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

[Serge Petrenko]

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)
 {
 	wal_set_watcher(&gc.wal_watcher, "tx", gc_process_wal_event,
-			cbus_process, WAL_EVENT_GC);
+			NULL, WAL_EVENT_GC);
 }
 
 void
-- 
2.17.2 (Apple Git-113)