Implement "synology" compat option for Synology NAS

By default, the Synology DSM "btrfs send" command outputs ACL attributes that are not compatible
with a normal Btrfs implementation. The option compat "synology" allows passing the --without-syno-features
option, which makes "btrfs send" compatible with standard Btrfs implementations.

ChangeLog

@@ -1,3 +1,7 @@
+ unreleased
+
+  * Add "synology" compat option for Synology NAS.
+
 btrbk-0.32.6
 
   * Fix backup of unrelated (by parent_uuid) snapshots (close #339).

btrbk

@@ -142,9 +142,9 @@ my %config_options = (
   backend_remote              => { default => undef,         accept => [qw( no btrfs-progs btrfs-progs-btrbk btrfs-progs-sudo btrfs-progs-doas )] },
   backend_local_user          => { default => undef,         accept => [qw( no btrfs-progs btrfs-progs-btrbk btrfs-progs-sudo btrfs-progs-doas )] },
 
-  compat                      => { default => undef,         accept => [qw( no busybox ignore_receive_errors )], split => 1 },
+  compat                      => { default => undef,         accept => [qw( no busybox ignore_receive_errors synology )], split => 1 },
-  compat_local                => { default => undef,         accept => [qw( no busybox ignore_receive_errors )], split => 1 },
+  compat_local                => { default => undef,         accept => [qw( no busybox ignore_receive_errors synology )], split => 1 },
-  compat_remote               => { default => undef,         accept => [qw( no busybox ignore_receive_errors )], split => 1 },
+  compat_remote               => { default => undef,         accept => [qw( no busybox ignore_receive_errors synology )], split => 1 },
   safe_commands               => { default => undef,         accept => [qw( yes no )], context => [qw( global )] },
   btrfs_commit_delete         => { default => undef,         accept => [qw( yes no after each )],
                                                              deprecated => { MATCH => { regex => qr/^(?:after|each)$/, warn => 'Please use "btrfs_commit_delete yes|no"', replace_key => "btrfs_commit_delete", replace_value => "yes" } } },
@@ -1461,11 +1461,13 @@ sub _btrfs_send_options($$;$$)
   my $clone_src = shift // [];
   my $send_protocol = config_key($target, "send_protocol");
   my $send_compressed_data = config_key($target, "send_compressed_data");
+  my $compat_synology = config_key_lru($target, "compat", "synology");
   my @send_options;
   push(@send_options, '-p', { unsafe => $parent->{PATH} } ) if($parent);
   push(@send_options, '-c', { unsafe => $_ } ) foreach(map { $_->{PATH} } @$clone_src);
   push(@send_options, '--proto', $send_protocol ) if($send_protocol);
   push(@send_options, '--compressed-data' ) if($send_compressed_data);
+  push(@send_options, '--without-syno-features' ) if($compat_synology);
   #push(@send_options, '-v') if($loglevel >= 3);
   return \@send_options;
 }

btrbk.conf.example

@@ -202,3 +202,18 @@ volume ssh://my-remote-host.com/mnt/btr_pool
   snapshot_preserve_min  all
   subvolume home
     target /mnt/btr_backup/my-remote-host.com
+
+# Archive all shared folders from a Synology NAS btrfs.
+# Needs an admin account with SSH key login.
+volume ssh://my-synology-host.com/volume1
+  subvolume *
+  compat synology
+
+  snapshot_dir           btrbk_snapshots
+  snapshot_create        ondemand
+  snapshot_preserve_min  latest
+  snapshot_preserve      no
+
+  target_preserve_min    6m
+  target_preserve        10d 10w 12m
+  target /mnt/btr_backup/my-synology-host.com

doc/btrbk.conf.5.asciidoc

@@ -417,6 +417,12 @@ with "ERROR: attribute 12 requested but not present".
 Note that there is *no guarantee that backups created with this
 option enabled can be restored at all*.
 
+*synology*::
+    Useful when transfering snapshots from Synology DSM NAS systems, that
+    are normally not compatible with standard Btrfs systems and sending
+    would cause a lot of ACL errors. When this is enabled, the option
+    `--without-syno-features` is passed to `btrfs send`.
+
 If you want to set this option for local or remote hosts only, set
 *compat_local* or *compat_remote* (e.g. "compat_remote busybox").
 --