Add ErrorCodeOverride to error structures

Adding the `ErrorCodeOverride` field allows protocols to
override the default `ErrorCode()` implementation (which
is the name of the error) with a custom string.

If a smithy model uses the `ErrorCodeOverride` field in
an error structure, it will be renamed to
`ErrorCodeOverride_`.

Also, the writer for the switch-statement default block
is now pluggable, which means the default block can be
overwritten with a custom writer.

codegen/smithy-go-codegen/src/main/java/software/amazon/smithy/go/codegen/StructureGenerator.java

@@ -38,7 +38,7 @@ final class StructureGenerator implements Runnable {
             "ErrorMessage", "string",
             "ErrorFault", "string"
     );
-    private static final Set<String> ERROR_MESSAGE_MEMBER_NAMES = SetUtils.of("ErrorMessage", "Message");
+    private static final Set<String> ERROR_MEMBER_NAMES = SetUtils.of("ErrorMessage", "Message", "ErrorCodeOverride");
 
     private final Model model;
     private final SymbolProvider symbolProvider;
@@ -144,12 +144,12 @@ private void renderErrorStructure() {
             // The message is the only part of the standard APIError interface that isn't known ahead of time.
             // Message is a pointer mostly for the sake of consistency.
             writer.write("Message *string").write("");
-
+            writer.write("ErrorCodeOverride *string").write("");
 
             for (MemberShape member : shape.getAllMembers().values()) {
                 String memberName = symbolProvider.toMemberName(member);
                 // error messages are represented under Message for consistency
-                if (!ERROR_MESSAGE_MEMBER_NAMES.contains(memberName)) {
+                if (!ERROR_MEMBER_NAMES.contains(memberName)) {
                     writer.write("$L $P", memberName, symbolProvider.toSymbol(member));
                 }
             }
@@ -174,7 +174,12 @@ private void renderErrorStructure() {
 
         String errorCode = protocolGenerator == null ? shape.getId().getName(service)
                 : protocolGenerator.getErrorCode(service, shape);
-        writer.write("func (e *$L) ErrorCode() string { return $S }", structureSymbol.getName(), errorCode);
+        writer.openBlock("func (e *$L) ErrorCode() string {", "}", structureSymbol.getName(), () -> {
+            writer.openBlock("if e.ErrorCodeOverride == nil {", "}", () -> {
+                writer.write("return $S", errorCode);
+            });
+            writer.write("return *e.ErrorCodeOverride");
+        });
 
         String fault = "smithy.FaultUnknown";
         if (errorTrait.isClientError()) {

codegen/smithy-go-codegen/src/main/java/software/amazon/smithy/go/codegen/SymbolVisitor.java

@@ -122,6 +122,7 @@ final class SymbolVisitor implements SymbolProvider, ShapeVisitor<Symbol> {
                 .put("ErrorFault", "ErrorFault_")
                 .put("Unwrap", "Unwrap_")
                 .put("Error", "Error_")
+                .put("ErrorCodeOverride", "ErrorCodeOverride_")
                 .build();
 
         errorMemberEscaper = ReservedWordSymbolProvider.builder()

codegen/smithy-go-codegen/src/main/java/software/amazon/smithy/go/codegen/integration/HttpProtocolGeneratorUtils.java

@@ -59,12 +59,45 @@ private HttpProtocolGeneratorUtils() {
      *                                  and {@code errorMessage} variables from the http response.
      * @return A set of all error structure shapes for the operation that were dispatched to.
      */
-    static Set<StructureShape> generateErrorDispatcher(
+    public static Set<StructureShape> generateErrorDispatcher(
             GenerationContext context,
             OperationShape operation,
             Symbol responseType,
             Consumer<GenerationContext> errorMessageCodeGenerator,
             BiFunction<GenerationContext, OperationShape, Map<String, ShapeId>> operationErrorsToShapes
+    ) {
+        return generateErrorDispatcher(
+            context, operation, responseType,
+            errorMessageCodeGenerator, operationErrorsToShapes, writer -> defaultBlock(writer));
+    }
+
+    private static void defaultBlock(GoWriter writer) {
+        writer.openBlock("default:", "", () -> {
+            writer.openBlock("genericError := &smithy.GenericAPIError{", "}", () -> {
+                writer.write("Code: errorCode,");
+                writer.write("Message: errorMessage,");
+            });
+            writer.write("return genericError");
+        });
+    }
+
+    @FunctionalInterface
+    public interface DefaultBlockWriter {
+        void write(GoWriter writer);
+    }
+
+    /**
+     * Generates a function that handles error deserialization by getting the error code then
+     * dispatching to the error-specific deserializer. Provies an option to write custom default
+     * error block.
+     */
+    public static Set<StructureShape> generateErrorDispatcher(
+            GenerationContext context,
+            OperationShape operation,
+            Symbol responseType,
+            Consumer<GenerationContext> errorMessageCodeGenerator,
+            BiFunction<GenerationContext, OperationShape, Map<String, ShapeId>> operationErrorsToShapes,
+            DefaultBlockWriter defaultBlockWriter
     ) {
         GoWriter writer = context.getWriter().get();
         ServiceShape service = context.getService();
@@ -111,13 +144,7 @@ static Set<StructureShape> generateErrorDispatcher(
 
                         // Create a generic error
                         writer.addUseImports(SmithyGoDependency.SMITHY);
-                        writer.openBlock("default:", "", () -> {
-                            writer.openBlock("genericError := &smithy.GenericAPIError{", "}", () -> {
-                                writer.write("Code: errorCode,");
-                                writer.write("Message: errorMessage,");
-                            });
-                            writer.write("return genericError");
-                        });
+                        defaultBlockWriter.write(writer);
                     });
                 }).write("");
 

codegen/smithy-go-codegen/src/main/java/software/amazon/smithy/go/codegen/integration/HttpRpcProtocolGenerator.java

@@ -368,13 +368,18 @@ private void generateOperationDeserializer(GenerationContext context, OperationS
         });
         writer.write("");
 
-        Set<StructureShape> errorShapes = HttpProtocolGeneratorUtils.generateErrorDispatcher(
-                context, operation, responseType, this::writeErrorMessageCodeDeserializer,
-                this::getOperationErrors);
+        Set<StructureShape> errorShapes = generateErrorShapes(context, operation, responseType);
         deserializingErrorShapes.addAll(errorShapes);
         deserializingDocumentShapes.addAll(errorShapes);
     }
 
+    protected Set<StructureShape> generateErrorShapes(
+        GenerationContext context, OperationShape operation, Symbol responseType) {
+        return HttpProtocolGeneratorUtils.generateErrorDispatcher(
+                context, operation, responseType, this::writeErrorMessageCodeDeserializer,
+                this::getOperationErrors);
+    }
+
     /**
      * Generate the document deserializer logic for the deserializer middleware body.
      *