Selectively shallow merge certain mappings or sequences when including configuration files (#672).

NEWS

@@ -7,6 +7,9 @@
    "match_archives" configuration option for the "transfer", "list", "rlist", and "info" actions.
  * #668: Fix error when running the "prune" action with both "archive_name_format" and "prefix"
    options set.
+ * #672: Selectively shallow merge certain mappings or sequences when including configuration files.
+   See the documentation for more information:
+   https://torsion.org/borgmatic/docs/how-to/make-per-application-backups/#shallow-merge
  * #673: View the results of configuration file merging via "validate-borgmatic-config --show" flag.
    See the documentation for more information:
    https://torsion.org/borgmatic/docs/how-to/make-per-application-backups/#debugging-includes

borgmatic/config/load.py

@@ -38,6 +38,24 @@ def include_configuration(loader, filename_node, include_directory):
     return load_configuration(include_filename)
 
 
+def retain_node_error(loader, node):
+    '''
+    Given a ruamel.yaml.loader.Loader and a YAML node, raise an error.
+
+    Raise ValueError if a mapping or sequence node is given, as that indicates that "!retain" was
+    used in a configuration file without a merge. In configuration files with a merge, mapping and
+    sequence nodes with "!retain" tags are handled by deep_merge_nodes() below.
+
+    Also raise ValueError if a scalar node is given, as "!retain" is not supported on scalar nodes.
+    '''
+    if isinstance(node, (ruamel.yaml.nodes.MappingNode, ruamel.yaml.nodes.SequenceNode)):
+        raise ValueError(
+            'The !retain tag may only be used within a configuration file containing a merged !include tag.'
+        )
+
+    raise ValueError('The !retain tag may only be used on a YAML mapping or sequence.')
+
+
 class Include_constructor(ruamel.yaml.SafeConstructor):
     '''
     A YAML "constructor" (a ruamel.yaml concept) that supports a custom "!include" tag for including
@@ -50,6 +68,7 @@ class Include_constructor(ruamel.yaml.SafeConstructor):
             '!include',
             functools.partial(include_configuration, include_directory=include_directory),
         )
+        self.add_constructor('!retain', retain_node_error)
 
     def flatten_mapping(self, node):
         '''
@@ -176,6 +195,8 @@ def deep_merge_nodes(nodes):
             ),
         ]
 
+    If a mapping or sequence node has a YAML "!retain" tag, then that node is not merged.
+
     The purpose of deep merging like this is to support, for instance, merging one borgmatic
     configuration file into another for reuse, such that a configuration section ("retention",
     etc.) does not completely replace the corresponding section in a merged file.
@@ -198,32 +219,42 @@ def deep_merge_nodes(nodes):
 
                 # If we're dealing with MappingNodes, recurse and merge its values as well.
                 if isinstance(b_value, ruamel.yaml.nodes.MappingNode):
-                    replaced_nodes[(b_key, b_value)] = (
-                        b_key,
-                        ruamel.yaml.nodes.MappingNode(
-                            tag=b_value.tag,
-                            value=deep_merge_nodes(a_value.value + b_value.value),
-                            start_mark=b_value.start_mark,
-                            end_mark=b_value.end_mark,
-                            flow_style=b_value.flow_style,
-                            comment=b_value.comment,
-                            anchor=b_value.anchor,
-                        ),
-                    )
+                    # A "!retain" tag says to skip deep merging for this node. Replace the tag so
+                    # downstream schema validation doesn't break on our application-specific tag.
+                    if b_value.tag == '!retain':
+                        b_value.tag = 'tag:yaml.org,2002:map'
+                    else:
+                        replaced_nodes[(b_key, b_value)] = (
+                            b_key,
+                            ruamel.yaml.nodes.MappingNode(
+                                tag=b_value.tag,
+                                value=deep_merge_nodes(a_value.value + b_value.value),
+                                start_mark=b_value.start_mark,
+                                end_mark=b_value.end_mark,
+                                flow_style=b_value.flow_style,
+                                comment=b_value.comment,
+                                anchor=b_value.anchor,
+                            ),
+                        )
                 # If we're dealing with SequenceNodes, merge by appending one sequence to the other.
                 elif isinstance(b_value, ruamel.yaml.nodes.SequenceNode):
-                    replaced_nodes[(b_key, b_value)] = (
-                        b_key,
-                        ruamel.yaml.nodes.SequenceNode(
-                            tag=b_value.tag,
-                            value=a_value.value + b_value.value,
-                            start_mark=b_value.start_mark,
-                            end_mark=b_value.end_mark,
-                            flow_style=b_value.flow_style,
-                            comment=b_value.comment,
-                            anchor=b_value.anchor,
-                        ),
-                    )
+                    # A "!retain" tag says to skip deep merging for this node. Replace the tag so
+                    # downstream schema validation doesn't break on our application-specific tag.
+                    if b_value.tag == '!retain':
+                        b_value.tag = 'tag:yaml.org,2002:seq'
+                    else:
+                        replaced_nodes[(b_key, b_value)] = (
+                            b_key,
+                            ruamel.yaml.nodes.SequenceNode(
+                                tag=b_value.tag,
+                                value=a_value.value + b_value.value,
+                                start_mark=b_value.start_mark,
+                                end_mark=b_value.end_mark,
+                                flow_style=b_value.flow_style,
+                                comment=b_value.comment,
+                                anchor=b_value.anchor,
+                            ),
+                        )
 
     return [
         replaced_nodes.get(node, node) for node in nodes if replaced_nodes.get(node) != DELETED_NODE

docs/how-to/make-per-application-backups.md

@@ -276,6 +276,65 @@ include, the local file's option takes precedence.
 list values are appended together.
 
 
+### Shallow merge
+
+Even though deep merging is generally pretty handy for included files,
+sometimes you want specific sections in the local file to take precedence over
+included sections—without any merging occuring for them.
+
+<span class="minilink minilink-addedin">New in version 1.7.12</span> That's
+where the `!retain` tag comes in. Whenever you're merging an included file
+into your configuration file, you can optionally add the `!retain` tag to
+particular local mappings or sequences to retain the local values and ignore
+included values.
+
+For instance, start with this configuration file containing the `!retain` tag
+on the `retention` mapping:
+
+```yaml
+<<: !include /etc/borgmatic/common.yaml
+
+location:
+   repositories:
+     - repo.borg
+
+retention: !retain
+    keep_daily: 5
+```
+
+And `common.yaml` like this:
+
+```yaml
+location:
+   repositories:
+     - common.borg
+
+retention:
+    keep_hourly: 24
+    keep_daily: 7
+```
+
+Once this include gets merged in, the resulting configuration will have a
+`keep_daily` value of `5` and nothing else in the `retention` section. That's
+because the `!retain` tag says to retain the local version of `retention` and
+ignore any values coming in from the include. But because the `repositories`
+sequence doesn't have a `!retain` tag, that sequence still gets merged
+together to contain both `common.borg` and `repo.borg`.
+
+The `!retain` tag can only be placed on mapping and sequence nodes, and it
+goes right after the name of the option (and its colon) on the same line. The
+effects of `!retain` are recursive, meaning that if you place a `!retain` tag
+on a top-level mapping, even deeply nested values within it will not be
+merged.
+
+Additionally, the `!retain` tag only works in a configuration file that also
+performs a merge include with `<<: !include`. It doesn't make sense within,
+for instance, an included configuration file itself (unless it in turn
+performs its own merge include). That's because `!retain` only applies to the
+file doing the include; it doesn't work in reverse or propagate through
+includes.
+
+
 ## Debugging includes
 
 <span class="minilink minilink-addedin">New in version 1.7.12</span> If you'd

tests/integration/config/test_load.py

@@ -2,7 +2,6 @@ import io
 import sys
 
 import pytest
-import ruamel.yaml
 from flexmock import flexmock
 
 from borgmatic.config import load as module
@@ -150,6 +149,99 @@ def test_load_configuration_merges_include():
     assert module.load_configuration('config.yaml') == {'foo': 'override', 'baz': 'quux'}
 
 
+def test_load_configuration_with_retain_tag_merges_include_but_keeps_local_values():
+    builtins = flexmock(sys.modules['builtins'])
+    flexmock(module.os).should_receive('getcwd').and_return('/tmp')
+    flexmock(module.os.path).should_receive('isabs').and_return(False)
+    flexmock(module.os.path).should_receive('exists').and_return(True)
+    include_file = io.StringIO(
+        '''
+        stuff:
+          foo: bar
+          baz: quux
+
+        other:
+          a: b
+          c: d
+        '''
+    )
+    include_file.name = 'include.yaml'
+    builtins.should_receive('open').with_args('/tmp/include.yaml').and_return(include_file)
+    config_file = io.StringIO(
+        '''
+        stuff: !retain
+          foo: override
+
+        other:
+          a: override
+        <<: !include include.yaml
+        '''
+    )
+    config_file.name = 'config.yaml'
+    builtins.should_receive('open').with_args('config.yaml').and_return(config_file)
+
+    assert module.load_configuration('config.yaml') == {
+        'stuff': {'foo': 'override'},
+        'other': {'a': 'override', 'c': 'd'},
+    }
+
+
+def test_load_configuration_with_retain_tag_but_without_merge_include_raises():
+    builtins = flexmock(sys.modules['builtins'])
+    flexmock(module.os).should_receive('getcwd').and_return('/tmp')
+    flexmock(module.os.path).should_receive('isabs').and_return(False)
+    flexmock(module.os.path).should_receive('exists').and_return(True)
+    include_file = io.StringIO(
+        '''
+        stuff: !retain
+          foo: bar
+          baz: quux
+        '''
+    )
+    include_file.name = 'include.yaml'
+    builtins.should_receive('open').with_args('/tmp/include.yaml').and_return(include_file)
+    config_file = io.StringIO(
+        '''
+        stuff:
+          foo: override
+        <<: !include include.yaml
+        '''
+    )
+    config_file.name = 'config.yaml'
+    builtins.should_receive('open').with_args('config.yaml').and_return(config_file)
+
+    with pytest.raises(ValueError):
+        assert module.load_configuration('config.yaml')
+
+
+def test_load_configuration_with_retain_tag_on_scalar_raises():
+    builtins = flexmock(sys.modules['builtins'])
+    flexmock(module.os).should_receive('getcwd').and_return('/tmp')
+    flexmock(module.os.path).should_receive('isabs').and_return(False)
+    flexmock(module.os.path).should_receive('exists').and_return(True)
+    include_file = io.StringIO(
+        '''
+        stuff:
+          foo: bar
+          baz: quux
+        '''
+    )
+    include_file.name = 'include.yaml'
+    builtins.should_receive('open').with_args('/tmp/include.yaml').and_return(include_file)
+    config_file = io.StringIO(
+        '''
+        stuff:
+          foo: !retain override
+        <<: !include include.yaml
+        '''
+    )
+    config_file.name = 'config.yaml'
+    builtins.should_receive('open').with_args('config.yaml').and_return(config_file)
+
+    with pytest.raises(ValueError):
+        assert module.load_configuration('config.yaml')
+
+
 def test_load_configuration_does_not_merge_include_list():
     builtins = flexmock(sys.modules['builtins'])
     flexmock(module.os).should_receive('getcwd').and_return('/tmp')
@@ -173,42 +265,59 @@ def test_load_configuration_does_not_merge_include_list():
     config_file.name = 'config.yaml'
     builtins.should_receive('open').with_args('config.yaml').and_return(config_file)
 
-    with pytest.raises(ruamel.yaml.error.YAMLError):
+    with pytest.raises(module.ruamel.yaml.error.YAMLError):
         assert module.load_configuration('config.yaml')
 
 
+@pytest.mark.parametrize(
+    'node_class',
+    (
+        module.ruamel.yaml.nodes.MappingNode,
+        module.ruamel.yaml.nodes.SequenceNode,
+        module.ruamel.yaml.nodes.ScalarNode,
+    ),
+)
+def test_retain_node_error_raises(node_class):
+    with pytest.raises(ValueError):
+        module.retain_node_error(
+            loader=flexmock(), node=node_class(tag=flexmock(), value=flexmock())
+        )
+
+
 def test_deep_merge_nodes_replaces_colliding_scalar_values():
     node_values = [
         (
-            ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:str', value='retention'),
-            ruamel.yaml.nodes.MappingNode(
+            module.ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:str', value='retention'),
+            module.ruamel.yaml.nodes.MappingNode(
                 tag='tag:yaml.org,2002:map',
                 value=[
                     (
-                        ruamel.yaml.nodes.ScalarNode(
+                        module.ruamel.yaml.nodes.ScalarNode(
                             tag='tag:yaml.org,2002:str', value='keep_hourly'
                         ),
-                        ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:int', value='24'),
+                        module.ruamel.yaml.nodes.ScalarNode(
+                            tag='tag:yaml.org,2002:int', value='24'
+                        ),
                     ),
                     (
-                        ruamel.yaml.nodes.ScalarNode(
+                        module.ruamel.yaml.nodes.ScalarNode(
                             tag='tag:yaml.org,2002:str', value='keep_daily'
                         ),
-                        ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:int', value='7'),
+                        module.ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:int', value='7'),
                     ),
                 ],
             ),
         ),
         (
-            ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:str', value='retention'),
-            ruamel.yaml.nodes.MappingNode(
+            module.ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:str', value='retention'),
+            module.ruamel.yaml.nodes.MappingNode(
                 tag='tag:yaml.org,2002:map',
                 value=[
                     (
-                        ruamel.yaml.nodes.ScalarNode(
+                        module.ruamel.yaml.nodes.ScalarNode(
                             tag='tag:yaml.org,2002:str', value='keep_daily'
                         ),
-                        ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:int', value='5'),
+                        module.ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:int', value='5'),
                     ),
                 ],
             ),
@@ -230,35 +339,39 @@ def test_deep_merge_nodes_replaces_colliding_scalar_values():
 def test_deep_merge_nodes_keeps_non_colliding_scalar_values():
     node_values = [
         (
-            ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:str', value='retention'),
-            ruamel.yaml.nodes.MappingNode(
+            module.ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:str', value='retention'),
+            module.ruamel.yaml.nodes.MappingNode(
                 tag='tag:yaml.org,2002:map',
                 value=[
                     (
-                        ruamel.yaml.nodes.ScalarNode(
+                        module.ruamel.yaml.nodes.ScalarNode(
                             tag='tag:yaml.org,2002:str', value='keep_hourly'
                         ),
-                        ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:int', value='24'),
+                        module.ruamel.yaml.nodes.ScalarNode(
+                            tag='tag:yaml.org,2002:int', value='24'
+                        ),
                     ),
                     (
-                        ruamel.yaml.nodes.ScalarNode(
+                        module.ruamel.yaml.nodes.ScalarNode(
                             tag='tag:yaml.org,2002:str', value='keep_daily'
                         ),
-                        ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:int', value='7'),
+                        module.ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:int', value='7'),
                     ),
                 ],
             ),
         ),
         (
-            ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:str', value='retention'),
-            ruamel.yaml.nodes.MappingNode(
+            module.ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:str', value='retention'),
+            module.ruamel.yaml.nodes.MappingNode(
                 tag='tag:yaml.org,2002:map',
                 value=[
                     (
-                        ruamel.yaml.nodes.ScalarNode(
+                        module.ruamel.yaml.nodes.ScalarNode(
                             tag='tag:yaml.org,2002:str', value='keep_minutely'
                         ),
-                        ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:int', value='10'),
+                        module.ruamel.yaml.nodes.ScalarNode(
+                            tag='tag:yaml.org,2002:int', value='10'
+                        ),
                     ),
                 ],
             ),
@@ -282,28 +395,28 @@ def test_deep_merge_nodes_keeps_non_colliding_scalar_values():
 def test_deep_merge_nodes_keeps_deeply_nested_values():
     node_values = [
         (
-            ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:str', value='storage'),
-            ruamel.yaml.nodes.MappingNode(
+            module.ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:str', value='storage'),
+            module.ruamel.yaml.nodes.MappingNode(
                 tag='tag:yaml.org,2002:map',
                 value=[
                     (
-                        ruamel.yaml.nodes.ScalarNode(
+                        module.ruamel.yaml.nodes.ScalarNode(
                             tag='tag:yaml.org,2002:str', value='lock_wait'
                         ),
-                        ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:int', value='5'),
+                        module.ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:int', value='5'),
                     ),
                     (
-                        ruamel.yaml.nodes.ScalarNode(
+                        module.ruamel.yaml.nodes.ScalarNode(
                             tag='tag:yaml.org,2002:str', value='extra_borg_options'
                         ),
-                        ruamel.yaml.nodes.MappingNode(
+                        module.ruamel.yaml.nodes.MappingNode(
                             tag='tag:yaml.org,2002:map',
                             value=[
                                 (
-                                    ruamel.yaml.nodes.ScalarNode(
+                                    module.ruamel.yaml.nodes.ScalarNode(
                                         tag='tag:yaml.org,2002:str', value='init'
                                     ),
-                                    ruamel.yaml.nodes.ScalarNode(
+                                    module.ruamel.yaml.nodes.ScalarNode(
                                         tag='tag:yaml.org,2002:str', value='--init-option'
                                     ),
                                 ),
@@ -314,22 +427,22 @@ def test_deep_merge_nodes_keeps_deeply_nested_values():
             ),
         ),
         (
-            ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:str', value='storage'),
-            ruamel.yaml.nodes.MappingNode(
+            module.ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:str', value='storage'),
+            module.ruamel.yaml.nodes.MappingNode(
                 tag='tag:yaml.org,2002:map',
                 value=[
                     (
-                        ruamel.yaml.nodes.ScalarNode(
+                        module.ruamel.yaml.nodes.ScalarNode(
                             tag='tag:yaml.org,2002:str', value='extra_borg_options'
                         ),
-                        ruamel.yaml.nodes.MappingNode(
+                        module.ruamel.yaml.nodes.MappingNode(
                             tag='tag:yaml.org,2002:map',
                             value=[
                                 (
-                                    ruamel.yaml.nodes.ScalarNode(
+                                    module.ruamel.yaml.nodes.ScalarNode(
                                         tag='tag:yaml.org,2002:str', value='prune'
                                     ),
-                                    ruamel.yaml.nodes.ScalarNode(
+                                    module.ruamel.yaml.nodes.ScalarNode(
                                         tag='tag:yaml.org,2002:str', value='--prune-option'
                                     ),
                                 ),
@@ -361,32 +474,32 @@ def test_deep_merge_nodes_keeps_deeply_nested_values():
 def test_deep_merge_nodes_appends_colliding_sequence_values():
     node_values = [
         (
-            ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:str', value='hooks'),
-            ruamel.yaml.nodes.MappingNode(
+            module.ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:str', value='hooks'),
+            module.ruamel.yaml.nodes.MappingNode(
                 tag='tag:yaml.org,2002:map',
                 value=[
                     (
-                        ruamel.yaml.nodes.ScalarNode(
+                        module.ruamel.yaml.nodes.ScalarNode(
                             tag='tag:yaml.org,2002:str', value='before_backup'
                         ),
-                        ruamel.yaml.nodes.SequenceNode(
-                            tag='tag:yaml.org,2002:int', value=['echo 1', 'echo 2']
+                        module.ruamel.yaml.nodes.SequenceNode(
+                            tag='tag:yaml.org,2002:seq', value=['echo 1', 'echo 2']
                         ),
                     ),
                 ],
             ),
         ),
         (
-            ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:str', value='hooks'),
-            ruamel.yaml.nodes.MappingNode(
+            module.ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:str', value='hooks'),
+            module.ruamel.yaml.nodes.MappingNode(
                 tag='tag:yaml.org,2002:map',
                 value=[
                     (
-                        ruamel.yaml.nodes.ScalarNode(
+                        module.ruamel.yaml.nodes.ScalarNode(
                             tag='tag:yaml.org,2002:str', value='before_backup'
                         ),
-                        ruamel.yaml.nodes.SequenceNode(
-                            tag='tag:yaml.org,2002:int', value=['echo 3', 'echo 4']
+                        module.ruamel.yaml.nodes.SequenceNode(
+                            tag='tag:yaml.org,2002:seq', value=['echo 3', 'echo 4']
                         ),
                     ),
                 ],
@@ -402,3 +515,101 @@ def test_deep_merge_nodes_appends_colliding_sequence_values():
     assert len(options) == 1
     assert options[0][0].value == 'before_backup'
     assert options[0][1].value == ['echo 1', 'echo 2', 'echo 3', 'echo 4']
+
+
+def test_deep_merge_nodes_keeps_mapping_values_tagged_with_retain():
+    node_values = [
+        (
+            module.ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:str', value='retention'),
+            module.ruamel.yaml.nodes.MappingNode(
+                tag='tag:yaml.org,2002:map',
+                value=[
+                    (
+                        module.ruamel.yaml.nodes.ScalarNode(
+                            tag='tag:yaml.org,2002:str', value='keep_hourly'
+                        ),
+                        module.ruamel.yaml.nodes.ScalarNode(
+                            tag='tag:yaml.org,2002:int', value='24'
+                        ),
+                    ),
+                    (
+                        module.ruamel.yaml.nodes.ScalarNode(
+                            tag='tag:yaml.org,2002:str', value='keep_daily'
+                        ),
+                        module.ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:int', value='7'),
+                    ),
+                ],
+            ),
+        ),
+        (
+            module.ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:str', value='retention'),
+            module.ruamel.yaml.nodes.MappingNode(
+                tag='!retain',
+                value=[
+                    (
+                        module.ruamel.yaml.nodes.ScalarNode(
+                            tag='tag:yaml.org,2002:str', value='keep_daily'
+                        ),
+                        module.ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:int', value='5'),
+                    ),
+                ],
+            ),
+        ),
+    ]
+
+    result = module.deep_merge_nodes(node_values)
+    assert len(result) == 1
+    (section_key, section_value) = result[0]
+    assert section_key.value == 'retention'
+    assert section_value.tag == 'tag:yaml.org,2002:map'
+    options = section_value.value
+    assert len(options) == 1
+    assert options[0][0].value == 'keep_daily'
+    assert options[0][1].value == '5'
+
+
+def test_deep_merge_nodes_keeps_sequence_values_tagged_with_retain():
+    node_values = [
+        (
+            module.ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:str', value='hooks'),
+            module.ruamel.yaml.nodes.MappingNode(
+                tag='tag:yaml.org,2002:map',
+                value=[
+                    (
+                        module.ruamel.yaml.nodes.ScalarNode(
+                            tag='tag:yaml.org,2002:str', value='before_backup'
+                        ),
+                        module.ruamel.yaml.nodes.SequenceNode(
+                            tag='tag:yaml.org,2002:seq', value=['echo 1', 'echo 2']
+                        ),
+                    ),
+                ],
+            ),
+        ),
+        (
+            module.ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:str', value='hooks'),
+            module.ruamel.yaml.nodes.MappingNode(
+                tag='tag:yaml.org,2002:map',
+                value=[
+                    (
+                        module.ruamel.yaml.nodes.ScalarNode(
+                            tag='tag:yaml.org,2002:str', value='before_backup'
+                        ),
+                        module.ruamel.yaml.nodes.SequenceNode(
+                            tag='!retain', value=['echo 3', 'echo 4']
+                        ),
+                    ),
+                ],
+            ),
+        ),
+    ]
+
+    result = module.deep_merge_nodes(node_values)
+    assert len(result) == 1
+    (section_key, section_value) = result[0]
+    assert section_key.value == 'hooks'
+    options = section_value.value
+    assert len(options) == 1
+    assert options[0][0].value == 'before_backup'
+    assert options[0][1].tag == 'tag:yaml.org,2002:seq'
+    assert options[0][1].value == ['echo 3', 'echo 4']