Allow server decorators to postprocess ValidationException not atta…

…ched error messages (smithy-lang#2338)

Should they want to, a server decorator can now postprocess the error
message that arises when a constrained operation does not have the
`ValidationException` shape attached to its errors.

This commit adds a test to ensure that when such a decorator is
registered, the `ValidationResult` can indeed be altered, but no such
decorator is added to the `rust-server-codegen` plugin.

codegen-server/src/main/kotlin/software/amazon/smithy/rust/codegen/server/smithy/ServerCodegenVisitor.kt

@@ -200,10 +200,12 @@ open class ServerCodegenVisitor(
 
         val validationExceptionShapeId = validationExceptionConversionGenerator.shapeId
         for (validationResult in listOf(
-            validateOperationsWithConstrainedInputHaveValidationExceptionAttached(
-                model,
-                service,
-                validationExceptionShapeId,
+            codegenDecorator.postprocessValidationExceptionNotAttachedErrorMessage(
+                validateOperationsWithConstrainedInputHaveValidationExceptionAttached(
+                    model,
+                    service,
+                    validationExceptionShapeId,
+                ),
             ),
             validateUnsupportedConstraints(model, service, codegenContext.settings.codegenConfig),
         )) {
@@ -212,7 +214,7 @@ open class ServerCodegenVisitor(
                 logger.log(logMessage.level, logMessage.message)
             }
             if (validationResult.shouldAbort) {
-                throw CodegenException("Unsupported constraints feature used; see error messages above for resolution")
+                throw CodegenException("Unsupported constraints feature used; see error messages above for resolution", validationResult)
             }
         }
 

codegen-server/src/main/kotlin/software/amazon/smithy/rust/codegen/server/smithy/ValidateUnsupportedConstraints.kt

@@ -130,7 +130,8 @@ private data class UnsupportedUniqueItemsTraitOnShape(val shape: Shape, val uniq
     UnsupportedConstraintMessageKind()
 
 data class LogMessage(val level: Level, val message: String)
-data class ValidationResult(val shouldAbort: Boolean, val messages: List<LogMessage>)
+data class ValidationResult(val shouldAbort: Boolean, val messages: List<LogMessage>) :
+    Throwable(message = messages.joinToString("\n") { it.message })
 
 private val unsupportedConstraintsOnMemberShapes = allConstraintTraits - RequiredTrait::class.java
 

codegen-server/src/main/kotlin/software/amazon/smithy/rust/codegen/server/smithy/customize/ServerCodegenDecorator.kt

@@ -11,6 +11,7 @@ import software.amazon.smithy.rust.codegen.core.smithy.customize.CombinedCoreCod
 import software.amazon.smithy.rust.codegen.core.smithy.customize.CoreCodegenDecorator
 import software.amazon.smithy.rust.codegen.core.smithy.protocols.ProtocolMap
 import software.amazon.smithy.rust.codegen.server.smithy.ServerCodegenContext
+import software.amazon.smithy.rust.codegen.server.smithy.ValidationResult
 import software.amazon.smithy.rust.codegen.server.smithy.generators.ValidationExceptionConversionGenerator
 import software.amazon.smithy.rust.codegen.server.smithy.generators.protocol.ServerProtocolGenerator
 import java.util.logging.Logger
@@ -23,6 +24,12 @@ typealias ServerProtocolMap = ProtocolMap<ServerProtocolGenerator, ServerCodegen
 interface ServerCodegenDecorator : CoreCodegenDecorator<ServerCodegenContext> {
     fun protocols(serviceId: ShapeId, currentProtocols: ServerProtocolMap): ServerProtocolMap = currentProtocols
     fun validationExceptionConversion(codegenContext: ServerCodegenContext): ValidationExceptionConversionGenerator? = null
+
+    /**
+     * Injection point to allow a decorator to postprocess the error message that arises when an operation is
+     * constrained but the `ValidationException` shape is not attached to the operation's errors.
+     */
+    fun postprocessValidationExceptionNotAttachedErrorMessage(validationResult: ValidationResult) = validationResult
 }
 
 /**
@@ -33,6 +40,9 @@ interface ServerCodegenDecorator : CoreCodegenDecorator<ServerCodegenContext> {
 class CombinedServerCodegenDecorator(private val decorators: List<ServerCodegenDecorator>) :
     CombinedCoreCodegenDecorator<ServerCodegenContext, ServerCodegenDecorator>(decorators),
     ServerCodegenDecorator {
+
+    private val orderedDecorators = decorators.sortedBy { it.order }
+
     override val name: String
         get() = "CombinedServerCodegenDecorator"
     override val order: Byte
@@ -46,7 +56,12 @@ class CombinedServerCodegenDecorator(private val decorators: List<ServerCodegenD
     override fun validationExceptionConversion(codegenContext: ServerCodegenContext): ValidationExceptionConversionGenerator =
         // We use `firstNotNullOf` instead of `firstNotNullOfOrNull` because the [SmithyValidationExceptionDecorator]
         // is registered.
-        decorators.sortedBy { it.order }.firstNotNullOf { it.validationExceptionConversion(codegenContext) }
+        orderedDecorators.firstNotNullOf { it.validationExceptionConversion(codegenContext) }
+
+    override fun postprocessValidationExceptionNotAttachedErrorMessage(validationResult: ValidationResult): ValidationResult =
+        orderedDecorators.foldRight(validationResult) { decorator, accumulated ->
+            decorator.postprocessValidationExceptionNotAttachedErrorMessage(accumulated)
+        }
 
     companion object {
         fun fromClasspath(

codegen-server/src/test/kotlin/software/amazon/smithy/rust/codegen/server/smithy/customizations/PostprocessValidationExceptionNotAttachedErrorMessageDecoratorTest.kt

@@ -0,0 +1,73 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package software.amazon.smithy.rust.codegen.server.smithy.customizations
+
+import io.kotest.matchers.shouldBe
+import io.kotest.matchers.string.shouldContain
+import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.assertThrows
+import software.amazon.smithy.codegen.core.CodegenException
+import software.amazon.smithy.rust.codegen.core.testutil.asSmithyModel
+import software.amazon.smithy.rust.codegen.server.smithy.LogMessage
+import software.amazon.smithy.rust.codegen.server.smithy.ValidationResult
+import software.amazon.smithy.rust.codegen.server.smithy.customize.ServerCodegenDecorator
+import software.amazon.smithy.rust.codegen.server.smithy.testutil.serverIntegrationTest
+
+internal class PostprocessValidationExceptionNotAttachedErrorMessageDecoratorTest {
+    @Test
+    fun `validation exception not attached error message is postprocessed if decorator is registered`() {
+        val model =
+            """
+            namespace test
+            use aws.protocols#restJson1
+            
+            @restJson1
+            service TestService {
+                operations: ["ConstrainedOperation"],
+            }
+            
+            operation ConstrainedOperation { 
+                input: ConstrainedOperationInput 
+            }
+            
+            structure ConstrainedOperationInput {
+                @required
+                requiredString: String
+            }
+            """.asSmithyModel()
+
+        val validationExceptionNotAttachedErrorMessageDummyPostprocessorDecorator = object : ServerCodegenDecorator {
+            override val name: String
+                get() = "ValidationExceptionNotAttachedErrorMessageDummyPostprocessorDecorator"
+            override val order: Byte
+                get() = 69
+
+            override fun postprocessValidationExceptionNotAttachedErrorMessage(validationResult: ValidationResult): ValidationResult {
+                check(validationResult.messages.size == 1)
+
+                val level = validationResult.messages.first().level
+                val message =
+                    """
+${validationResult.messages.first().message}
+
+There are three things all wise men fear: the sea in storm, a night with no moon, and the anger of a gentle man.
+                    """
+
+                return validationResult.copy(messages = listOf(LogMessage(level, message)))
+            }
+        }
+
+        val exception = assertThrows<CodegenException> {
+            serverIntegrationTest(
+                model,
+                additionalDecorators = listOf(validationExceptionNotAttachedErrorMessageDummyPostprocessorDecorator),
+            )
+        }
+        val exceptionCause = (exception.cause!! as ValidationResult)
+        exceptionCause.messages.size shouldBe 1
+        exceptionCause.messages.first().message shouldContain "There are three things all wise men fear: the sea in storm, a night with no moon, and the anger of a gentle man."
+    }
+}