How to add a locally override the message function?

Question

I would like to add a suffix to a message reported by a function called in some source I don't maintain.

Is there a way to locally override the message function so I can manipulate it before it's displayed? (add a prefix or suffix for example).

I know I can read the message log and log new message, but I'd rather avoid flooding the log, or manipulate the message buffer.

Answer 1

This macro adds support for adding a suffix to messages.

Using advice allows this to be nested, so multiple functions can add their own suffixes which accumulate onto the end.

(defmacro with-temp-advice (fn-orig where fn-advice &rest body)
  (declare (indent 3))
  (let ((function-var (gensym)))
    `(let ((,function-var ,fn-advice))
       (unwind-protect
           (progn
             (advice-add ,fn-orig ,where ,function-var)
             ,@body)
         (advice-remove ,fn-orig ,function-var)))))

(defmacro with-message-suffix (suffix &rest body)
  "Add text after the message output.
Argument SUFFIX is the text to add at the start of the message.
Optional argument BODY runs with the message suffix."
  (declare (indent 1))
  `(with-temp-advice
       'message
       :around
       (lambda (fn-orig arg &rest args)
         (apply fn-orig (append (list (concat arg "%s")) args (list ,suffix))))
     ,@body))