Libguestfs - Nov 2021 - [PATCH libnbd 2/2] socket-activation: Allow multiple connections

On Sun, Nov 07, 2021 at 03:27:47PM +0200, Nir Soffer
wrote:
> When using qemu-nbd with systemd socket activation, trying to connect
> additional clients concurrently fails with:
> 
>     nbd_connect_uri: connect: server backlog overflowed, see
>     https://bugzilla.redhat.com/1925045: Resource temporarily
>     unavailable
> 
> But this bug was fixed in qemu-nbd long time ago; turns out that libnbd
> has the same bug, creating the server socket with:
> 
>     listen(s, 1);
> 
> Use SOMAXCONN so we can have multiple connections for better
> performance. I think this can be very useful for nbdcopy.
Apart from the problems in API mentioned in the other review, this
patch ordering isn't bisectable.

What's the motivation for this patch series?

Rich.
> Signed-off-by: Nir Soffer <nsoffer at redhat.com>
> ---
>  generator/states-connect-socket-activation.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/generator/states-connect-socket-activation.c
b/generator/states-connect-socket-activation.c
> index 800d963..2abf61b 100644
> --- a/generator/states-connect-socket-activation.c
> +++ b/generator/states-connect-socket-activation.c
> @@ -147,7 +147,7 @@ STATE_MACHINE {
>      return 0;
>    }
>  
> -  if (listen (s, 1) == -1) {
> +  if (listen (s, SOMAXCONN) == -1) {
>      SET_NEXT_STATE (%.DEAD);
>      set_error (errno, "listen");
>      close (s);
> -- 
> 2.31.1
-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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On Sun, Nov 7, 2021 at 4:00 PM Richard W.M. Jones <rjones at redhat.com>
wrote:
>
> On Sun, Nov 07, 2021 at 03:27:47PM +0200, Nir Soffer wrote:
> > When using qemu-nbd with systemd socket activation, trying to connect
> > additional clients concurrently fails with:
> >
> >     nbd_connect_uri: connect: server backlog overflowed, see
> >     https://bugzilla.redhat.com/1925045: Resource temporarily
> >     unavailable
> >
> > But this bug was fixed in qemu-nbd long time ago; turns out that
libnbd
> > has the same bug, creating the server socket with:
> >
> >     listen(s, 1);
> >
> > Use SOMAXCONN so we can have multiple connections for better
> > performance. I think this can be very useful for nbdcopy.
>
> Apart from the problems in API mentioned in the other review, this
> patch ordering isn't bisectable.
>
> What's the motivation for this patch series?
Why is it not bissectable?

In general, I like to add first tests before I modify code. In the best case
the test also reproduces the issue. Having a failing test in the build is
better than having a bug.

In this case the test is too simple, we need to use multiple threads to
reproduce the issue.

We can switch the order if you don't like this approach.

Nir