When we are checking if we need to delay the rename operation for an
inode we not checking if a parent inode that exists in the send and
parent snapshots is really the same inode or not, that is, we are not
comparing the generation number of the parent inode in the send and
parent snapshots. Not only this results in unnecessarily delaying a
rename operation but also can later on make us generate an incorrect
name for a new inode in the send snapshot that has the same number
as another inode in the parent snapshot but a different generation.

Here follows an example where this happens.

Parent snapshot:

 .                                                  (ino 256, gen 3)
 |--- dir258/                                       (ino 258, gen 7)
 |       |--- dir257/                               (ino 257, gen 7)
 |
 |--- dir259/                                       (ino 259, gen 7)

Send snapshot:

 .                                                  (ino 256, gen 3)
 |--- file258                                       (ino 258, gen 10)
 |
 |--- new_dir259/                                   (ino 259, gen 10)
          |--- dir257/                              (ino 257, gen 7)

The following steps happen when computing the incremental send stream:

1) When processing inode 257, its new parent is created using its orphan
   name (o257-21-0), and the rename operation for inode 257 is delayed
   because its new parent (inode 259) was not yet processed - this
   decision to delay the rename operation does not make much sense
   because the inode 259 in the send snapshot is a new inode, it's not
   the same as inode 259 in the parent snapshot.

2) When processing inode 258 we end up delaying its rmdir operation,
   because inode 257 was not yet renamed (moved away from the directory
   inode 258 represents). We also create the new inode 258 using its
   orphan name "o258-10-0", then rename it to its final name of "file258"
   and then issue a truncate operation for it. However this truncate
   operation contains an incorrect name, which corresponds to the orphan
   name and not to the final name, which makes the receiver fail. This
   happens because when we attempt to compute the inode's current name
   we verify that there's another inode with the same number (258) that
   has its rmdir operation pending and because of that we generate an
   orphan name for the new inode 258 (we do this in the function
   get_cur_path()).

Fix this by not delayed the rename operation of an inode if it has parents
with the same number but different generations in both snapshots.

The following steps reproduce this example scenario.

 $ mkfs.btrfs -f /dev/sdb
 $ mount /dev/sdb /mnt
 $ mkdir /mnt/dir257
 $ mkdir /mnt/dir258
 $ mkdir /mnt/dir259
 $ mv /mnt/dir257 /mnt/dir258/dir257
 $ btrfs subvolume snapshot -r /mnt /mnt/snap1

 $ mv /mnt/dir258/dir257 /mnt/dir257
 $ rmdir /mnt/dir258
 $ rmdir /mnt/dir259

 # Remount the filesystem so that the next created inodes will have the
 # numbers 258 and 259. This is because when a filesystem is mounted,
 # btrfs sets the subvolume's inode counter to a value corresponding to
 # the highest inode number in the subvolume plus 1. This inode counter
 # is used to assign a unique number to each new inode and it's
 # incremented by 1 after very inode creation.
 # Note: we unmount and then mount instead of doing a mount with
 # "-o remount" because otherwise the inode counter remains at value 260.
 $ umount /mnt
 $ mount /dev/sdb /mnt
 $ touch /mnt/file258
 $ mkdir /mnt/new_dir259
 $ mv /mnt/dir257 /mnt/new_dir259/dir257
 $ btrfs subvolume snapshot -r /mnt /mnt/snap2

 $ btrfs send /mnt/snap1 -f /tmp/1.snap
 $ btrfs send -p /mnt/snap1 /mnt/snap2 -f /tmp/2.snap

 $ umount /mnt
 $ mkfs.btrfs -f /dev/sdc
 $ mount /dev/sdc /mnt
 $ btrfs receive /mnt -f /tmo/1.snap
 $ btrfs receive /mnt -f /tmo/2.snap -vv
 receiving snapshot mysnap2 uuid=e059b6d1-7f55-f140-8d7c-9a3039d23c97, ctransid=10 parent_uuid=77e98cb6-8762-814f-9e05-e8ba877fc0b0, parent_ctransid=7
 utimes
 mkdir o259-10-0
 rename dir258 -> o258-7-0
 utimes
 mkfile o258-10-0
 rename o258-10-0 -> file258
 utimes
 truncate o258-10-0 size=0
 ERROR: truncate o258-10-0 failed: No such file or directory
Reported-by: Robbie Ko <robbieko@synology.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>