errors: don't print an empty line for Exit with empty message

When HandleExitCoder received an ExitCoder whose Error() returned an
empty string (e.g. cli.Exit("", code)), it still called
fmt.Fprintln(ErrWriter, err) which wrote a bare newline to stderr.

Guard the print with a check on the error message so that an empty
message produces no output at all.

Fixes #2263

Signed-off-by: alliasgher <alliasgher123@gmail.com>

Author: alliasgher

errors.go

@@ -150,10 +150,12 @@ func HandleExitCoder(err error) {
 	}
 
 	if exitErr, ok := err.(ExitCoder); ok {
-		if _, ok := exitErr.(ErrorFormatter); ok {
-			_, _ = fmt.Fprintf(ErrWriter, "%+v\n", err)
-		} else {
-			_, _ = fmt.Fprintln(ErrWriter, err)
+		if msg := err.Error(); msg != "" {
+			if _, ok := exitErr.(ErrorFormatter); ok {
+				_, _ = fmt.Fprintf(ErrWriter, "%+v\n", err)
+			} else {
+				_, _ = fmt.Fprintln(ErrWriter, err)
+			}
 		}
 		OsExiter(exitErr.ExitCode())
 		return

errors_test.go

@@ -232,3 +232,28 @@ func TestErrRequiredFlags_Error(t *testing.T) {
 	expectedMsg = "Required flag \"flag1\" not set"
 	assert.Equal(t, expectedMsg, err.Error())
 }
+
+// TestHandleExitCoder_EmptyMessage is a regression test for
+// https://github.com/urfave/cli/issues/2263.
+// HandleExitCoder must not print an empty line to ErrWriter when the ExitCoder
+// message is empty (e.g. cli.Exit("", code)).
+func TestHandleExitCoder_EmptyMessage(t *testing.T) {
+	called := false
+
+	OsExiter = func(rc int) {
+		if !called {
+			called = true
+		}
+	}
+	ErrWriter = &bytes.Buffer{}
+
+	defer func() {
+		OsExiter = fakeOsExiter
+		ErrWriter = fakeErrWriter
+	}()
+
+	HandleExitCoder(Exit("", 2))
+
+	assert.True(t, called)
+	assert.Empty(t, ErrWriter.(*bytes.Buffer).String(), "expected no output for empty-message exit")
+}