Example scenario:
Parent snapshot:
. (ino 277)
|---- tmp/ (ino 278)
|---- pre/ (ino 280)
| |---- wait_dir/ (ino 281)
|
|---- desc/ (ino 282)
|---- ance/ (ino 283)
| |---- below_ance/ (ino 279)
|
|---- other_dir/ (ino 284)
Send snapshot:
. (ino 277)
|---- tmp/ (ino 278)
|---- other_dir/ (ino 284)
|---- below_ance/ (ino 279)
| |---- pre/ (ino 280)
|
|---- wait_dir/ (ino 281)
|---- desc/ (ino 282)
|---- ance/ (ino 283)
While computing the send stream the following steps happen:
1) While processing inode 279 we end up delaying its rename operation
because its new parent in the send snapshot, inode 284, was not
yet processed and therefore not yet renamed;
2) Later when processing inode 280 we end up renaming it immediately to
"ance/below_once/pre" and not delay its rename operation because its
new parent (inode 279 in the send snapshot) has its rename operation
delayed and inode 280 is not an encestor of inode 279 (its parent in
the send snapshot) in the parent snapshot;
3) When processing inode 281 we end up delaying its rename operation
because its new parent in the send snapshot, inode 284, was not yet
processed and therefore not yet renamed;
4) When processing inode 282 we do not delay its rename operation because
its parent in the send snapshot, inode 281, already has its own rename
operation delayed and our current inode (282) is not an ancestor of
inode 281 in the parent snapshot. Therefore inode 282 is renamed to
"ance/below_ance/pre/wait_dir";
5) When processing inode 283 we realize that we can rename it because one
of its ancestors in the send snapshot, inode 281, has its rename
operation delayed and inode 283 is not an ancestor of inode 281 in the
parent snapshot. So a rename operation to rename inode 283 to
"ance/below_ance/pre/wait_dir/desc/ance" is issued. This path is
invalid due to a missing path building loop that was undetected by
the incremental send implementation, as inode 283 ends up getting
included twice in the path (once with its path in the parent snapshot).
Therefore its rename operation must wait before the ancestor inode 284
is renamed.
Fix this by not terminating the rename dependency checks when we find an
ancestor, in the send snapshot, that has its rename operation delayed. So
that we continue doing the same checks if the current inode is not an
ancestor, in the parent snapshot, of an ancestor in the send snapshot we
are processing in the loop.
The problem and reproducer were reported by Robbie Ko, as part of a patch
titled "Btrfs: incremental send, avoid ancestor rename to descendant".
However the fix was unnecessarily complicated and can be addressed with
much less code and effort.
Reported-by: Robbie Ko <robbieko@synology.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
4122ea64f8