Add validation for paths in config

README.md

@@ -190,6 +190,10 @@ with key suffixes as follows:
  * `_required`: `true` means that the key must be set
  * `_values`: the value, if set, must be within the space-separated list
    of values here
+ * `_type`: `path` means the value, if set, must be the path to a file which
+   exists
+ * `_type`: `paths` means the value, if set, must be a space-separated list of
+   path to files which exist
 
 Merged options
 --------------

config/schema.ini

@@ -4,10 +4,20 @@
 #  _required = true means that the key must be set
 #  _values means the value, if set, must be within the space-separated list of
 #          values here
+#  _type = path means the value, if set, must be the path to a file which exists
+#  _type = paths means the value, if set, must be a space-separated list of
+#                path to files which exist
 
 [image]
 compression_required = true
 compression_values = gz
                      xz
 partition_table_values = gpt
                          dos
+branding_fbe_config_type = path
+icon_grid_type = paths
+initramfs_plymouth_watermark = path
+chromium_policies_managed = path
+chromium_policies_recommended = path
+settings = paths
+settings_locks = paths

run-build

@@ -515,6 +515,26 @@ class ImageBuilder(object):
                 raise eib.ImageBuildError(
                     'Configuration key [%s] %s has invalid value: %s'
                     % (section, option, self.config[section][option]))
+        elif option.endswith('_type'):
+            real_option = option[:-len('_type')]
+            if real_option in self.config[section]:
+                real_value = self.config[section][real_option].strip()
+                if value == "path":
+                    paths = [real_value] if real_value else []
+                elif value == "paths":
+                    paths = real_value.split()
+                else:
+                    raise eib.ImageBuildError(
+                        f'Schema key [{section}] {option} has invalid value: '
+                        f'{value}'
+                    )
+
+                for path in paths:
+                    if not os.path.exists(path):
+                        raise eib.ImageBuildError(
+                            f'Configuration key [{section}] {real_option} refers to '
+                            f'nonexistent path: {path}'
+                        )
 
     def check_config(self):
         """Check loaded configuration against schema for validity."""

tests/test_image_builder.py

@@ -327,3 +327,52 @@ def test_localdir(make_builder, tmp_path, tmp_builder_paths, caplog):
     assert (builder.config['image']['branding_desktop_logo'] ==
             str(localdir / 'data' / 'desktop.png'))
     assert builder.config['image']['signing_key'] == 'foobar'
+
+
+def test_path_validation(make_builder, tmp_path):
+    configdir = tmp_path / 'config'
+    configdir.mkdir()
+
+    # Schema
+    schema = configdir / 'schema.ini'
+    schema.write_text(dedent("""\
+    [image]
+    singular_type = path
+    plural_type = paths
+    """))
+
+    # Some config
+    defaults = configdir / 'defaults.ini'
+    defaults.write_text(dedent("""\
+    [image]
+    singular = ${build:localdatadir}/a.txt
+    plural = ${build:localdatadir}/b.txt ${build:localdatadir}/c.txt
+    """))
+
+    localdir = tmp_path / 'local'
+    localdatadir = localdir / 'data'
+    localdatadir.mkdir(parents=True)
+
+    a = localdatadir / 'a.txt'
+    a.touch()
+
+    b = localdatadir / 'b.txt'
+    b.touch()
+
+    c = localdatadir / 'c.txt'
+    c.touch()
+
+    builder = make_builder(configdir=str(configdir), localdir=str(localdir))
+    builder.configure()
+
+    # All paths exist
+    builder.check_config()
+
+    b.unlink()
+    with pytest.raises(eib.ImageBuildError, match=r'plural.*b.txt'):
+        builder.check_config()
+
+    b.touch()
+    a.unlink()
+    with pytest.raises(eib.ImageBuildError, match=r'singular.*a.txt'):
+        builder.check_config()