block: improve permission conflict error message

Now permissions are updated as follows:
 1. do graph modifications ignoring permissions
 2. do permission update

 (of course, we rollback [1] if [2] fails)

So, on stage [2] we can't say which users are "old" and which are
"new" and exist only since [1]. And current error message is a bit
outdated. Let's improve it, to make everything clean.

While being here, add also a comment and some good assertions.

iotests 283, 307, qsd-jobs outputs are updated.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210601075218.79249-7-vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/tests/qemu-iotests/283.out b/tests/qemu-iotests/283.out
index c9397bf..c6e12b1 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 backup-top filter: Conflicts with use by node 'source' as 'image', which does not allow 'write' on base"}}
+{"error": {"class": "GenericError", "desc": "Cannot append backup-top filter: Permission conflict on node 'base': permissions 'write' are both required by node 'other' (uses node 'base' as 'image' child) and unshared by node 'source' (uses node 'base' as 'image' child)."}}
 
 === backup-top should be gone after job-finalize ===