[Libguestfs] [PATCH libnbd 2/2] interop: Retry TCP connections to qemu-nbd.

[Eric Blake]

On 9/12/19 5:02 AM, Richard W.M. Jones wrote:
> The test interop-qemu-nbd-tls-certs frequently fails on slow (32 bit)
> machines in Fedora Koji.  (Is crypto slow on these already overloaded
> machines?)
> 
> As we cannot wait for a signal when qemu-nbd is ready start serving,
> we have to use a sleep.  The current sleep is 5 seconds, which is not
> long enough.  Making the sleep longer would work but is inconsiderate
> for people using faster machines.  Therefore replace this with a retry
> loop with exponential backoff.
> 
> I tested this with a simple wrapper around qemu-nbd which did:
> 
>   sleep 5; exec /usr/bin/qemu-nbd "$@"
> ---
>  interop/interop.c | 19 +++++++++++++------
>  1 file changed, 13 insertions(+), 6 deletions(-)
> 

>    /* Unfortunately there's no good way to wait for qemu-nbd to start
> -   * serving, so ...
> +   * serving, so we need to retry here.
>     */

qemu 4.1 added --pid-file, where you can test for that file existing to
know when the server is up and running.  But we still want an adaptive
exponential backoff whether we test for a file or just try connections,
so this patch is fine.

> -  sleep (5);
> -
> -  if (nbd_connect_tcp (nbd, "localhost", port_str) == -1) {
> -    fprintf (stderr, "%s\n", nbd_get_error ());
> -    goto out;
> +  for (retry = 0; retry < 5; ++retry) {
> +    sleep (1 << retry);
> +    if (nbd_connect_tcp (nbd, "localhost", port_str) == -1) {
> +      fprintf (stderr, "%s\n", nbd_get_error ());
> +      if (nbd_get_errno () != ECONNREFUSED)
> +        goto out;
> +    }
> +    else break;
>    }
> +  if (retry == 5)
> +    goto out;
>  
>  #else /* !SERVE_OVER_TCP */
>  
> 

ACK series

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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