btrbk: add options "{snapshot,target,archive}_qgroup_destroy": destroy qgroups whenever a subvolume is deleted

Btrfs does not destroy qgroups when subvolumes are deleted (see
https://bugzilla.kernel.org/show_bug.cgi?id=91751). As a workaround
for this, btrbk can be configured to always destroy the corresponding
default qgroup "0/<subvol-id>" whenever a subvolume (snapshot, backup
or archive) is deleted.

Added configuration options:
 - snapshot_qgroup_destroy
 - target_qgroup_destroy
 - archive_qgroup_destroy

ChangeLog

@@ -12,6 +12,8 @@ btrbk-current
   * Change raw backup format (sidecar file instead of uuid in file).
   * Honor target_preserve for raw targets (delete raw targets).
   * Add symmetric encryption for raw targets (close #157).
+  * Add "{snapshot,target,archive}_qgroup_destroy" configuration
+    options (close #49, #189).
   * Do not run in "perl taint mode" by default: remove "perl -T" in
     hashbang; hardcode $PATH only if taint mode is enabled.
   * Remove "duration" column from transaction_log/transaction_syslog.

btrbk

@@ -124,6 +124,10 @@ my %config_options = (
   backend_local               => { default => undef,         accept => [ "no", "btrfs-progs", "btrfs-progs-btrbk", "btrfs-progs-sudo" ] },
   backend_remote              => { default => undef,         accept => [ "no", "btrfs-progs", "btrfs-progs-btrbk", "btrfs-progs-sudo" ] },
 
+  snapshot_qgroup_destroy     => { default => undef,     accept => [ "yes", "no" ], context => [ "root", "volume", "subvolume" ] },
+  target_qgroup_destroy       => { default => undef,     accept => [ "yes", "no" ] },
+  archive_qgroup_destroy      => { default => undef,     accept => [ "yes", "no" ], context => [ "root" ] },
+
   # deprecated options
   btrfs_progs_compat          => { default => undef, accept => [ "yes", "no" ],
                                    deprecated => { DEFAULT => { ABORT => 1, warn => 'This feature has been dropped in btrbk-v0.23.0. Please update to newest btrfs-progs, AT LEAST >= $BTRFS_PROGS_MIN' } } },
@@ -214,6 +218,7 @@ my %backend_cmd_map = (
                            "btrfs subvolume delete"   => [ "btrfs-subvolume-delete"   ],
                            "btrfs send"               => [ "btrfs-send"               ],
                            "btrfs receive"            => [ "btrfs-receive"            ],
+                           "btrfs qgroup destroy"     => [ "btrfs-qgroup-destroy"     ],
                          },
   "btrfs-progs-sudo" =>  { "btrfs subvolume list"     => [ "sudo", "-n", "btrfs", "subvolume", "list"     ],
                            "btrfs subvolume show"     => [ "sudo", "-n", "btrfs", "subvolume", "show"     ],
@@ -221,6 +226,7 @@ my %backend_cmd_map = (
                            "btrfs subvolume delete"   => [ "sudo", "-n", "btrfs", "subvolume", "delete"   ],
                            "btrfs send"               => [ "sudo", "-n", "btrfs", "send"                  ],
                            "btrfs receive"            => [ "sudo", "-n", "btrfs", "receive"               ],
+                           "btrfs qgroup destroy"     => [ "sudo", "-n", "btrfs", "qgroup", "destroy"     ],
                          },
 );
 
@@ -1280,6 +1286,33 @@ sub btrfs_subvolume_delete($@)
 }
 
 
+sub btrfs_qgroup_destroy($@)
+{
+  my $vol = shift // die;
+  my %opts = @_;
+  my $vol_id = $vol->{node}{id};
+  unless($vol_id) {
+    ERROR "Unknown subvolume_id for: $vol->{PRINT}";
+    return undef;
+  }
+  my $path = $vol->{PATH} // die;
+  my $qgroup_id = "0/$vol_id";
+  INFO "[qgroup-destroy] qgroup_id: $qgroup_id";
+  INFO "[qgroup-destroy] subvolume: $vol->{PRINT}";
+  start_transaction($opts{type} // "qgroup_destroy",
+                    vinfo_prefixed_keys("target", $vol));
+  my $ret = run_cmd(cmd => vinfo_cmd($vol, "btrfs qgroup destroy", $qgroup_id, { unsafe => $path }),
+                    rsh => vinfo_rsh($vol),
+                   );
+  end_transaction($opts{type} // "qgroup_destroy", defined($ret));
+  unless(defined($ret)) {
+    ERROR "Failed to destroy qgroup \"$qgroup_id\" for subvolume: $vol->{PRINT}";
+    return undef;
+  }
+  return $vol;
+}
+
+
 sub btrfs_send_receive($$$$)
 {
   my $snapshot = shift || die;
@@ -3512,6 +3545,11 @@ sub macro_delete($$$$$;@)
     preserve_date_in_future => 1,
    );
 
+  if($delete_options{qgroup}->{destroy}) {
+    # NOTE: we do not abort on qgroup destroy errors
+    btrfs_qgroup_destroy($_, %{$delete_options{qgroup}}) foreach(@$delete);
+  }
+
   my @delete_success = btrfs_subvolume_delete($delete, %delete_options);
   $subvol_dir .= '/' if($subvol_dir ne "");
   INFO "Deleted " . scalar(@delete_success) . " subvolumes in: $root_subvol->{PRINT}/$subvol_dir$subvol_basename.*";
@@ -4605,6 +4643,8 @@ MAIN:
                        },
                        commit => config_key($droot, "btrfs_commit_delete"),
                        type   => "delete_archive",
+                       qgroup => { destroy => config_key($droot, "archive_qgroup_destroy"),
+                                   type => "qgroup_destroy_archive" },
                       );
         }
       }
@@ -5751,6 +5791,8 @@ MAIN:
                                   },
                                   commit => config_key($droot, "btrfs_commit_delete"),
                                   type   => "delete_target",
+                                  qgroup => { destroy => config_key($droot, "target_qgroup_destroy"),
+                                              type => "qgroup_destroy_target" },
                                  ))
               {
                 $target_aborted = 1;
@@ -5780,6 +5822,8 @@ MAIN:
                          },
                          commit => config_key($svol, "btrfs_commit_delete"),
                          type   => "delete_snapshot",
+                         qgroup => { destroy => config_key($svol, "snapshot_qgroup_destroy"),
+                                     type => "qgroup_destroy_snapshot" },
                         );
           }
         }

doc/btrbk.conf.5

@@ -320,6 +320,19 @@ For convenience, it is also possible to set \fIbackend_local\fR or
 local or remote sources/targets (e.g. "backend_remote
 btrfs-progs-btrbk").
 .RE
+.PP
+\fBsnapshot_qgroup_destroy\fR  yes|no  \fI*experimental*\fR
+.PD 0
+.PP
+\fBtarget_qgroup_destroy\fR  yes|no  \fI*experimental*\fR
+.PP
+\fBarchive_qgroup_destroy\fR  yes|no  \fI*experimental*\fR
+.PD
+.RS 4
+Whenever a subvolume is deleted, also destroy corresponding default
+qgroup "0/<subvol-id>". Only useful if you have enabled btrfs quota
+support. See also: <https://bugzilla.kernel.org/show_bug.cgi?id=91751>
+.RE
 .SH RETENTION POLICY
 btrbk uses separate retention policies for snapshots and backups,
 which are defined by the \fIsnapshot_preserve_min\fR,