On Sat, Nov 08, 2025 at 04:59:28PM -0600, Eric Blake wrote:
> The point of QIONetListener is to allow a server to listen to more
> than one socket address at a time, and respond to clients in a
> first-come-first-serve order across any of those addresses. While
> some servers (like NBD) really do want to serve multiple simultaneous
> clients, others only care about the first client to connect, and will
> immediately deregister the callback, possibly by dropping their
> reference to the QIONetListener. The existing code ensures that the
> reference count on the listener will not drop to zero while there are
> any pending GSource (since each GSource will not call the notify of
> object_unref() until it is no longer servicing a callback); however,
> it is also possible to guarantee the same setup by merely holding an
> overall reference to the listener as long as there is at least one
> GSource, as well as also holding a local reference around any callback
> function that has not yet run to completion.
>
> Hoisting the reference like this will make it easier for an upcoming
> patch to still ensure the listener cannot be prematurely finalized
> during the user's callback, even when using AioContext (where we have
> not plumbed in support for a notify function) rather than GSource.
>
> Signed-off-by: Eric Blake <[email protected]>
>
> ---
> v2: also grab reference around callback execution
Still not comfortable with this. I've sent a reply to your v1
patch though, so we keep the analysis in one thread.
> ---
> io/net-listener.c | 9 +++++++--
> 1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/io/net-listener.c b/io/net-listener.c
> index ad097175eba..dd3522c9b3c 100644
> --- a/io/net-listener.c
> +++ b/io/net-listener.c
> @@ -54,7 +54,9 @@ static gboolean qio_net_listener_channel_func(QIOChannel
> *ioc,
> trace_qio_net_listener_callback(listener, listener->io_func,
> listener->context);
> if (listener->io_func) {
> + object_ref(OBJECT(listener));
> listener->io_func(listener, sioc, listener->io_data);
> + object_unref(OBJECT(listener));
> }
>
> object_unref(OBJECT(sioc));
> @@ -120,12 +122,14 @@ qio_net_listener_watch(QIONetListener *listener, size_t
> i, const char *caller)
>
> trace_qio_net_listener_watch(listener, listener->io_func,
> listener->context, caller);
> - for ( ; i < listener->nsioc; i++) {
> + if (i == 0) {
> object_ref(OBJECT(listener));
> + }
> + for ( ; i < listener->nsioc; i++) {
> listener->io_source[i] = qio_channel_add_watch_source(
> QIO_CHANNEL(listener->sioc[i]), G_IO_IN,
> qio_net_listener_channel_func,
> - listener, (GDestroyNotify)object_unref, listener->context);
> + listener, NULL, listener->context);
> }
> }
>
> @@ -147,6 +151,7 @@ qio_net_listener_unwatch(QIONetListener *listener, const
> char *caller)
> listener->io_source[i] = NULL;
> }
> }
> + object_unref(OBJECT(listener));
> }
>
> void qio_net_listener_add(QIONetListener *listener,
> --
> 2.51.1
>
With regards,
Daniel
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