status: Allow external packages to produce status-compatible errors (#1927)

Embues the status package with the ability to create statuses
from generic errors that implement the interface:

type StatusError interface {
    Status() *Status
}

This was designed with the github.com/gogo/protobuf project in mind,
but is implemented in a fashion that makes it accessible to arbitrary
payloads.

Fixes #1885.

status/status.go

@@ -46,7 +46,7 @@ func (se *statusError) Error() string {
 	return fmt.Sprintf("rpc error: code = %s desc = %s", codes.Code(p.GetCode()), p.GetMessage())
 }
 
-func (se *statusError) status() *Status {
+func (se *statusError) Status() *Status {
 	return &Status{s: (*spb.Status)(se)}
 }
 
@@ -120,14 +120,14 @@ func FromProto(s *spb.Status) *Status {
 }
 
 // FromError returns a Status representing err if it was produced from this
-// package. Otherwise, ok is false and a Status is returned with codes.Unknown
+// package or has a method Status() *Status. Otherwise, ok is false and a
-// and the original error message.
+// Status is returned with codes.Unknown and the original error message.
 func FromError(err error) (s *Status, ok bool) {
 	if err == nil {
 		return &Status{s: &spb.Status{Code: int32(codes.OK)}}, true
 	}
-	if se, ok := err.(*statusError); ok {
+	if se, ok := err.(interface{ Status() *Status }); ok {
-		return se.status(), true
+		return se.Status(), true
 	}
 	return New(codes.Unknown, err.Error()), false
 }
@@ -182,8 +182,8 @@ func Code(err error) codes.Code {
 	if err == nil {
 		return codes.OK
 	}
-	if se, ok := err.(*statusError); ok {
+	if se, ok := err.(interface{ Status() *Status }); ok {
-		return se.status().Code()
+		return se.Status().Code()
 	}
 	return codes.Unknown
 }

status/status_test.go

@@ -24,6 +24,8 @@ import (
 	"reflect"
 	"testing"
 
+	"github.com/golang/protobuf/ptypes/any"
+
 	"github.com/golang/protobuf/proto"
 	"github.com/golang/protobuf/ptypes"
 	apb "github.com/golang/protobuf/ptypes/any"
@@ -119,6 +121,47 @@ func TestFromErrorOK(t *testing.T) {
 	}
 }
 
+type customError struct {
+	Code    codes.Code
+	Message string
+	Details []*any.Any
+}
+
+func (c customError) Error() string {
+	return fmt.Sprintf("rpc error: code = %s desc = %s", c.Code, c.Message)
+}
+
+func (c customError) Status() *Status {
+	return &Status{
+		s: &spb.Status{
+			Code:    int32(c.Code),
+			Message: c.Message,
+			Details: c.Details,
+		},
+	}
+}
+
+func TestFromErrorImplementsInterface(t *testing.T) {
+	code, message := codes.Internal, "test description"
+	details := []*any.Any{{
+		TypeUrl: "testUrl",
+		Value:   []byte("testValue"),
+	}}
+	err := customError{
+		Code:    code,
+		Message: message,
+		Details: details,
+	}
+	s, ok := FromError(err)
+	if !ok || s.Code() != code || s.Message() != message || s.Err() == nil {
+		t.Fatalf("FromError(%v) = %v, %v; want <Code()=%s, Message()=%q, Err()!=nil>, true", err, s, ok, code, message)
+	}
+	pd := s.Proto().GetDetails()
+	if len(pd) != 1 || !reflect.DeepEqual(pd[0], details[0]) {
+		t.Fatalf("s.Proto.GetDetails() = %v; want <Details()=%s>", pd, details)
+	}
+}
+
 func TestFromErrorUnknownError(t *testing.T) {
 	code, message := codes.Unknown, "unknown error"
 	err := errors.New("unknown error")