block: introduce bdrv_can_set_read_only()

Introduce check function for setting read_only flags.  Will return < 0 on
error, with appropriate Error value set.  Does not alter any flags.

Signed-off-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: e2bba34ac3bc76a0c42adc390413f358ae0566e8.1491597120.git.jcody@redhat.com

block.c

@@ -197,7 +197,7 @@ bool bdrv_is_read_only(BlockDriverState *bs)
     return bs->read_only;
 }
 
-int bdrv_set_read_only(BlockDriverState *bs, bool read_only, Error **errp)
+int bdrv_can_set_read_only(BlockDriverState *bs, bool read_only, Error **errp)
 {
     /* Do not set read_only if copy_on_read is enabled */
     if (bs->copy_on_read && read_only) {
@@ -213,6 +213,18 @@ int bdrv_set_read_only(BlockDriverState *bs, bool read_only, Error **errp)
         return -EPERM;
     }
 
+    return 0;
+}
+
+int bdrv_set_read_only(BlockDriverState *bs, bool read_only, Error **errp)
+{
+    int ret = 0;
+
+    ret = bdrv_can_set_read_only(bs, read_only, errp);
+    if (ret < 0) {
+        return ret;
+    }
+
     bs->read_only = read_only;
     return 0;
 }

include/block/block.h

@@ -434,6 +434,7 @@ int bdrv_is_allocated_above(BlockDriverState *top, BlockDriverState *base,
                             int64_t sector_num, int nb_sectors, int *pnum);
 
 bool bdrv_is_read_only(BlockDriverState *bs);
+int bdrv_can_set_read_only(BlockDriverState *bs, bool read_only, Error **errp);
 int bdrv_set_read_only(BlockDriverState *bs, bool read_only, Error **errp);
 bool bdrv_is_sg(BlockDriverState *bs);
 bool bdrv_is_inserted(BlockDriverState *bs);