Allow statuses from compatible errors

Embues the status package with the ability to create statuses
from generic errors that implement a special interface.

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

@@ -33,6 +33,7 @@ import (
 
 	"github.com/golang/protobuf/proto"
 	"github.com/golang/protobuf/ptypes"
+	"github.com/golang/protobuf/ptypes/any"
 	spb "google.golang.org/genproto/googleapis/rpc/status"
 	"google.golang.org/grpc/codes"
 )
@@ -119,16 +120,55 @@ func FromProto(s *spb.Status) *Status {
 	return &Status{s: proto.Clone(s).(*spb.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
-// and the original error message.
+/*
+FromError returns a Status representing err.
+It converts an error to a status as follows:
+
+	1. If the error is produced by this package, it will
+	extract the status from the error.
+
+	2. If the error implements the following interface, it will
+	construct a status out of the information exposed:
+
+		type StatusError interface {
+			GetMessage() string
+			GetCode() codes.Code
+			GetDetails() []interface {
+				GetTypeURL() string
+				GetValue() []byte
+			}
+		}
+
+	3. Otherwise, ok is false and a 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 {
 		return se.status(), true
 	}
+	if e, ok := err.(interface {
+		GetMessage() string
+		GetCode() codes.Code
+		GetDetails() []interface {
+			GetTypeURL() string
+			GetValue() []byte
+		}
+	}); ok {
+		sp := &spb.Status{
+			Code:    int32(e.GetCode()),
+			Message: e.GetMessage(),
+		}
+		for _, detail := range e.GetDetails() {
+			sp.Details = append(sp.Details, &any.Any{
+				TypeUrl: detail.GetTypeURL(),
+				Value:   detail.GetValue(),
+			})
+		}
+		return FromProto(sp), true
+	}
 	return New(codes.Unknown, err.Error()), false
 }
 

status/status_test.go

@@ -19,6 +19,7 @@
 package status
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
 	"reflect"
@@ -119,6 +120,70 @@ func TestFromErrorOK(t *testing.T) {
 	}
 }
 
+type customError struct {
+	Code    codes.Code
+	Message string
+	Details []interface {
+		GetTypeURL() string
+		GetValue() []byte
+	}
+}
+
+func (c customError) Error() string {
+	return fmt.Sprintf("rpc error: code = %s desc = %s", c.GetCode(), c.GetMessage())
+}
+
+func (c customError) GetCode() codes.Code {
+	return c.Code
+}
+
+func (c customError) GetMessage() string {
+	return c.Message
+}
+
+func (c customError) GetDetails() []interface {
+	GetTypeURL() string
+	GetValue() []byte
+} {
+	return c.Details
+}
+
+type detail struct {
+	TypeURL string
+	Value   []byte
+}
+
+func (d detail) GetTypeURL() string {
+	return d.TypeURL
+}
+
+func (d detail) GetValue() []byte {
+	return d.Value
+}
+
+func TestFromErrorImplementsInterface(t *testing.T) {
+	code, message := codes.Internal, "test description"
+	details := []interface {
+		GetTypeURL() string
+		GetValue() []byte
+	}{
+		detail{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 || !bytes.Equal(pd[0].GetValue(), details[0].GetValue()) || pd[0].GetTypeUrl() != details[0].GetTypeURL() {
+		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")