btrbk: disallow unrelated (by parent_uuid) parents for btrfs-send if incremental=strict

Makes sure that if, for whatever reason, a subvolume having correct
btrbk name scheme does NOT share any extents with previous snapshots
is never used as parent.

Note that if a related parent is found, the unrelated closest
older/newer (by btrbk timestamp) subvolumes are still added as clone
sources.

btrbk

@@ -3107,6 +3107,7 @@ sub get_best_parent($$$;@)
   my $ret_clone_src          = $opts{clone_src};
   my $ret_clone_src_extra    = $opts{clone_src_extra};
   my $ret_target_parent_node = $opts{target_parent_node};
+  my $strict_related         = $opts{strict_related};
 
   TRACE "get_best_parent: resolving best common parent for subvolume: $svol->{PRINT}  (droot=$droot->{PRINT})";
 
@@ -3229,6 +3230,12 @@ sub get_best_parent($$$;@)
     DEBUG("No common parents of \"$svol->{PRINT}\" found in src=\"$resolve_sroot->{PRINT}/\", target=\"$resolve_droot->{PRINT}/\"");
     return undef;
   }
+  if($strict_related && (not scalar(@c_related))) {
+    # all parents come from c_snapdir (btrbk_direct_leaf), no relations by parent_uuid found
+    WARN "No related common parent found (by parent_uuid relationship) for: $svol->{PRINT}";
+    WARN "Hint: setting option \"incremental\" to \"yes\" (instead of \"strict\") will use parent: " . join(", ", map { $_->[0]{PRINT} } @parent);
+    return undef;
+  }
 
   my @extra_clones;
   foreach my $cc (@c_related_older, @c_related_newer, grep { not exists($c_rel_id{$_->[0]{node}{id}}) } (@c_snapdir_older, @c_snapdir_newer)) {
@@ -4023,6 +4030,7 @@ sub macro_archive_target($$$;$)
   {
     my ($clone_src, $clone_src_extra, $target_parent_node);
     my $parent = get_best_parent($svol, $sroot, $droot,
+                                 strict_related     => 0,
                                  clone_src          => \$clone_src,
                                  clone_src_extra    => \$clone_src_extra,
                                  target_parent_node => \$target_parent_node);
@@ -6157,6 +6165,7 @@ MAIN:
                 INFO "Creating subvolume backup (send-receive) for: $child->{PRINT}";
                 my ($clone_src, $clone_src_extra, $target_parent_node);
                 my $parent = get_best_parent($child, $snaproot, $droot,
+                                             strict_related     => ((config_key($droot, "incremental") // "") eq "strict"),
                                              clone_src          => \$clone_src,
                                              clone_src_extra    => \$clone_src_extra,
                                              target_parent_node => \$target_parent_node);

doc/btrbk.conf.5.asciidoc

@@ -150,9 +150,20 @@ Note that using ``long-iso'' has implications on the scheduling, see
     snapshot creation). Defaults to ``always''.
 
 *incremental* yes|no|strict::
+
     If set, incremental backups are created. If set to ``strict'',
-    non-incremental (initial) backups are never created. Defaults to
+    non-incremental (initial) backups are never created, and
-    ``yes''.
+    incremental backups are restricted to 'related parents' (by
+    parent-uuid relationship). Defaults to ``yes''.
++
+--
+Note that even if the parent-uuid chain is broken, snapshots and
+backups can still share data (which is especially true for backups
+created with 'incremental' option enabled), and are perfectly suitable
+as parents for incremental send-receive operations. But as btrbk can
+not be certain about this, such operations are disallowed in
+"incremental strict" mode.
+--
 
 === Grouping Options