feature: Add errors.ConjureErrorDecoder interface & instantiable regi…

…stry type for customizing error deserialization (#724)

changelog/@unreleased/pr-724.v2.yml

@@ -0,0 +1,6 @@
+type: feature
+feature:
+  description: Add errors.ConjureErrorDecoder interface & instantiable registry type
+    for customizing error deserialization
+  links:
+  - https://github.com/palantir/conjure-go-runtime/pull/724

conjure-go-client/httpclient/request_params.go

@@ -23,6 +23,7 @@ import (
 	"time"
 
 	"github.com/palantir/conjure-go-runtime/v2/conjure-go-contract/codecs"
+	"github.com/palantir/conjure-go-runtime/v2/conjure-go-contract/errors"
 	werror "github.com/palantir/witchcraft-go-error"
 )
 
@@ -243,3 +244,14 @@ func WithRequestTimeout(timeout time.Duration) RequestParam {
 		return nil
 	})
 }
+
+func WithRequestConjureErrorDecoder(ced errors.ConjureErrorDecoder) RequestParam {
+	return requestParamFunc(func(b *requestBuilder) error {
+		b.errorDecoderMiddleware = errorDecoderMiddleware{
+			errorDecoder: restErrorDecoder{
+				conjureErrorDecoder: ced,
+			},
+		}
+		return nil
+	})
+}

conjure-go-client/httpclient/response_error_decoder_middleware.go

@@ -66,7 +66,9 @@ func (e errorDecoderMiddleware) RoundTrip(req *http.Request, next http.RoundTrip
 // If the response has a Content-Type containing 'application/json', we attempt
 // to unmarshal the error as a conjure error. See TestErrorDecoderMiddlewares for
 // example error messages and parameters.
-type restErrorDecoder struct{}
+type restErrorDecoder struct {
+	conjureErrorDecoder errors.ConjureErrorDecoder
+}
 
 var _ ErrorDecoder = restErrorDecoder{}
 
@@ -102,7 +104,13 @@ func (d restErrorDecoder) DecodeError(resp *http.Response) error {
 	if isJSON := strings.Contains(resp.Header.Get("Content-Type"), codecs.JSON.ContentType()); !isJSON {
 		return werror.Error(resp.Status, wSafeParams, wUnsafeParams, werror.UnsafeParam("responseBody", string(body)))
 	}
-	conjureErr, jsonErr := errors.UnmarshalError(body)
+	var conjureErr errors.Error
+	var jsonErr error
+	if d.conjureErrorDecoder != nil {
+		conjureErr, jsonErr = errors.UnmarshalErrorWithDecoder(d.conjureErrorDecoder, body)
+	} else {
+		conjureErr, jsonErr = errors.UnmarshalError(body)
+	}
 	if jsonErr != nil {
 		return werror.Error(resp.Status, wSafeParams, wUnsafeParams, werror.UnsafeParam("responseBody", string(body)))
 	}

conjure-go-contract/errors/conjure_error_decoder.go

@@ -0,0 +1,19 @@
+// Copyright (c) 2024 Palantir Technologies. 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.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License 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 errors
+
+type ConjureErrorDecoder interface {
+	DecodeConjureError(name string, body []byte) (Error, error)
+}

conjure-go-contract/errors/error_type_registry.go

@@ -17,21 +17,59 @@ package errors
 import (
 	"fmt"
 	"reflect"
+
+	"github.com/palantir/conjure-go-runtime/v2/conjure-go-contract/codecs"
+	werror "github.com/palantir/witchcraft-go-error"
 )
 
-var registry = map[string]reflect.Type{}
+var globalRegistry = NewReflectTypeConjureErrorDecoder()
 
 var errorInterfaceType = reflect.TypeOf((*Error)(nil)).Elem()
 
 // RegisterErrorType registers an error name and its go type in a global registry.
 // The type should be a struct type whose pointer implements Error.
 // Panics if name is already registered or *type does not implement Error.
 func RegisterErrorType(name string, typ reflect.Type) {
-	if existing, exists := registry[name]; exists {
-		panic(fmt.Sprintf("ErrorName %v already registered as %v", name, existing))
+	if err := globalRegistry.RegisterErrorType(name, typ); err != nil {
+		panic(err.Error())
+	}
+}
+
+// NewReflectTypeConjureErrorDecoder returns a new ConjureErrorDecoder that uses reflection to convert JSON errors to their go types.
+func NewReflectTypeConjureErrorDecoder() *ReflectTypeConjureErrorDecoder {
+	return &ReflectTypeConjureErrorDecoder{registry: make(map[string]reflect.Type)}
+}
+
+// ReflectTypeConjureErrorDecoder is a ConjureErrorDecoder that uses reflection to convert JSON errors to their go types.
+// It stores a mapping of serialized error name to the go type that should be used to unmarshal the error.
+type ReflectTypeConjureErrorDecoder struct {
+	registry map[string]reflect.Type
+}
+
+func (d *ReflectTypeConjureErrorDecoder) RegisterErrorType(name string, typ reflect.Type) error {
+	if existing, exists := d.registry[name]; exists {
+		return fmt.Errorf("ErrorName %v already registered as %v", name, existing)
+	}
+	if ptr := reflect.PointerTo(typ); !ptr.Implements(errorInterfaceType) {
+		return fmt.Errorf("Error type %v does not implement errors.Error interface", ptr)
+	}
+	d.registry[name] = typ
+	return nil
+}
+
+func (d *ReflectTypeConjureErrorDecoder) DecodeConjureError(errorName string, body []byte) (Error, error) {
+	typ, ok := d.registry[errorName]
+	if !ok {
+		// Unrecognized error name, fall back to genericError
+		typ = reflect.TypeOf(genericError{})
+	}
+	instance := reflect.New(typ).Interface()
+	if err := codecs.JSON.Unmarshal(body, &instance); err != nil {
+		return nil, werror.Wrap(err, "failed to unmarshal body using registered type", werror.SafeParam("type", typ.String()))
 	}
-	if ptr := reflect.PtrTo(typ); !ptr.Implements(errorInterfaceType) {
-		panic(fmt.Sprintf("Error type %v does not implement errors.Error interface", ptr))
+	cerr, ok := instance.(Error)
+	if !ok {
+		return nil, werror.Error("unmarshaled type does not implement errors.Error interface", werror.SafeParam("type", typ.String()))
 	}
-	registry[name] = typ
+	return cerr, nil
 }

conjure-go-contract/errors/unmarshal.go

@@ -15,8 +15,6 @@
 package errors
 
 import (
-	"reflect"
-
 	"github.com/palantir/conjure-go-runtime/v2/conjure-go-contract/codecs"
 	werror "github.com/palantir/witchcraft-go-error"
 )
@@ -26,23 +24,21 @@ import (
 // If the ErrorName is not recognized, a genericError is returned with all params marked unsafe.
 // If we fail to unmarshal to a generic SerializableError or to the type specified by ErrorName, an error is returned.
 func UnmarshalError(body []byte) (Error, error) {
+	return UnmarshalErrorWithDecoder(globalRegistry, body)
+}
+
+// UnmarshalErrorWithDecoder attempts to deserialize the message to a known implementation of Error
+// using the provided ConjureErrorDecoder.
+func UnmarshalErrorWithDecoder(ced ConjureErrorDecoder, body []byte) (Error, error) {
 	var name struct {
 		Name string `json:"errorName"`
 	}
 	if err := codecs.JSON.Unmarshal(body, &name); err != nil {
 		return nil, werror.Wrap(err, "failed to unmarshal body as conjure error")
 	}
-	typ, ok := registry[name.Name]
-	if !ok {
-		// Unrecognized error name, fall back to genericError
-		typ = reflect.TypeOf(genericError{})
+	cErr, err := ced.DecodeConjureError(name.Name, body)
+	if err != nil {
+		return nil, werror.Convert(err)
 	}
-
-	instance := reflect.New(typ).Interface()
-	if err := codecs.JSON.Unmarshal(body, &instance); err != nil {
-		return nil, werror.Wrap(err, "failed to unmarshal body using registered type", werror.SafeParam("type", typ.String()))
-	}
-
-	// Cast should never panic, as we've verified in RegisterErrorType
-	return instance.(Error), nil
+	return cErr, nil
 }