Fix #422 added support for error unwrapping for errors with a single …

…sub-error (#433)

openapi3/schema.go

@@ -1489,6 +1489,8 @@ type SchemaError struct {
 	Origin      error
 }
 
+var _ interface{ Unwrap() error } = SchemaError{}
+
 func markSchemaErrorKey(err error, key string) error {
 	if v, ok := err.(*SchemaError); ok {
 		v.reversePath = append(v.reversePath, key)
@@ -1564,6 +1566,10 @@ func (err *SchemaError) Error() string {
 	return buf.String()
 }
 
+func (err SchemaError) Unwrap() error {
+	return err.Origin
+}
+
 func isSliceOfUniqueItems(xs []interface{}) bool {
 	s := len(xs)
 	m := make(map[string]struct{}, s)

openapi3filter/errors.go

@@ -17,6 +17,8 @@ type RequestError struct {
 	Err         error
 }
 
+var _ interface{ Unwrap() error } = RequestError{}
+
 func (err *RequestError) Error() string {
 	reason := err.Reason
 	if e := err.Err; e != nil {
@@ -35,6 +37,10 @@ func (err *RequestError) Error() string {
 	}
 }
 
+func (err RequestError) Unwrap() error {
+	return err.Err
+}
+
 var _ error = &ResponseError{}
 
 // ResponseError is returned by ValidateResponse when response does not match OpenAPI spec
@@ -44,6 +50,8 @@ type ResponseError struct {
 	Err    error
 }
 
+var _ interface{ Unwrap() error } = ResponseError{}
+
 func (err *ResponseError) Error() string {
 	reason := err.Reason
 	if e := err.Err; e != nil {
@@ -56,6 +64,10 @@ func (err *ResponseError) Error() string {
 	return reason
 }
 
+func (err ResponseError) Unwrap() error {
+	return err.Err
+}
+
 var _ error = &SecurityRequirementsError{}
 
 // SecurityRequirementsError is returned by ValidateSecurityRequirements

openapi3filter/req_resp_decoder.go

@@ -4,7 +4,6 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
-	"gopkg.in/yaml.v2"
 	"io"
 	"io/ioutil"
 	"mime"
@@ -15,6 +14,8 @@ import (
 	"strconv"
 	"strings"
 
+	"gopkg.in/yaml.v2"
+
 	"github.com/getkin/kin-openapi/openapi3"
 )
 
@@ -42,6 +43,8 @@ type ParseError struct {
 	path []interface{}
 }
 
+var _ interface{ Unwrap() error } = ParseError{}
+
 func (e *ParseError) Error() string {
 	var msg []string
 	if p := e.Path(); len(p) > 0 {
@@ -81,6 +84,10 @@ func (e *ParseError) RootCause() error {
 	return e.Cause
 }
 
+func (e ParseError) Unwrap() error {
+	return e.Cause
+}
+
 // Path returns a path to the root cause.
 func (e *ParseError) Path() []interface{} {
 	var path []interface{}