errors: enhance fatalError type to include underlying errors

internal/errors/fatal.go

@@ -7,25 +7,48 @@ import (
 
 // fatalError is an error that should be printed to the user, then the program
 // should exit with an error code.
-type fatalError string
+type fatalError struct {
+	msg string
+	err error // Underlying error
+}
 
-func (e fatalError) Error() string {
-	return string(e)
+func (e *fatalError) Error() string {
+	return e.msg
+}
+
+func (e *fatalError) Unwrap() error {
+	return e.err
 }
 
 // IsFatal returns true if err is a fatal message that should be printed to the
 // user. Then, the program should exit.
 func IsFatal(err error) bool {
-	var fatal fatalError
+	var fatal *fatalError
 	return errors.As(err, &fatal)
 }
 
 // Fatal returns an error that is marked fatal.
 func Fatal(s string) error {
-	return Wrap(fatalError(s), "Fatal")
+	return Wrap(&fatalError{msg: s}, "Fatal")
 }
 
-// Fatalf returns an error that is marked fatal.
+// Fatalf returns an error that is marked fatal, preserving an underlying error if passed.
 func Fatalf(s string, data ...interface{}) error {
-	return Wrap(fatalError(fmt.Sprintf(s, data...)), "Fatal")
+	// Use the last error found.
+	var underlyingErr error
+	for i := len(data) - 1; i >= 0; i-- {
+		if err, ok := data[i].(error); ok {
+			underlyingErr = err
+			break
+		}
+	}
+
+	msg := fmt.Sprintf(s, data...)
+
+	fatal := &fatalError{
+		msg: msg,
+		err: underlyingErr,
+	}
+
+	return Wrap(fatal, "Fatal")
 }