There is a bug sending link commands for existing file paths. When we're
processing an inode, we go over all references. All the new file paths are
added to the "new_refs" list. And all the deleted file paths are added to
the "deleted_refs" list. In the end, when we finish processing the inode,
we iterate over all the items in the "new_refs" list and send link commands
for those file paths. After that, we go over all the items in the
"deleted_refs" list and send unlink commands for them. If there are
duplicated file paths in both lists, we will try to create them before we
remove them. Then the receiver gets an -EEXIST error when trying the link
operations.

Example for having duplicated file paths in both list:

  $ btrfs subvolume create vol

  # create a file and 2000 hard links to the same inode
  $ touch vol/foo
  $ for i in {1..2000}; do link vol/foo vol/$i ; done

  # take a snapshot for a parent snapshot
  $ btrfs subvolume snapshot -r vol snap1

  # remove 2000 hard links and re-create the last 1000 links
  $ for i in {1..2000}; do rm vol/$i; done;
  $ for i in {1001..2000}; do link vol/foo vol/$i; done

  # take another one for a send snapshot
  $ btrfs subvolume snapshot -r vol snap2

  $ mkdir receive_dir
  $ btrfs send snap2 -p snap1 | btrfs receive receive_dir/
  At subvol snap2
  link 1238 -> foo
  ERROR: link 1238 -> foo failed: File exists

In this case, we will have the same file paths added to both lists. In the
parent snapshot, reference paths {1..1237} are stored in inode references,
but reference paths {1238..2000} are stored in inode extended references.
In the send snapshot, all reference paths {1001..2000} are stored in inode
references. During the incremental send, we process their inode references
first. In record_changed_ref(), we iterate all its inode references in the
send/parent snapshot. For every inode reference, we also use find_iref() to
check whether the same file path also appears in the parent/send snapshot
or not. Inode references {1238..2000} which appear in the send snapshot but
not in the parent snapshot are added to the "new_refs" list. On the other
hand, Inode references {1..1000} which appear in the parent snapshot but
not in the send snapshot are added to the "deleted_refs" list. Next, when
we process their inode extended references, reference paths {1238..2000}
are added to the "deleted_refs" list because all of them only appear in the
parent snapshot. Now two lists contain items as below:
"new_refs" list: {1238..2000}
"deleted_refs" list: {1..1000}, {1238..2000}

Reference paths {1238..2000} appear in both lists. And as the processing
order mentioned about before, the receiver gets an -EEXIST error when trying
the link operations.

To fix the bug, the idea is to process the "deleted_refs" list before
the "new_refs" list. However, it's not easy to reshuffle the processing
order. For one reason, if we do so, we may unlink all the existing paths
first, there's no valid path anymore for links. And it's inefficient
because we do a bunch of unlinks followed by links for the same paths.
Moreover, it makes less sense to have duplications in both lists. A
reference path cannot not only be regarded as new but also has been seen in
the past, or we won't call it a new path. However, it's also not a good
idea to make find_iref() check a reference against all inode references
and all inode extended references because it may result in large disk
reads.

So we introduce two rbtrees to make the references easier for lookups.
And we also introduce record_new_ref_if_needed() and
record_deleted_ref_if_needed() for changed_ref() to check and remove
duplicated references early.
Reviewed-by: Robbie Ko <robbieko@synology.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: BingJing Chang <bingjingc@synology.com>
Signed-off-by: David Sterba <dsterba@suse.com>