block: Add bdrv_child_refresh_perms()

If a block node uses bdrv_child_try_set_perm() to change the permission
it takes on its child, the result may be very short-lived.  If anything
makes the block layer recalculate the permissions internally, it will
invoke the node driver's .bdrv_child_perm() implementation.  The
permission/shared permissions masks that returns will then override the
values previously passed to bdrv_child_try_set_perm().

If drivers want a child edge to have specific values for the
permissions/shared permissions mask, it must return them in
.bdrv_child_perm().  Consequentially, there is no need for them to pass
the same values to bdrv_child_try_set_perm() then: It is better to have
a function that invokes .bdrv_child_perm() and calls
bdrv_child_try_set_perm() with the result.  This patch adds such a
function under the name of bdrv_child_refresh_perms().

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/block.c b/block.c
index ceb2ea23c5..013369851b 100644
--- a/block.c
+++ b/block.c
@@ -2083,6 +2083,18 @@ int bdrv_child_try_set_perm(BdrvChild *c, uint64_t perm, uint64_t shared,
     return 0;
 }
 
+int bdrv_child_refresh_perms(BlockDriverState *bs, BdrvChild *c, Error **errp)
+{
+    uint64_t parent_perms, parent_shared;
+    uint64_t perms, shared;
+
+    bdrv_get_cumulative_perm(bs, &parent_perms, &parent_shared);
+    bdrv_child_perm(bs, c->bs, c, c->role, NULL, parent_perms, parent_shared,
+                    &perms, &shared);
+
+    return bdrv_child_try_set_perm(c, perms, shared, errp);
+}
+
 void bdrv_filter_default_perms(BlockDriverState *bs, BdrvChild *c,
                                const BdrvChildRole *role,
                                BlockReopenQueue *reopen_queue,