Re: [PATCH v2] migration: Setup pre-listened cpr.sock to remove race-condition.

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

On Tue, Jun 10, 2025 at 10:08:49AM -0500, Jaehoon Kim wrote:
> When the source VM attempts to connect to the destination VM's Unix
> domain socket (cpr.sock) during a cpr-transfer test, race conditions can
> occur if the socket file isn't ready. This can lead to connection
> failures when running tests.
> 
> This patch creates and listens on the socket in advance, and passes the
> pre-listened FD directly. This avoids timing issues and improves the
> reliability of CPR tests.
> 
> Reviewed-by: Jason J. Herne <jjherne@linux.ibm.com>
> Signed-off-by: Jaehoon Kim <jhkim@linux.ibm.com>
> 
> ---
> Changes since v1:
> - In v1, the patch added a wait loop to poll the existence of the socket
>   file (cpr_validate_socket_path()).
> 
> - This version instead creates the socket beforehand and passes its FD
>   to the destination QEMU, eliminating the race condition entirely.
> 
> - Commit title and message changed accordingly.
> ---
>  migration/cpr-transfer.c          |  3 +-
>  tests/qtest/migration/cpr-tests.c | 72 ++++++++++++++++++++++++++++++-
>  2 files changed, 72 insertions(+), 3 deletions(-)
> 
> diff --git a/migration/cpr-transfer.c b/migration/cpr-transfer.c
> index e1f140359c..7c9de70bad 100644
> --- a/migration/cpr-transfer.c
> +++ b/migration/cpr-transfer.c
> @@ -46,7 +46,8 @@ QEMUFile *cpr_transfer_input(MigrationChannel *channel, Error **errp)
>      MigrationAddress *addr = channel->addr;
>  
>      if (addr->transport == MIGRATION_ADDRESS_TYPE_SOCKET &&
> -        addr->u.socket.type == SOCKET_ADDRESS_TYPE_UNIX) {
> +        (addr->u.socket.type == SOCKET_ADDRESS_TYPE_UNIX ||
> +            addr->u.socket.type == SOCKET_ADDRESS_TYPE_FD)) {
>  
>          g_autoptr(QIOChannelSocket) sioc = NULL;
>          SocketAddress *saddr = &addr->u.socket;

This will likely cause a crash in tracing code that is below this diff:

        trace_cpr_transfer_input(addr->u.socket.u.q_unix.path);

q_unix.path is NOT valid to access with TYPE_FD.


With regards,
Daniel
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