Make selector attributes stricter

docs/source/1.0/spec/core/selectors.rst

@@ -370,19 +370,6 @@ above selector:
     [trait | range
         | min = 1 ]
 
-Accessing an attribute or nested attribute property that does not exist
-returns an *empty value*. An empty value does not satisfy existence checks,
-returns an empty string when used with string comparators, and returns an
-empty value when attempting to access any properties.
-
-The following selector attempts to descend into non-existent properties of
-the :ref:`documentation-trait`. This example MUST NOT cause an error and
-MUST NOT match any shapes:
-
-.. code-block:: none
-
-    [trait|documentation|invalid|child = Hi]
-
 
 .. _id-attribute:
 
@@ -544,10 +531,11 @@ to a shape. The ``trait`` attribute supports the following properties:
 
         [trait|(length) > 10]
 ``*``
-    Other values are treated as shape IDs, where a relative shape ID is
+    Any other value is treated as a shape ID, where a relative shape ID is
     resolved to the ``smithy.api`` namespace. If a matching trait with the
     given shape ID is attached to the shape, it's :ref:`node value <node-attribute>`
-    is returned. An empty value is returned if the trait does not exist.
+    is returned. An :ref:`empty value <empty-attributes>` is returned if the
+    trait does not exist.
 
     The following selector matches shapes that have the
     :ref:`deprecated-trait`:
@@ -661,6 +649,30 @@ values return empty strings when used by string comparators.
 
         [trait|externalDocumentation|'Reference Docs']
 
+    Attempting to access a nested property that does not exist or
+    attempting to descend into nested values of a scalar type returns
+    an :ref:`empty value <empty-attributes>`.
+
+
+.. _empty-attributes:
+
+Empty attribute
+---------------
+
+Attempting to access a trait that does not exist, a variable that does
+not exist, or attempting to descend into node attribute values that do not
+exist returns an *empty value*. An empty value does not satisfy existence
+checks, returns an empty string when used with string comparators, and
+returns an empty value when attempting to access any properties.
+
+The following selector attempts to descend into non-existent properties of
+the :ref:`documentation-trait`. This example MUST NOT cause an error and
+MUST NOT match any shapes:
+
+.. code-block:: none
+
+    [trait|documentation|invalid|child = Hi]
+
 
 .. _projection-attribute:
 
@@ -1491,8 +1503,8 @@ A *var attribute* is an object accessible from a shape that provides
 access to the named :ref:`variables <selector-variables>` currently in scope.
 Variables are accessed by providing the variable name after ``var``. The
 values returned from ``var`` are :ref:`projections <projection-attribute>`
-that contain the set of shapes that were bound to the variable, or an empty
-value if the variable does not exist.
+that contain the set of shapes that were bound to the variable, or an
+:ref:`empty value <empty-attributes>` if the variable does not exist.
 
 The following selector finds all operations in the closure of a service
 where the operation has an :ref:`auth-trait` that is not a subset of the

smithy-model/src/main/java/software/amazon/smithy/model/selector/AttributeValueImpl.java

@@ -313,7 +313,7 @@ public AttributeValue getProperty(String key) {
             } else if (key.equals("id")) {
                 return AttributeValue.id(service.getId());
             } else {
-                return EMPTY;
+                throw new SelectorException("Invalid nested 'service' selector attribute property: " + key);
             }
         }
     }
@@ -357,7 +357,7 @@ public AttributeValue getProperty(String property) {
                     // Length returns the length of the shape ID.
                     return AttributeValue.literal(id.toString().length());
                 default:
-                    return EMPTY;
+                    throw new SelectorException("Invalid nested 'id' selector attribute property: " + property);
             }
         }
     }
@@ -445,8 +445,7 @@ public AttributeValue getProperty(String property) {
                 case "var":
                     return new VariableValue(vars);
                 default:
-                    LOGGER.warning("Unsupported shape selector attribute: " + property);
-                    return EMPTY;
+                    throw new SelectorException("Invalid shape selector attribute: " + property);
             }
         }
     }

smithy-model/src/main/java/software/amazon/smithy/model/selector/SelectorException.java

@@ -0,0 +1,29 @@
+/*
+ * Copyright 2021 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License").
+ * You may not use this file except in compliance with the License.
+ * A copy of the License is located at
+ *
+ *  http://aws.amazon.com/apache2.0
+ *
+ * or in the "license" file accompanying this file. This file is distributed
+ * on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
+ * express or implied. See the License for the specific language governing
+ * permissions and limitations under the License.
+ */
+
+package software.amazon.smithy.model.selector;
+
+/**
+ * Exception thrown when a selector evaluation is invalid.
+ */
+public class SelectorException extends RuntimeException {
+    public SelectorException(String message) {
+        super(message);
+    }
+
+    public SelectorException(String message, Throwable previous) {
+        super(message, previous);
+    }
+}

smithy-model/src/main/java/software/amazon/smithy/model/selector/SelectorSyntaxException.java

@@ -18,7 +18,7 @@
 /**
  * Exception thrown when a selector expression is invalid.
  */
-public final class SelectorSyntaxException extends RuntimeException {
+public final class SelectorSyntaxException extends SelectorException {
     SelectorSyntaxException(String message, String expression, int pos, int line, int column) {
         super(createMessage(message, expression, pos, line, column));
     }

smithy-model/src/test/java/software/amazon/smithy/model/selector/SelectorTest.java

@@ -593,8 +593,8 @@ public void invalidNumbersFailsGracefully() {
     }
 
     @Test
-    public void toleratesGettingNullPropertyFromString() {
-        assertThat(ids(traitModel, "[id|no|no=100]"), empty());
+    public void doesNotToleratesInvalidIdAccess() {
+        Assertions.assertThrows(SelectorException.class, () -> ids(traitModel, "[id|no|no=100]"));
     }
 
     @Test
@@ -633,12 +633,8 @@ public void selectsServiceVersions() {
     }
 
     @Test
-    public void toleratesUnknownServicePaths() {
-        Set<String> services1 = ids(traitModel, "[service|foo|baz='bam']");
-        Set<String> services2 = ids(traitModel, "[service|foo|baz]");
-
-        assertThat(services1, empty());
-        assertThat(services2, empty());
+    public void doesNotTolerateUnknownServicePaths() {
+        Assertions.assertThrows(SelectorException.class, () -> ids(traitModel, "[service|foo|baz='bam']"));
     }
 
     @Test

smithy-model/src/test/resources/software/amazon/smithy/model/errorfiles/validators/linters/emit-each-selector-validator.json

@@ -400,14 +400,21 @@
                 "id": "ignored",
                 "name": "EmitEachSelector",
                 "configuration": {
-                    "selector": "[foo]"
+                    "selector": "[trait|notRealTrait|notFound]"
                 }
             },
             {
                 "id": "ignored",
                 "name": "EmitEachSelector",
                 "configuration": {
-                    "selector": "[foo=baz]"
+                    "selector": "[var|notRealVar|notFound]"
+                }
+            },
+            {
+                "id": "ignored",
+                "name": "EmitEachSelector",
+                "configuration": {
+                    "selector": "[trait|error|nesting|into|things|is|tolerated]"
                 }
             },
             {