Re: [PATCH] sockets: update SOCKET_ADDRESS_TYPE_FD listen(2) backlog

[Stefano Garzarella]

On Wed, Mar 10, 2021 at 05:30:04PM +0000, Stefan Hajnoczi wrote:
> socket_get_fd() fails with the error "socket_get_fd: too many
> connections" if the given listen backlog value is not 1.
>
> Not all callers set the backlog to 1. For example, commit
> 582d4210eb2f2ab5baac328fe4b479cd86da1647 ("qemu-nbd: Use SOMAXCONN for
> socket listen() backlog") uses SOMAXCONN. This will always fail with in
> socket_get_fd().
>
> This patch calls listen(2) on the fd to update the backlog value. The
> socket may already be in the listen state. I have tested that this works
> on Linux 5.10 and macOS Catalina.

Confirmed also from the listen(2) man page of FreeBSD:
"A subsequent listen() system call on the listening socket allows the 
caller to change the maximum queue length using a new backlog argument."

> As a bonus this allows us to detect when the fd cannot listen. Now we'll
> be able to catch unbound or connected fds in socket_listen().
>
> Drop the num argument from socket_get_fd() since this function is also
> called by socket_connect() where a listen backlog value does not make
> sense.
>
> Fixes: e5b6353cf25c99c3f08bf51e29933352f7140e8f ("socket: Add backlog parameter to 
> socket_listen")
> Reported-by: Richard W.M. Jones <rjones@redhat.com>
> Cc: Juan Quintela <quintela@redhat.com>
> Cc: Eric Blake <eblake@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
> util/qemu-sockets.c | 29 ++++++++++++++++++++++-------
> 1 file changed, 22 insertions(+), 7 deletions(-)

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>

Thanks,
Stefano