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

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.

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

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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