Fix initialization of defaults in yaml schema validation (#7959)

Fixes #7924

Spack's yaml schema validator was initializing all instances of
unspecified properties with the same object, so that updating the
property for one entry was updating it for others (e.g. updating
versions for one package would update it for other packages).

This updates the schema validator to instantiate defaults with
separate object instances and adds a test to confirm this behavior
(and also confirms #7924 without this change).

lib/spack/spack/config.py

@@ -116,7 +116,8 @@ def extend_with_default(validator_class):
     def set_defaults(validator, properties, instance, schema):
         for property, subschema in iteritems(properties):
             if "default" in subschema:
-                instance.setdefault(property, subschema["default"])
+                instance.setdefault(
+                    property, copy.deepcopy(subschema["default"]))
         for err in validate_properties(
                 validator, properties, instance, schema):
             yield err
@@ -127,7 +128,7 @@ def set_pp_defaults(validator, properties, instance, schema):
                 if isinstance(instance, dict):
                     for key, val in iteritems(instance):
                         if re.match(property, key) and val is None:
-                            instance[key] = subschema["default"]
+                            instance[key] = copy.deepcopy(subschema["default"])
 
         for err in validate_pattern_properties(
                 validator, properties, instance, schema):

lib/spack/spack/test/config.py

@@ -167,6 +167,50 @@
         'build_stage:': ['patha', 'pathb']}}
 
 
+packages_merge_low = {
+    'packages': {
+        'foo': {
+            'variants': ['+v1']
+        },
+        'bar': {
+            'variants': ['+v2']
+        }
+    }
+}
+
+packages_merge_high = {
+    'packages': {
+        'foo': {
+            'version': ['a']
+        },
+        'bar': {
+            'version': ['b'],
+            'variants': ['+v3']
+        },
+        'baz': {
+            'version': ['c']
+        }
+    }
+}
+
+
+@pytest.mark.regression('7924')
+def test_merge_with_defaults(config, write_config_file):
+    """This ensures that specified preferences merge with defaults as
+       expected. Originally all defaults were initialized with the
+       exact same object, which led to aliasing problems. Therefore
+       the test configs used here leave 'version' blank for multiple
+       packages in 'packages_merge_low'.
+    """
+    write_config_file('packages', packages_merge_low, 'low')
+    write_config_file('packages', packages_merge_high, 'high')
+    cfg = spack.config.get_config('packages')
+
+    assert cfg['foo']['version'] == ['a']
+    assert cfg['bar']['version'] == ['b']
+    assert cfg['baz']['version'] == ['c']
+
+
 def check_compiler_config(comps, *compiler_names):
     """Check that named compilers in comps match Spack's config."""
     config = spack.config.get_config('compilers')