btrbk: bugfix: do not read target btrfs tree on "btrbk snapshot --preserve"

If running "btrbk snapshot --preserve", there is no need to initialize
targets, and we don't want to warn and fail (exitcode=10) on missing
targets.

ChangeLog

@@ -8,6 +8,8 @@ btrbk-0.27.0-dev
     (close #217). Note that this change may result in (previously
     preserved) backups to be deleted!
   * Bugfix: add "btrfs filesystem usage" backend mapping (close #222).
+  * Bugfix: do not fail (exitcode=10) if targets are not accessible on
+    "btrfs snapshot --preserve".
   * Enhance internal data structures:
     - Allow snapshot_dir to be a mountpoint.
     - Search complete target tree for correlated subvolumes.

btrbk

@@ -5313,6 +5313,12 @@ MAIN:
   }
 
   # read target btrfs tree
+  if($action_run && $skip_backups && $preserve_snapshots && $preserve_backups) {
+    # if running "btrbk snapshot --preserve", there is no need to
+    # initialize targets, and we don't want to fail on missing targets.
+    DEBUG "Skipping target tree readin (preserving all snapshots and backups)";
+  }
+  else {
     foreach my $sroot (vinfo_subsection($config, 'volume')) {
       foreach my $svol (vinfo_subsection($sroot, 'subvolume')) {
         foreach my $droot (vinfo_subsection($svol, 'target')) {
@@ -5342,6 +5348,7 @@ MAIN:
         }
       }
     }
+  }
 
   # check for duplicate snapshot locations
   my %snapshot_check;