Add message member to DocumentException toString

Fixes #815

Author: mtdowling

client/dynamic-client/src/main/java/software/amazon/smithy/java/dynamicclient/DocumentException.java

@@ -14,6 +14,7 @@
 import software.amazon.smithy.java.dynamicschemas.SchemaConverter;
 import software.amazon.smithy.java.dynamicschemas.StructDocument;
 import software.amazon.smithy.model.shapes.ShapeId;
+import software.amazon.smithy.model.shapes.ShapeType;
 
 /**
  * A {@link ModeledException} that provides access to the contents of the exception as a document.
@@ -23,10 +24,25 @@ public final class DocumentException extends ModeledException {
     private final StructDocument document;
 
     DocumentException(Schema schema, String message, StructDocument document) {
-        super(schema, message);
+        super(schema, createMessage(message, document));
         this.document = document;
     }
 
+    // Inject the "message" member into the exception message if we can find it, and it isn't there already.
+    private static String createMessage(String message, StructDocument document) {
+        var messageMember = document.getMember("message");
+        if (messageMember == null) {
+            messageMember = document.getMember("Message");
+        }
+        if (messageMember != null && messageMember.isType(ShapeType.STRING)) {
+            var messageMemberValue = messageMember.asString();
+            if (!message.contains(messageMemberValue)) {
+                message = message + ": " + messageMemberValue;
+            }
+        }
+        return message;
+    }
+
     @Override
     public void serialize(ShapeSerializer encoder) {
         document.serialize(encoder);

client/dynamic-client/src/test/java/software/amazon/smithy/java/dynamicclient/DocumentExceptionTest.java

@@ -0,0 +1,55 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package software.amazon.smithy.java.dynamicclient;
+
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.endsWith;
+
+import java.util.Map;
+import org.junit.jupiter.api.Test;
+import software.amazon.smithy.java.core.schema.Schema;
+import software.amazon.smithy.java.core.serde.document.Document;
+import software.amazon.smithy.java.dynamicschemas.StructDocument;
+import software.amazon.smithy.model.shapes.ShapeId;
+import software.amazon.smithy.model.traits.ErrorTrait;
+
+public class DocumentExceptionTest {
+
+    private static final Schema SCHEMA = Schema.structureBuilder(ShapeId.from("foo#Error"), new ErrorTrait("client"))
+            .putMember("message", Schema.createString(ShapeId.from("foo#Str")))
+            .build();
+
+    @Test
+    public void addsMessageWhenFound() {
+        var ex = new DocumentException(SCHEMA,
+                "hello: oh my!",
+                StructDocument.of(SCHEMA,
+                        Document.of(Map.of(
+                                "message",
+                                Document.of("oh my!")))));
+
+        assertThat(ex.toString(), endsWith("hello: oh my!"));
+    }
+
+    @Test
+    public void doesNotAddMessageTwice() {
+        var ex = new DocumentException(SCHEMA,
+                "hello: oh my!",
+                StructDocument.of(SCHEMA,
+                        Document.of(Map.of(
+                                "message",
+                                Document.of("oh my!")))));
+
+        assertThat(ex.toString(), endsWith("hello: oh my!"));
+    }
+
+    @Test
+    public void addsNothingWhenNoMessage() {
+        var ex = new DocumentException(SCHEMA, "hello", StructDocument.of(SCHEMA, Document.of(Map.of())));
+
+        assertThat(ex.toString(), endsWith("hello"));
+    }
+}