[PATCH] box: ensure fiber processing box.cfg doesn't process messages from iproto

[Vladimir Davydov]

On Sat, Nov 10, 2018 at 02:24:36PM +0300, Serge Petrenko wrote:
> In box_cfg() we have a call to gc_set_wal_watcher(), which creates pipes
> between 'wal' and 'tx' under the hood using cbus_pair().
> While pipes are being created, the fiber calling gc_set_wal_watcher()
> will process all the messages coming to 'tx' thread from iproto. This is
> wrong, since we have a separate fiber pool to handle iproto messages,
> and background fibers shouldn't participate in these messages
> processing. For example, this causes occasional credential corruption in
> the fiber executing box_cfg().
> 
> Since tx fiber pool is already created at the time gc_set_wal_watcher()
> is called, we may forbid message processing for the fiber which calls
> the function, one of the tx fiber pool fibers will wake us up when the
> pipes are created.
> 
> Closes #3779
> ---
> https://github.com/tarantool/tarantool/tree/sp/gh-3779-fix
> https://github.com/tarantool/tarantool/issues/3779
> 
>  src/box/gc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/src/box/gc.c b/src/box/gc.c
> index 467eecb91..8f0ecb0b9 100644
> --- a/src/box/gc.c
> +++ b/src/box/gc.c
> @@ -110,7 +110,7 @@ void
>  gc_set_wal_watcher(void)
>  {

Please add a comment to the code, otherwise the patch looks OK.

>  	wal_set_watcher(&gc.wal_watcher, "tx", gc_process_wal_event,
> -			cbus_process, WAL_EVENT_GC);
> +			NULL, WAL_EVENT_GC);
>  }
>  
>  void