We are going to publish copy-before-write filter. So, user should be
able to create it with blockdev-add first, specifying both filtered and
target children. And then do blockdev-reopen, to actually insert the
filter where needed.

Currently, filter unshares write permission unconditionally on source
node. It's good, but it will not allow to do blockdev-add. So, let's
relax restrictions when filter doesn't have any parent.

Test output is modified, as now permission conflict happens only when
job creates a blk parent for filter node.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
---
 block/copy-before-write.c  | 8 +++++---
 tests/qemu-iotests/283.out | 2 +-
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/block/copy-before-write.c b/block/copy-before-write.c
index a7996d54db..2a51cc64e4 100644
--- a/block/copy-before-write.c
+++ b/block/copy-before-write.c
@@ -142,10 +142,12 @@ static void cbw_child_perm(BlockDriverState *bs, BdrvChild *c,
         bdrv_default_perms(bs, c, role, reopen_queue,
                            perm, shared, nperm, nshared);
 
-        if (perm & BLK_PERM_WRITE) {
-            *nperm = *nperm | BLK_PERM_CONSISTENT_READ;
+        if (!QLIST_EMPTY(&bs->parents)) {
+            if (perm & BLK_PERM_WRITE) {
+                *nperm = *nperm | BLK_PERM_CONSISTENT_READ;
+            }
+            *nshared &= ~(BLK_PERM_WRITE | BLK_PERM_RESIZE);
         }
-        *nshared &= ~(BLK_PERM_WRITE | BLK_PERM_RESIZE);
     }
 }
 
diff --git a/tests/qemu-iotests/283.out b/tests/qemu-iotests/283.out
index e08f807dab..d5350ce7a7 100644
--- a/tests/qemu-iotests/283.out
+++ b/tests/qemu-iotests/283.out
@@ -5,7 +5,7 @@
 {"execute": "blockdev-add", "arguments": {"driver": "blkdebug", "image": "base", "node-name": "other", "take-child-perms": ["write"]}}
 {"return": {}}
 {"execute": "blockdev-backup", "arguments": {"device": "source", "sync": "full", "target": "target"}}
-{"error": {"class": "GenericError", "desc": "Cannot append copy-before-write filter: Conflicts with use by source as 'image', which does not allow 'write' on base"}}
+{"error": {"class": "GenericError", "desc": "Conflicts with use by source as 'image', which does not allow 'write' on base"}}
 
 === copy-before-write filter should be gone after job-finalize ===
 
-- 
2.29.2