Adhere to new root find behavior

Between versions 4.6.1 and 4.10.2 the output emitted in case 'btrfs subvolume show' is executed on the root volume changed again. We no longer see a special string but the normal subvolume output. An additional complication is that sometimes this output contains the special name '<FS_TREE>' and sometimes it does not. All that causes our find-root-directory algorithm to no longer work. This change adjusts the program to correctly handle this new behavior. It does so by using the 'btrfs filesystem show' command instead of the subvolume related counterpart. Note that we also drop compatibility with btrfs-progs versions earlier than 3.17.3 which exhibited yet another output.

Author: d-e-s-o

btrfs-backup/src/deso/btrfs/command.py

@@ -1,7 +1,7 @@
 # command.py
 
 #/***************************************************************************
-# * Copyright (C) 2015 Daniel Mueller (deso@posteo.net)  *
+# * Copyright (C) 2015,2017 Daniel Mueller (deso@posteo.net) *
 # * *
 # * This program is free software: you can redistribute it and/or modify *
 # * it under the terms of the GNU General Public License as published by *
@@ -123,3 +123,8 @@ def diff(subvolume, generation):
  previous generation ID, determines the files that have been changed.
  """
  return [_BTRFS, "subvolume", "find-new", subvolume, generation]
+
+
+def showFilesystem(filesystem):
+ """Retrieve the command to show information about a btrfs file system."""
+ return [_BTRFS, "filesystem", "show", filesystem]

btrfs-backup/src/deso/btrfs/repository.py

@@ -1,7 +1,7 @@
 # repository.py
 
 #/***************************************************************************
-# * Copyright (C) 2015-2016 Daniel Mueller (deso@posteo.net) *
+# * Copyright (C) 2015-2017 Daniel Mueller (deso@posteo.net) *
 # * *
 # * This program is free software: you can redistribute it and/or modify *
 # * it under the terms of the GNU General Public License as published by *
@@ -39,6 +39,7 @@
  deserialize,
  diff,
  show,
+ showFilesystem,
  serialize,
  snapshot as mkSnapshot,
  snapshots as listSnapshots,
@@ -94,13 +95,9 @@
 # ID A gen B top level C path PATH
 _LIST_STRING = r"^ID {nums} gen ({nums}) top level {nums} path ({path})$"
 _LIST_REGEX = regex(_LIST_STRING.format(nums=_NUMS_STRING, path=_PATH_STRING))
-# The first line in the output generated by executing the command
-# returned by the show() should contain the subvolume path if the
-# given directory is a subvolume. However, if it is the root of the
-# btrfs file system then it will end in 'is btrfs root'. We need to
-# detect this case to determine the btrfs root.
-_SHOW_IS_ROOT = "is toplevel subvolume"
-_SHOW_SUBVOL_NAME_REGEX = regex(r"\s*Name:\s*(.*)")
+_SHOW_SUBVOL_NAME = r"\s*Name:\s*(%s)"
+_SHOW_SUBVOL_NAME_REGEX = regex(_SHOW_SUBVOL_NAME % ".*")
+_SHOW_SUBVOL_ROOT_REGEX = regex(_SHOW_SUBVOL_NAME % "<FS_TREE>")
 # The marker ending the file list reported by the diff() function. If
 # this marker is the only thing reported then no files have changed.
 _DIFF_END_MARKER = "transid marker"
@@ -238,18 +235,10 @@ def _doShow(directory, repository):
 def _isRoot(directory, repository):
  """Check if a given directory represents the root of a btrfs file system."""
  try:
- output = _doShow(directory, repository)
-
- # The output of show() contains multiple lines in case the given
- # directory is a subvolume. In case it is an ordinary directory the
- # output is a single line and begins with "ERROR:" (but the command
- # actually succeeds), and in case of the root directory it will be
- # matched here.
- return len(output) == 1 and output[0].endswith(_SHOW_IS_ROOT)
+ cmd = repository.command(showFilesystem, directory)
+ execute(*cmd, stdout=b"", stderr=repository.stderr)
+ return True
  except ProcessError:
- # Starting with btrfs-progs-3.17.3 the show command will return an
- # error code in case the supplied directory is not a btrfs
- # subvolume which will manifest as an exception here.
  return False
 
 
@@ -264,7 +253,9 @@ def _isSubvolume(directory, repository):
  for line in output:
  # We are looking for a line containing the subvolume's name.
  m = _SHOW_SUBVOL_NAME_REGEX.match(line)
- if m is not None:
+ # In case of the root we need special treatment. It must not be
+ # reported as a normal subvolume.
+ if m is not None and not _isRoot(directory, repository):
  name, = m.groups()
  return True, name