[PATCH v3 6/6] block: improve permission conflict error message

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

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outdated.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>
---
 block.c                               | 29 ++++++++++++++++++++-------
 tests/qemu-iotests/283.out            |  2 +-
 tests/qemu-iotests/307.out            |  2 +-
 tests/qemu-iotests/tests/qsd-jobs.out |  2 +-
 4 files changed, 25 insertions(+), 10 deletions(-)

diff --git a/block.c b/block.c
index 3c0c3964ec..3f456892d0 100644
--- a/block.c
+++ b/block.c
@@ -2029,20 +2029,35 @@ static char *bdrv_child_user_desc(BdrvChild *c)
     return c->klass->get_parent_desc(c);
 }
 
+/*
+ * Check that @a allows everything that @b needs. @a and @b must reference same
+ * child node.
+ */
 static bool bdrv_a_allow_b(BdrvChild *a, BdrvChild *b, Error **errp)
 {
-    g_autofree char *user = NULL;
-    g_autofree char *perm_names = NULL;
+    const char *child_bs_name;
+    g_autofree char *a_user = NULL;
+    g_autofree char *b_user = NULL;
+    g_autofree char *perms = NULL;
+
+    assert(a->bs);
+    assert(a->bs == b->bs);
 
     if ((b->perm & a->shared_perm) == b->perm) {
         return true;
     }
 
-    perm_names = bdrv_perm_names(b->perm & ~a->shared_perm);
-    user = bdrv_child_user_desc(a);
-    error_setg(errp, "Conflicts with use by %s as '%s', which does not "
-               "allow '%s' on %s",
-               user, a->name, perm_names, bdrv_get_node_name(b->bs));
+    child_bs_name = bdrv_get_node_name(b->bs);
+    a_user = bdrv_child_user_desc(a);
+    b_user = bdrv_child_user_desc(b);
+    perms = bdrv_perm_names(b->perm & ~a->shared_perm);
+
+    error_setg(errp, "Permission conflict on node '%s': permissions '%s' are "
+               "both required by %s (uses node '%s' as '%s' child) and "
+               "unshared by %s (uses node '%s' as '%s' child).",
+               child_bs_name, perms,
+               b_user, child_bs_name, b->name,
+               a_user, child_bs_name, a->name);
 
     return false;
 }
diff --git a/tests/qemu-iotests/283.out b/tests/qemu-iotests/283.out
index c9397bfc44..c6e12b15c5 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 ===
 
diff --git a/tests/qemu-iotests/307.out b/tests/qemu-iotests/307.out
index 66bf2ddb74..4b0c7e155a 100644
--- a/tests/qemu-iotests/307.out
+++ b/tests/qemu-iotests/307.out
@@ -53,7 +53,7 @@ exports available: 1
 
 === Add a writable export ===
 {"execute": "block-export-add", "arguments": {"description": "This is the writable second export", "id": "export1", "name": "export1", "node-name": "fmt", "type": "nbd", "writable": true, "writethrough": true}}
-{"error": {"class": "GenericError", "desc": "Conflicts with use by block device 'sda' as 'root', which does not allow 'write' on fmt"}}
+{"error": {"class": "GenericError", "desc": "Permission conflict on node 'fmt': permissions 'write' are both required by an unnamed block device (uses node 'fmt' as 'root' child) and unshared by block device 'sda' (uses node 'fmt' as 'root' child)."}}
 {"execute": "device_del", "arguments": {"id": "sda"}}
 {"return": {}}
 {"data": {"device": "sda", "path": "/machine/peripheral/sda"}, "event": "DEVICE_DELETED", "timestamp": {"microseconds": "USECS", "seconds": "SECS"}}
diff --git a/tests/qemu-iotests/tests/qsd-jobs.out b/tests/qemu-iotests/tests/qsd-jobs.out
index 9f52255da8..189423354b 100644
--- a/tests/qemu-iotests/tests/qsd-jobs.out
+++ b/tests/qemu-iotests/tests/qsd-jobs.out
@@ -16,7 +16,7 @@ QMP_VERSION
 {"return": {}}
 {"timestamp": {"seconds":  TIMESTAMP, "microseconds":  TIMESTAMP}, "event": "JOB_STATUS_CHANGE", "data": {"status": "created", "id": "job0"}}
 {"timestamp": {"seconds":  TIMESTAMP, "microseconds":  TIMESTAMP}, "event": "JOB_STATUS_CHANGE", "data": {"status": "null", "id": "job0"}}
-{"error": {"class": "GenericError", "desc": "Conflicts with use by stream job 'job0' as 'intermediate node', which does not allow 'write' on fmt_base"}}
+{"error": {"class": "GenericError", "desc": "Permission conflict on node 'fmt_base': permissions 'write' are both required by an unnamed block device (uses node 'fmt_base' as 'root' child) and unshared by stream job 'job0' (uses node 'fmt_base' as 'intermediate node' child)."}}
 {"return": {}}
 {"timestamp": {"seconds":  TIMESTAMP, "microseconds":  TIMESTAMP}, "event": "BLOCK_EXPORT_DELETED", "data": {"id": "export1"}}
 *** done
-- 
2.29.2