From 79591889a97af09f177601e3e9e61f9c7e359359 Mon Sep 17 00:00:00 2001
From: mh-cbon <mh-cbon@users.noreply.github.com>
Date: Sat, 5 Nov 2016 10:27:46 +0100
Subject: [PATCH] Close #558: detect FormattedError and print their stack trace

---
 errors.go      | 14 ++++++++++----
 errors_test.go | 30 ++++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+), 4 deletions(-)

diff --git a/errors.go b/errors.go
index ddef369..e0f295e 100644
--- a/errors.go
+++ b/errors.go
@@ -34,6 +34,10 @@ func (m MultiError) Error() string {
 	return strings.Join(errs, "\n")
 }
 
+type ErrorFormatter interface {
+	Format(s fmt.State, verb rune)
+}
+
 // ExitCoder is the interface checked by `App` and `Command` for a custom exit
 // code
 type ExitCoder interface {
@@ -44,11 +48,11 @@ type ExitCoder interface {
 // ExitError fulfills both the builtin `error` interface and `ExitCoder`
 type ExitError struct {
 	exitCode int
-	message  string
+	message  interface{}
 }
 
 // NewExitError makes a new *ExitError
-func NewExitError(message string, exitCode int) *ExitError {
+func NewExitError(message interface{}, exitCode int) *ExitError {
 	return &ExitError{
 		exitCode: exitCode,
 		message:  message,
@@ -58,7 +62,7 @@ func NewExitError(message string, exitCode int) *ExitError {
 // Error returns the string message, fulfilling the interface required by
 // `error`
 func (ee *ExitError) Error() string {
-	return ee.message
+	return fmt.Sprintf("%v", ee.message)
 }
 
 // ExitCode returns the exit code, fulfilling the interface required by
@@ -77,7 +81,9 @@ func HandleExitCoder(err error) {
 	}
 
 	if exitErr, ok := err.(ExitCoder); ok {
-		if err.Error() != "" {
+		if _, ok := exitErr.(ErrorFormatter); ok {
+			fmt.Fprintf(ErrWriter, "%+v\n", err)
+		} else {
 			fmt.Fprintln(ErrWriter, err)
 		}
 		OsExiter(exitErr.ExitCode())
diff --git a/errors_test.go b/errors_test.go
index 04df031..e6dfc16 100644
--- a/errors_test.go
+++ b/errors_test.go
@@ -3,6 +3,7 @@ package cli
 import (
 	"bytes"
 	"errors"
+	"fmt"
 	"testing"
 )
 
@@ -104,3 +105,32 @@ func TestHandleExitCoder_ErrorWithoutMessage(t *testing.T) {
 	expect(t, called, true)
 	expect(t, ErrWriter.(*bytes.Buffer).String(), "")
 }
+
+// make a stub to not import pkg/errors
+type ErrorWithFormat struct {
+	error
+}
+
+func (f *ErrorWithFormat) Format(s fmt.State, verb rune) {
+	fmt.Fprintf(s, "This the format: %v", f.error)
+}
+
+func TestHandleExitCoder_ErrorWithFormat(t *testing.T) {
+	called := false
+
+	OsExiter = func(rc int) {
+		called = true
+	}
+	ErrWriter = &bytes.Buffer{}
+
+	defer func() {
+		OsExiter = fakeOsExiter
+		ErrWriter = fakeErrWriter
+	}()
+
+	err := &ErrorWithFormat{error: errors.New("I am formatted")}
+	HandleExitCoder(err)
+
+	expect(t, called, true)
+	expect(t, ErrWriter.(*bytes.Buffer).String(), "This the format: I am formatted\n")
+}