Fix findmnt command error in the Btrfs hook by switching to parsing JSON output (#954).

NEWS

@@ -1,3 +1,6 @@
+1.9.5.dev0
+ * #954: Fix findmnt command error in the Btrfs hook by switching to parsing JSON output.
+
 1.9.4
  * #80 (beta): Add an LVM hook for snapshotting and backing up LVM logical volumes. See the
    documentation for more information:

borgmatic/hooks/data_source/btrfs.py

@@ -1,5 +1,6 @@
 import collections
 import glob
+import json
 import logging
 import os
 import shutil
@@ -26,13 +27,21 @@ def get_filesystem_mount_points(findmnt_command):
     findmnt_output = borgmatic.execute.execute_command_and_capture_output(
         tuple(findmnt_command.split(' '))
         + (
-            '-n',  # No headings.
             '-t',  # Filesystem type.
             'btrfs',
+            '--json',
+            '--list',  # Request a flat list instead of a nested subvolume hierarchy.
         )
     )
 
-    return tuple(line.rstrip().split(' ')[0] for line in findmnt_output.splitlines())
+    try:
+        return tuple(
+            filesystem['target'] for filesystem in json.loads(findmnt_output)['filesystems']
+        )
+    except json.JSONDecodeError as error:
+        raise ValueError(f'Invalid {findmnt_command} JSON output: {error}')
+    except KeyError as error:
+        raise ValueError(f'Invalid {findmnt_command} output: Missing key "{error}"')
 
 
 def get_subvolumes_for_filesystem(btrfs_command, filesystem_mount_point):

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "borgmatic"
-version = "1.9.4"
+version = "1.9.5.dev0"
 authors = [
   { name="Dan Helfman", email="witten@torsion.org" },
 ]

tests/end-to-end/commands/fake_findmnt.py

@@ -4,29 +4,38 @@ import sys
 
 def parse_arguments(*unparsed_arguments):
     parser = argparse.ArgumentParser(add_help=False)
-    parser.add_argument('-n', dest='headings', action='store_false', default=True)
     parser.add_argument('-t', dest='type')
+    parser.add_argument('--json', action='store_true')
+    parser.add_argument('--list', action='store_true')
 
     return parser.parse_args(unparsed_arguments)
 
 
-BUILTIN_FILESYSTEM_MOUNT_LINES = (
-    '/mnt/subvolume /dev/loop1 btrfs rw,relatime,ssd,space_cache=v2,subvolid=5,subvol=/',
-)
+BUILTIN_FILESYSTEM_MOUNT_OUTPUT = '''{
+       "filesystems": [
+          {
+             "target": "/mnt/subvolume",
+             "source": "/dev/loop0",
+             "fstype": "btrfs",
+             "options": "rw,relatime,ssd,space_cache=v2,subvolid=5,subvol=/"
+          }
+       ]
+    }
+    '''
 
 
-def print_filesystem_mounts(arguments):
-    for line in BUILTIN_FILESYSTEM_MOUNT_LINES:
-        print(line)
+def print_filesystem_mounts():
+    print(BUILTIN_FILESYSTEM_MOUNT_OUTPUT)
 
 
 def main():
     arguments = parse_arguments(*sys.argv[1:])
 
-    assert not arguments.headings
     assert arguments.type == 'btrfs'
+    assert arguments.json
+    assert arguments.list
 
-    print_filesystem_mounts(arguments)
+    print_filesystem_mounts()
 
 
 if __name__ == '__main__':

tests/unit/hooks/data_source/test_btrfs.py

@@ -1,3 +1,4 @@
+import pytest
 from flexmock import flexmock
 
 from borgmatic.hooks.data_source import btrfs as module
@@ -7,13 +8,46 @@ def test_get_filesystem_mount_points_parses_findmnt_output():
     flexmock(module.borgmatic.execute).should_receive(
         'execute_command_and_capture_output'
     ).and_return(
-        '/mnt0   /dev/loop0 btrfs  rw,relatime,ssd,space_cache=v2,subvolid=5,subvol=/\n'
-        '/mnt1   /dev/loop1 btrfs  rw,relatime,ssd,space_cache=v2,subvolid=5,subvol=/\n'
+        '''{
+           "filesystems": [
+              {
+                 "target": "/mnt0",
+                 "source": "/dev/loop0",
+                 "fstype": "btrfs",
+                 "options": "rw,relatime,ssd,space_cache=v2,subvolid=5,subvol=/"
+              },
+              {
+                 "target": "/mnt1",
+                 "source": "/dev/loop0",
+                 "fstype": "btrfs",
+                 "options": "rw,relatime,ssd,space_cache=v2,subvolid=5,subvol=/"
+              }
+           ]
+        }
+        '''
     )
 
     assert module.get_filesystem_mount_points('findmnt') == ('/mnt0', '/mnt1')
 
 
+def test_get_filesystem_mount_points_with_invalid_findmnt_json_errors():
+    flexmock(module.borgmatic.execute).should_receive(
+        'execute_command_and_capture_output'
+    ).and_return('{')
+
+    with pytest.raises(ValueError):
+        module.get_filesystem_mount_points('findmnt')
+
+
+def test_get_filesystem_mount_points_with_findmnt_json_missing_filesystems_errors():
+    flexmock(module.borgmatic.execute).should_receive(
+        'execute_command_and_capture_output'
+    ).and_return('{"wtf": "something is wrong here"}')
+
+    with pytest.raises(ValueError):
+        module.get_filesystem_mount_points('findmnt')
+
+
 def test_get_subvolumes_for_filesystem_parses_subvolume_list_output():
     flexmock(module.borgmatic.execute).should_receive(
         'execute_command_and_capture_output'