error: Avoid error_propagate() after migrate_add_blocker()

When migrate_add_blocker(blocker, &errp) is followed by
error_propagate(errp, err), we can often just as well do
migrate_add_blocker(..., errp).

Do that with this Coccinelle script:

    @@
    expression blocker, err, errp;
    expression ret;
    @@
    -    ret = migrate_add_blocker(blocker, &err);
    -    if (err) {
    +    ret = migrate_add_blocker(blocker, errp);
    +    if (ret < 0) {
             ... when != err;
    -        error_propagate(errp, err);
             ...
         }

    @@
    expression blocker, err, errp;
    @@
    -    migrate_add_blocker(blocker, &err);
    -    if (err) {
    +    if (migrate_add_blocker(blocker, errp) < 0) {
             ... when != err;
    -        error_propagate(errp, err);
             ...
         }

Double-check @err is not used afterwards.  Dereferencing it would be
use after free, but checking whether it's null would be legitimate.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20200707160613.848843-43-armbru@redhat.com>

1 files changed, 2 insertions, 4 deletions

diff --git a/block/vvfat.c b/block/vvfat.c
index 24d36220d3..36b53c8757 100644
--- a/block/vvfat.c
+++ b/block/vvfat.c
@@ -1141,7 +1141,6 @@ static int vvfat_open(BlockDriverState *bs, QDict *options, int flags,
     bool floppy;
     const char *dirname, *label;
     QemuOpts *opts;
-    Error *local_err = NULL;
     int ret;
 
 #ifdef DEBUG
@@ -1267,9 +1266,8 @@ static int vvfat_open(BlockDriverState *bs, QDict *options, int flags,
                    "The vvfat (rw) format used by node '%s' "
                    "does not support live migration",
                    bdrv_get_device_or_node_name(bs));
-        ret = migrate_add_blocker(s->migration_blocker, &local_err);
-        if (local_err) {
-            error_propagate(errp, local_err);
+        ret = migrate_add_blocker(s->migration_blocker, errp);
+        if (ret < 0) {
             error_free(s->migration_blocker);
             goto fail;
         }