Re: [PATCH v1] migration: Wait for cpr.sock file to appear before connecting

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

On Mon, Jun 09, 2025 at 09:12:27AM -0400, Steven Sistare wrote:
> On 6/9/2025 4:06 AM, Daniel P. Berrangé wrote:
> > On Fri, Jun 06, 2025 at 03:37:56PM -0400, Steven Sistare wrote:
> > > 
> > > The easiest solution, with no interface changes, is adding wait_for_socket() in qtest,
> > > with this addition from Daniel:
> > > 
> > >    "With the busy wait you risk looping forever if the child (target) QEMU
> > >    already exited for some reason without ever creating the socket. You
> > >    can mitigate this by using 'kill($PID, 0)' in the loop and looking
> > >    for -ERSCH, but this only works if you know the pid involved."
> > > 
> > > Daniel also suggested:
> > >    "For the tests, passing a pre-opened UNIX socket FD could work"
> > > 
> > > Note we can not use any of the standard chardev options to specify such a socket,
> > > because the cpr socket is created before chardevs are created.
> > > 
> > > Perhaps we could specify the fd in an extension of the MigrationChannel MigrationAddress.
> > > { 'union': 'MigrationAddress',
> > >    'base': { 'transport' : 'MigrationAddressType'},
> > >    'discriminator': 'transport',
> > >    'data': {
> > >      'socket': 'SocketAddress',
> > >      'exec': 'MigrationExecCommand',
> > >      'rdma': 'InetSocketAddress',
> > >      'file': 'FileMigrationArgs',
> > >      'fd':   'FdMigrationArgs' } }           <-- add this
> > > 
> > > That would be useful for all clients, but this is asking a lot from you,
> > > when you are just trying to fix the tests.
> > 
> > Note, 'SocketAddress' already has an option for declaring a FD that
> > represents a socket.
> 
> Yes, but if I understand, you proposed passing an fd that represents a
> pre-listened socket, which requires target qemu to accept() first.  The
> existing FdSocketAddress is ready to read.  We could add a boolean to enable
> the new behavior.

It can do both actually - it depends on what APIs the QEMU uses the
SocketAddress with.

If it is used with qio_channel_socket_connect* the FD must be an
active peer connection.

If it is used with qio_channel_socket_listen*/qio_net_listener* the
FD must be listener socket.

With regards,
Daniel
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