btrbk: fix get_related_subvolume_nodes: add all parents and orphaned siblings

Old implementation was missing last readonly parent in chain, as well
as orphaned siblings.

Also sort all by cgen, not by distance, then cgen.

Also skip self.

btrbk

@@ -3031,57 +3031,78 @@ sub get_best_correlated_target($$;@)
 
 sub _push_related_children
 {
-  my $node = shift;
+  my $node = shift; # can be undef, siblings are considered related even if parent is gone
+  my $uuid = shift;
+  my $parent_uuid_hash = shift;
   my $related = shift;
   my $prune = shift;
-  my $distance = shift // 0;
-  my $cgen_ref = shift;
+  my $distance = shift;
 
   if($distance >= 256) {
     WARN "Maximum distance reached, aborting related subvolume search";
     return;
   }
-  return if(defined($prune) && ($node->{id} == $prune->{id}));
-
-  my $children = $node->{TREE_ROOT}{PARENT_UUID_HASH}->{$node->{uuid}} // [];
-  my @readonly = grep { $_->{readonly} } @$children;
-  TRACE "related_nodes: add " . scalar(@readonly) . " readonly children of uuid=$node->{uuid} (distance=$distance)" if(scalar(@readonly));
-
-  # sort by absolute cgen delta, favor older
-  push @$related, sort { (abs($cgen_ref - $a->{cgen}) <=> abs($cgen_ref - $b->{cgen})) ||
-                         ($a->{cgen} <=> $b->{cgen})
-                       } @readonly;
-
-  # recurse into all child subvolumes
-  foreach(@$children) {
-    _push_related_children($_, $related, $prune, $distance + 1, $cgen_ref);
+  if($node) {
+    if($distance == 0) {
+      # hacky, we want to skip ourself, and $node=$vol->{node} if distance=0
+      TRACE "related_nodes: d=$distance uuid=$uuid : related self: " . _fs_path($node);
+    } elsif($node->{readonly}) {
+      TRACE "related_nodes: d=$distance uuid=$uuid : push related readonly: " . _fs_path($node);
+      push @$related, $node;
+    } else {
+      TRACE "related_nodes: d=$distance uuid=$uuid : related not readonly: " . _fs_path($node);
+    }
+  } else {
+    TRACE "related_nodes: d=$distance uuid=$uuid : missing (deleted)";
   }
+
+  # recurse into all child subvolumes (even if parent is missing -> siblings)
+  my $children = $parent_uuid_hash->{$uuid} // [];
+  foreach(@$children) {
+    if(defined($prune) && ($_->{id} == $prune->{id})) {
+      TRACE "related_nodes: d=$distance uuid=$uuid : pruning processed uuid=$_->{uuid}";
+      next;
+    }
+    TRACE "related_nodes: d=$distance uuid=$uuid : processing child uuid=$_->{uuid}";
+    _push_related_children($_, $_->{uuid}, $parent_uuid_hash, $related, $prune, $distance + 1);
+  }
+  TRACE "related_nodes: d=$distance uuid=$uuid : processed " . (scalar @$children) . " children";
 }
 
 
-# returns subvolume nodes related to $vol (by parent_uuid relationship),
-# sorted by parent/child distance and cgen delta.
+# returns all related readonly nodes (by parent_uuid relationship),
+# sort by absolute cgen delta, favor older
 sub get_related_subvolume_nodes($)
 {
   my $vol = shift // die;
-  my $cgen_ref = $vol->{node}{readonly} ? $vol->{node}{cgen} : $vol->{node}{gen};
   TRACE "related_nodes: resolving related subvolumes of: $vol->{PATH}";
 
   # iterate parent chain
   my @related_nodes;
   my $uuid_hash = $vol->{node}{TREE_ROOT}{UUID_HASH};
-  my $parent_it = $vol->{node};
-  my $last_parent;
+  my $parent_uuid_hash = $vol->{node}{TREE_ROOT}{PARENT_UUID_HASH};
+  my $node = $vol->{node};
+  my $uuid = $node->{uuid};
+  my $last_node;
   my $distance = 0;
-  while($parent_it && ($distance <= 256)) {
-    _push_related_children($parent_it, \@related_nodes, $last_parent, $distance + 1, $cgen_ref);
-    $last_parent = $parent_it;
-    $parent_it = $uuid_hash->{$parent_it->{parent_uuid}};
+  while($distance < 256) {
+    _push_related_children($node, $uuid, $parent_uuid_hash, \@related_nodes, $last_node, $distance);
+    last unless $node;
+    $uuid = $node->{parent_uuid};
+    if($uuid eq "-") {
+      TRACE "related_nodes: d=$distance uuid=$node->{uuid} : no parent_uuid";
+      last;
+    }
+    $last_node = $node;
+    $node = $uuid_hash->{$uuid};
+    TRACE "related_nodes: d=$distance uuid=$last_node->{uuid} : processing parent uuid=$uuid";
     $distance++;
-    TRACE "related_nodes: found parent uuid=$parent_it->{uuid} (distance=$distance)" if($parent_it);
   }
+  WARN "Maximum distance reached, related subvolume search aborted" if($distance >= 256);
   TRACE "related_nodes: found total=" . scalar(@related_nodes) . " related readonly subvolumes";
-  return \@related_nodes;
+  my @sorted = sort { (abs($cgen_ref - $a->{cgen}) <=> abs($cgen_ref - $b->{cgen})) ||
+                      ($a->{cgen} <=> $b->{cgen}) } @related_nodes;
+  return \@sorted;
 }
 
 
@@ -3116,7 +3137,6 @@ sub get_best_parent($$$;@)
   # filter candidates
   my @candidate;  # candidates for parent, ordered by "best suited"
   foreach (@$all_related_nodes) {
-    next if($_->{id} == $svol->{node}{id}); # skip self
     my $vinfo = vinfo_resolved($_, $resolve_sroot);
     if((not $vinfo) && $source_fallback_all_mountpoints) { # related node is not under $resolve_sroot
       $vinfo = vinfo_resolved_all_mountpoints($_, $svol);