Add marshal and unmarshal methods for Error.

This is based off hashicorp#15, which was never merged due to cla.

multierror.go

@@ -2,6 +2,7 @@ package multierror
 
 import (
 	"errors"
+	"encoding/json"
 	"fmt"
 )
 
@@ -40,6 +41,32 @@ func (e *Error) GoString() string {
 	return fmt.Sprintf("*%#v", *e)
 }
 
+// MarshalJSON returns a valid json representation of a multierror,
+// as an object with an array of error strings.
+func (e *Error) MarshalJSON() ([]byte, error) {
+	j := map[string][]string{
+		"errors": []string{},
+	}
+	for _, err := range e.Errors {
+		j["errors"] = append(j["errors"], err.Error())
+	}
+	return json.Marshal(j)
+}
+
+// UnmarshalJSON from an array of strings.
+func (e *Error) UnmarshalJSON(b []byte) error {
+	j := make(map[string][]string)
+	if err := json.Unmarshal(b, &j); err != nil {
+		return err
+	}
+	if j["errors"] != nil {
+		for _, msg := range j["errors"] {
+			e.Errors = append(e.Errors, fmt.Errorf(msg))
+		}
+	}
+	return nil
+}
+
 // WrappedErrors returns the list of errors that this Error is wrapping. It is
 // an implementation of the errwrap.Wrapper interface so that multierror.Error
 // can be used with that library.

multierror_test.go

@@ -1,6 +1,7 @@
 package multierror
 
 import (
+	"encoding/json"
 	"errors"
 	"fmt"
 	"reflect"
@@ -45,6 +46,29 @@ func TestErrorError_default(t *testing.T) {
 	}
 }
 
+func TestError_json(t *testing.T) {
+	errs := []error{
+		errors.New("foo"),
+		errors.New("bar"),
+	}
+	multi := Error{Errors: errs}
+	b, err := json.Marshal(&multi)
+	if err != nil {
+		t.Fatalf("unexpected error; got %#v", err)
+	}
+	j := `{"errors":["foo","bar"]}`
+	if string(b) != j {
+		t.Errorf("bad representation; got: %s, want: %s", string(b), j)
+	}
+	rebuilt := Error{}
+	if err = json.Unmarshal(b, &rebuilt); err != nil {
+		t.Fatalf("unexpected error; go %#v", err)
+	}
+	if !reflect.DeepEqual(rebuilt, multi) {
+		t.Fatalf("mismatched types; got: %v, want: %v", rebuilt, multi)
+	}
+}
+
 func TestErrorErrorOrNil(t *testing.T) {
 	err := new(Error)
 	if err.ErrorOrNil() != nil {