When deep merging common configuration, merge colliding list values by appending them (#531).

NEWS

@@ -1,8 +1,10 @@
 1.6.1.dev0
  * #528: Improve the error message when a configuration override contains an invalid value.
+ * #531: BREAKING: When deep merging common configuration, merge colliding list values by appending
+   them. Previously, one list replaced the other.
  * #532: When a configuration include is a relative path, load it from either the current working
-   directory or from the directory containing the file doing the including. (Previously, only the
-   working directory was used.)
+   directory or from the directory containing the file doing the including. Previously, only the
+   working directory was used.
  * Add a randomized delay to the sample systemd timer to spread out the load on a server.
  * Add emojis to documentation table of contents to make it easier to find particular how-to and
    reference guides at a glance.

borgmatic/config/load.py

@@ -157,6 +157,20 @@ def deep_merge_nodes(nodes):
                             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,
+                        ),
+                    )
 
     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

@@ -125,9 +125,9 @@ Once this include gets merged in, the resulting configuration would have a
 When there's an option collision between the local file and the merged
 include, the local file's option takes precedence. And as of borgmatic 1.6.0,
 this feature performs a deep merge, meaning that values are merged at all
-levels in the two configuration files. This allows you to include common
-configuration—up to full borgmatic configuration files—while overriding only
-the parts you want to customize.
+levels in the two configuration files. Colliding list values are appended
+together. This allows you to include common configuration—up to full borgmatic
+configuration files—while overriding only the parts you want to customize.
 
 Note that this `<<` include merging syntax is only for merging in mappings
 (configuration options and their values). But if you'd like to include a

tests/integration/config/test_load.py

@@ -326,3 +326,49 @@ def test_deep_merge_nodes_keeps_deeply_nested_values():
     assert nested_options[0][1].value == '--init-option'
     assert nested_options[1][0].value == 'prune'
     assert nested_options[1][1].value == '--prune-option'
+
+
+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(
+                tag='tag:yaml.org,2002:map',
+                value=[
+                    (
+                        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']
+                        ),
+                    ),
+                ],
+            ),
+        ),
+        (
+            ruamel.yaml.nodes.ScalarNode(tag='tag:yaml.org,2002:str', value='hooks'),
+            ruamel.yaml.nodes.MappingNode(
+                tag='tag:yaml.org,2002:map',
+                value=[
+                    (
+                        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']
+                        ),
+                    ),
+                ],
+            ),
+        ),
+    ]
+
+    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].value == ['echo 1', 'echo 2', 'echo 3', 'echo 4']