[PATCH 04/16] block/cbw: Fix missing ERRP_GUARD() for error_prepend()

[Zhao Liu]

From: Zhao Liu <zhao1.liu@intel.com>

As the comment in qapi/error, passing @errp to error_prepend() requires
ERRP_GUARD():

* = Why, when and how to use ERRP_GUARD() =
*
* Without ERRP_GUARD(), use of the @errp parameter is restricted:
...
* - It should not be passed to error_prepend(), error_vprepend() or
*   error_append_hint(), because that doesn't work with &error_fatal.
* ERRP_GUARD() lifts these restrictions.
*
* To use ERRP_GUARD(), add it right at the beginning of the function.
* @errp can then be used without worrying about the argument being
* NULL or &error_fatal.

ERRP_GUARD() could avoid the case when @errp is the pointer of
error_fatal, the user can't see this additional information, because
exit() happens in error_setg earlier than information is added [1].

The cbw_open() passes @errp to error_prepend() without ERRP_GUARD().

Though it is the BlockDriver.bdrv_open() method, and currently its
@errp parameter only points to callers' local_err, to follow the
requirement of @errp, add missing ERRP_GUARD() at the beginning of this
function.

[1]: Issue description in the commit message of commit ae7c80a7bd73
     ("error: New macro ERRP_GUARD()").

Cc: John Snow <jsnow@redhat.com>
Cc: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Hanna Reitz <hreitz@redhat.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
---
 block/copy-before-write.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/block/copy-before-write.c b/block/copy-before-write.c
index 0842a1a6dfbe..8aba27a71d6d 100644
--- a/block/copy-before-write.c
+++ b/block/copy-before-write.c
@@ -407,6 +407,7 @@ out:
 static int cbw_open(BlockDriverState *bs, QDict *options, int flags,
                     Error **errp)
 {
+    ERRP_GUARD();
     BDRVCopyBeforeWriteState *s = bs->opaque;
     BdrvDirtyBitmap *bitmap = NULL;
     int64_t cluster_size;
-- 
2.34.1