel-socket: introduce qio_channel_socket_keep_nonblock()

[Peter Xu <peterx@redhat.com>]

On Wed, Aug 20, 2025 at 02:43:54PM +0100, Daniel P. Berrangé wrote:
> On Wed, Aug 20, 2025 at 09:27:09AM -0400, Peter Xu wrote:
> > On Wed, Aug 13, 2025 at 07:48:47PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> > > Add a possibility to keep socket non-block status when passing
> > > through qio channel. We need this to support migration of open
> > > fds through migration channel.
> > > 
> > > Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> > > ---
> > >  include/io/channel-socket.h |  3 +++
> > >  io/channel-socket.c         | 16 ++++++++++++----
> > >  2 files changed, 15 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/include/io/channel-socket.h b/include/io/channel-socket.h
> > > index a88cf8b3a9..0a4327d745 100644
> > > --- a/include/io/channel-socket.h
> > > +++ b/include/io/channel-socket.h
> > > @@ -49,6 +49,7 @@ struct QIOChannelSocket {
> > >      socklen_t remoteAddrLen;
> > >      ssize_t zero_copy_queued;
> > >      ssize_t zero_copy_sent;
> > > +    bool keep_nonblock;
> > >  };
> > >  
> > >  
> > > @@ -275,4 +276,6 @@ int qio_channel_socket_set_send_buffer(QIOChannelSocket *ioc,
> > >                                         size_t size,
> > >                                         Error **errp);
> > >  
> > > +void qio_channel_socket_keep_nonblock(QIOChannel *ioc);
> > > +
> > >  #endif /* QIO_CHANNEL_SOCKET_H */
> > > diff --git a/io/channel-socket.c b/io/channel-socket.c
> > > index 3b7ca924ff..cd93d7f180 100644
> > > --- a/io/channel-socket.c
> > > +++ b/io/channel-socket.c
> > > @@ -462,9 +462,16 @@ static void qio_channel_socket_finalize(Object *obj)
> > >  }
> > >  
> > >  
> > > +void qio_channel_socket_keep_nonblock(QIOChannel *ioc)
> > > +{
> > > +    QIOChannelSocket *sioc = QIO_CHANNEL_SOCKET(ioc);
> > > +    sioc->keep_nonblock = true;
> > > +}
> > > +
> > > +
> > >  #ifndef WIN32
> > >  static void qio_channel_socket_copy_fds(struct msghdr *msg,
> > > -                                        int **fds, size_t *nfds)
> > > +                                        int **fds, size_t *nfds, bool set_block)
> > >  {
> > >      struct cmsghdr *cmsg;
> > >  
> > > @@ -497,8 +504,9 @@ static void qio_channel_socket_copy_fds(struct msghdr *msg,
> > >                  continue;
> > >              }
> > >  
> > > -            /* O_NONBLOCK is preserved across SCM_RIGHTS so reset it */
> > > -            qemu_socket_set_block(fd);
> > > +            if (set_block) {
> > > +                qemu_socket_set_block(fd);
> > > +            }
> > 
> > "keep_nonblock" as a feature in iochannel is slightly hard to digest.  It
> > can also be read as "keep the fd to be always nonblocking".
> > 
> > Is this feature required, or can this also be done in a get() or
> > post_load() on the other side to set nonblock to whatever it should be
> > (that dest QEMU should be aware of)?
> 
> Either we preserve state of the flag when receiving the FD,
> or every QEMU backend that we're receiving FDs on behalf of
> needs to reset the flag when migration passes over the FD.
> 
> The latter might actually be a more robust scheme. If we're
> migrating across QEMU versions, there is not a strict
> guarantee that the new QEMU version's backend will want the
> O_NONBLOCK flag in the same state as the old QEMU version.
> The code might have been re-written to work in a different
> way than previously.

Good point.

Do you remember why we reset that in the very initial git commit?

Thanks,

-- 
Peter Xu