[Libguestfs] [PATCH nbdkit 8/9] nbd: Fix shared=true so it creates background thread after fork.

[Eric Blake]

On 7/1/20 2:17 AM, Richard W.M. Jones wrote:
> This option didn't work except when using --foreground (or using
> another option that implies this).  This is because it creates a
> background reader thread, but that thread is invalidated if nbdkit
> forks, resulting in deadlocks because the worker threads are
> fruitlessly sending notifications to a background thread that no
> longer exists.
> 
> The fix is to move this work to .after_fork.
> ---
>   plugins/nbd/nbd.c | 12 ++++++++++--
>   1 file changed, 10 insertions(+), 2 deletions(-)

Good catch.

> 
> diff --git a/plugins/nbd/nbd.c b/plugins/nbd/nbd.c
> index 05f78777..91ed91e0 100644
> --- a/plugins/nbd/nbd.c
> +++ b/plugins/nbd/nbd.c
> @@ -109,7 +109,7 @@ static void nbdplug_close_handle (struct handle *h);
>   static void
>   nbdplug_unload (void)
>   {
> -  if (shared)
> +  if (shared && shared_handle)
>       nbdplug_close_handle (shared_handle);
>     free (sockname);
>     free (tls_certificates);
> @@ -266,8 +266,15 @@ nbdplug_config_complete (void)
>       }
>       nbd_close (nbd);
>     }
> +  return 0;
> +}
>   
> -  /* Create the shared connection. */
> +/* Create the shared connection.  Because this may create a background
> + * thread it must be done after we fork.
> + */
> +static int
> +nbdplug_after_fork (void)
> +{
>     if (shared && (shared_handle = nbdplug_open_handle (false)) == NULL)
>       return -1;
>     return 0;
> @@ -858,6 +865,7 @@ static struct nbdkit_plugin plugin = {
>     .config_complete    = nbdplug_config_complete,
>     .config_help        = nbdplug_config_help,
>     .magic_config_key   = "uri",
> +  .after_fork         = nbdplug_after_fork,
>     .dump_plugin        = nbdplug_dump_plugin,
>     .open               = nbdplug_open,
>     .close              = nbdplug_close,
> 

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