Add config inheritance for personality field

It is often the case that we want to build an image which just applies a
few minor changes to an existing personality.  For instance, the apps we
want for most Spanish images are the same, but we have a few
country-specific applications, and we often wish to build a sub-variant
for a partner or region of a particular country.

Currently we need to copy the full list of apps & settings from our
es.ini personality config into (for instance) es_MX.ini; and then again
from es_MX.ini into (e.g.) es_MX_aguascalientes.ini to add some
region-specific apps or resources

Implement a form of inheritance where (e.g.) building the es_MX
personality reads es.ini and es_MX.ini, in that order; and (e.g.)
building the es_MX_aguascalientes personality reads es.ini, es_MX.ini
and es_MX_aguascalientes.ini, again in that order.  This matches the
convention that we have of naming personalities for the locale, and then
adding further underscore-separated suffixes for subvariants thereof.

This is only implemented for the personality field. This is where we do
most of our customisation and is the place where inheritance would be
most useful.

https://phabricator.endlessm.com/T34731

run-build

@@ -387,6 +387,27 @@ class ImageBuilder(object):
             datetime.datetime.utcnow().strftime('%y%m%d-%H%M%S')
         )
 
+    @staticmethod
+    def _get_prefixes(value):
+        """Yields all prefixes of value split by _, including the whole string.
+
+        >>> list(ImageBuilder._get_prefixes("en_GB_orkney"))
+        ["en", "en_GB", "en_GB_orkney"]
+        """
+        parts = value.split("_")
+        for i in range(1, len(parts) + 1):
+            yield "_".join(parts[:i])
+
+    def _get_attr_values(self, attr):
+        """Yields one or more values for attr. For most properties this is a single
+        value, but for "personality" this yields a series of values to implement
+        inheritance."""
+        value = getattr(self, attr)
+        if attr == "personality":
+            yield from self._get_prefixes(value)
+        else:
+            yield value
+
     def _get_config_paths(self, dir_path, dir_ns):
         config_files = []
 
@@ -398,20 +419,20 @@ class ImageBuilder(object):
         config_attrs = ('product', 'branch', 'arch', 'platform',
                         'personality')
         for attr in config_attrs:
-            path = os.path.join(dir_path, attr,
-                                getattr(self, attr) + '.ini')
-            namespace = dir_ns + '_'.join((attr, getattr(self, attr)))
-            config_files.append((path, namespace))
+            for value in self._get_attr_values(attr):
+                path = os.path.join(dir_path, attr, value + '.ini')
+                namespace = dir_ns + '_'.join((attr, value))
+                config_files.append((path, namespace))
 
         # Add combinations of per-attribute config directories
         for attr1, attr2 in itertools.combinations(config_attrs, 2):
-            ini_name = (getattr(self, attr1) + '-' +
-                        getattr(self, attr2) + '.ini')
-            path = os.path.join(dir_path, attr1 + '-' + attr2, ini_name)
-            namespace = (dir_ns +
-                         '_'.join((attr1, attr2, getattr(self, attr1),
-                                   getattr(self, attr2))))
-            config_files.append((path, namespace))
+            for (value1, value2) in itertools.product(self._get_attr_values(attr1),
+                                                      self._get_attr_values(attr2)):
+                ini_name = (value1 + '-' + value2 + '.ini')
+                path = os.path.join(dir_path, attr1 + '-' + attr2, ini_name)
+                namespace = (dir_ns +
+                             '_'.join((attr1, attr2, value1, value2)))
+                config_files.append((path, namespace))
 
         return config_files
 

tests/test_image_builder.py

@@ -281,6 +281,54 @@ def _run_test():
     _run_test()
 
 
+def test_config_inheritance(make_builder, tmp_path, tmp_builder_paths, caplog):
+    """Test that personality en_GB_orkney also loads settings from en and en_GB."""
+    configdir = tmp_path / 'config'
+    personalitydir = configdir / 'personality'
+    personalitydir.mkdir(parents=True, exist_ok=True)
+
+    en = personalitydir / 'en.ini'
+    en.write_text(dedent("""\
+    [image]
+    language = en_US.utf8
+
+    [flatpak-remote-eos-apps]
+    apps_add =
+      com.endlessm.encyclopedia.en
+      com.endlessm.football.en
+    """))
+
+    en_GB = personalitydir / 'en_GB.ini'
+    en_GB.write_text(dedent("""\
+    [image]
+    language = en_GB.utf8
+
+    [flatpak-remote-eos-apps]
+    # Football means something else in the UK
+    apps_del =
+      com.endlessm.football.en
+    apps_add =
+      com.endlessm.football.en_GB
+    """))
+
+    en_GB_orkney = personalitydir / 'en_GB_orkney.ini'
+    en_GB_orkney.write_text(dedent("""\
+    [flatpak-remote-eos-apps]
+    apps_add =
+      com.endlessm.orkneyingasaga
+    """))
+
+    builder = make_builder(configdir=str(configdir), personality="en_GB_orkney")
+    builder.configure()
+
+    assert builder.config['image']['language'] == "en_GB.utf8"
+    assert builder.config['flatpak-remote-eos-apps']['apps'] == "\n".join([
+      "com.endlessm.encyclopedia.en",
+      "com.endlessm.football.en_GB",
+      "com.endlessm.orkneyingasaga",
+    ])
+
+
 def test_localdir(make_builder, tmp_path, tmp_builder_paths, caplog):
     """Test use of local settings directory"""
     # Build without localdir