[PATCH] mirror: Avoid assertion failed in mirror_run

[Yi Wang <wang.yi59@zte.com.cn>]

From: Long YunJian <long.yunjian@zte.com.cn>

when blockcommit from active leaf node, sometimes, we get assertion failed with
"mirror_run: Assertion `QLIST_EMPTY(&bs->tracked_requests)' failed" messages.
According to the core file, we find bs->tracked_requests has IO request,
so assertion failed.
(gdb) bt
#0  0x00007f410df707cf in raise () from /lib64/libc.so.6
#1  0x00007f410df5ac05 in abort () from /lib64/libc.so.6
#2  0x00007f410df5aad9 in __assert_fail_base.cold.0 () from /lib64/libc.so.6
#3  0x00007f410df68db6 in __assert_fail () from /lib64/libc.so.6
#4  0x0000556915635371 in mirror_run (job=0x556916ff8600, errp=<optimized out>) at block/mirror.c:1092
#5  0x00005569155e6c53 in job_co_entry (opaque=0x556916ff8600) at job.c:904
#6  0x00005569156d9483 in coroutine_trampoline (i0=<optimized out>, i1=<optimized out>) at util/coroutine-ucontext.c:115
(gdb) p s->mirror_top_bs->backing->bs->tracked_requests
$12 = {lh_first = 0x7f3f07bfb8b0}
(gdb) p s->mirror_top_bs->backing->bs->tracked_requests->lh_first
$13 = (struct BdrvTrackedRequest *) 0x7f3f07bfb8b0

Actually, before excuting assert(QLIST_EMPTY(&bs->tracked_requests)),
it will excute mirror_flush(s). It may handle new I/O request and maybe
pending I/O during this flush. Just likes in bdrv_close fuction,
bdrv_drain(bs) followed by bdrv_flush(bs), we should add bdrv_drain fuction
to handle pending I/O after mirror_flush.

Signed-off-by: Long YunJian <long.yunjian@zte.com.cn>
Signed-off-by: Yi Wang <wang.yi59@zte.com.cn>
---
 block/mirror.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/block/mirror.c b/block/mirror.c
index efec2c7674..1eec356310 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -1079,6 +1079,8 @@ static int coroutine_fn mirror_run(Job *job, Error **errp)
                 s->in_drain = false;
                 continue;
             }
+            /* in case flush left pending I/O */
+            bdrv_drain(bs);
 
             /* The two disks are in sync.  Exit and report successful
              * completion.
-- 
2.18.1