Add validator for services with no auth trait

Adds a validator that warns when there's a service shape that has
multiple authDefinition traits applied, but no auth trait. This
encourages the use of the auth trait so auth scheme priority is
being modeled explicitly. It also allows for tooling to look for
and use this validation event.

Documentation was also updated to explain that without the auth
trait, auth scheme ordering is alphabetical.

A test for the auth trait (auth-trait-must-target-service-schemes)
was updated to avoid getting the warning message from the new
validator.

docs/source-2.0/spec/authentication-traits.rst

@@ -276,6 +276,11 @@ the ``auth`` trait, then the operation is expected to support each of the
 service. Each entry in the ``auth`` trait is a shape ID that MUST refer to an
 authentication scheme trait applied to the service in which it is bound.
 
+.. note::
+    When a service has multiple authentication scheme traits applied and no
+    ``auth`` trait, the ordering of authentication schemes is alphabetical
+    based on the absolute shape ID of each authentication scheme trait.
+
 The following example defines all combinations in which ``auth`` can be applied
 to services and operations:
 

smithy-model/src/main/java/software/amazon/smithy/model/validation/validators/ServiceWithNoAuthTraitValidator.java

@@ -0,0 +1,54 @@
+/*
+ * Copyright 2023 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.validation.validators;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.stream.Collectors;
+import software.amazon.smithy.model.Model;
+import software.amazon.smithy.model.knowledge.ServiceIndex;
+import software.amazon.smithy.model.shapes.ServiceShape;
+import software.amazon.smithy.model.traits.AuthTrait;
+import software.amazon.smithy.model.validation.AbstractValidator;
+import software.amazon.smithy.model.validation.ValidationEvent;
+
+/**
+ * Validates that services with multiple @authDefinition traits applied also
+ * have an @auth trait applied.
+ */
+public class ServiceWithNoAuthTraitValidator extends AbstractValidator {
+
+    @Override
+    public List<ValidationEvent> validate(Model model) {
+        List<ValidationEvent> events = new ArrayList<>();
+        ServiceIndex index = ServiceIndex.of(model);
+
+        List<ServiceShape> services = model.getServiceShapes().stream()
+                .filter(serviceShape -> !serviceShape.hasTrait(AuthTrait.ID))
+                .filter(serviceShape -> index.getAuthSchemes(serviceShape).size() > 1)
+                .collect(Collectors.toList());
+
+        for (ServiceShape service : services) {
+            events.add(warning(service, "This service uses multiple authentication schemes but does not have "
+                                        + "the `@auth` trait applied. The `@auth` trait defines a priority ordering "
+                                        + "of auth schemes for a client to use. Without it, the ordering of auth "
+                                        + "schemes is alphabetical based on the absolute shape ID of the auth "
+                                        + "schemes."));
+
+        }
+        return events;
+    }
+}

smithy-model/src/main/resources/META-INF/services/software.amazon.smithy.model.validation.Validator

@@ -36,6 +36,7 @@ software.amazon.smithy.model.validation.validators.ResourceIdentifierBindingVali
 software.amazon.smithy.model.validation.validators.ResourceIdentifierValidator
 software.amazon.smithy.model.validation.validators.ResourceLifecycleValidator
 software.amazon.smithy.model.validation.validators.ResourceOperationInputOutputValidator
+software.amazon.smithy.model.validation.validators.ServiceWithNoAuthTraitValidator
 software.amazon.smithy.model.validation.validators.ServiceValidator
 software.amazon.smithy.model.validation.validators.SetValidator
 software.amazon.smithy.model.validation.validators.ShapeIdConflictValidator

smithy-model/src/test/resources/software/amazon/smithy/model/errorfiles/validators/auth/auth-trait-must-target-service-schemes.json

@@ -10,7 +10,11 @@
             ],
             "traits": {
                 "smithy.api#httpBasicAuth": {},
-                "smithy.api#httpDigestAuth": {}
+                "smithy.api#httpDigestAuth": {},
+                "smithy.api#auth": [
+                    "smithy.api#httpDigestAuth",
+                    "smithy.api#httpBasicAuth"
+                ]
             }
         },
         "ns.foo#ValidOperation": {

smithy-model/src/test/resources/software/amazon/smithy/model/errorfiles/validators/service-with-no-auth-trait/multiple-auth-schemes.errors

@@ -0,0 +1 @@
+[WARNING] smithy.example#FooService: This service uses multiple authentication schemes but is not annotated with the @auth trait. The @auth trait defines a priority ordering of auth schemes for a client to use. Without it, the ordering of auth schemes is alphabetical based on the absolute shape id of the auth schemes. | ServiceWithNoAuthTrait

smithy-model/src/test/resources/software/amazon/smithy/model/errorfiles/validators/service-with-no-auth-trait/multiple-auth-schemes.smithy

@@ -0,0 +1,18 @@
+$version: "2"
+
+namespace smithy.example
+
+// Service shape with multiple auth schemes and no auth trait should cause warning
+@httpBasicAuth
+@httpDigestAuth
+@httpBearerAuth
+service FooService {
+    version: "2023-08-15"
+    operations: [GetFoo]
+}
+
+operation GetFoo {
+    output: GetFooOutput
+}
+
+structure GetFooOutput {}

smithy-model/src/test/resources/software/amazon/smithy/model/errorfiles/validators/service-with-no-auth-trait/one-auth-scheme.smithy

@@ -0,0 +1,16 @@
+$version: "2"
+
+namespace smithy.example
+
+// Service shape with one auth schemes and no auth trait should NOT cause warning
+@httpBasicAuth
+service FooService {
+    version: "2023-08-15"
+    operations: [GetFoo]
+}
+
+operation GetFoo {
+    output: GetFooOutput
+}
+
+structure GetFooOutput {}