Send process output to *Messages* buffer, but bypass the echo area

Question

Is it possible to send the output from a process-filter to the *Messages* buffer and suppress that message output from appearing in the echo area, such that I am able to simultaneously use interactive commands without having the minibuffer-prompt being erased by the ongoing subpress filter output?

(defun rsync-process-filter (proc string)
  (when (not (or
      (string-match "files...\r" string)
      (string-match "files to consider\n" string)))
    (message "%s" string)))

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EDIT (January 3, 2015):  The following is a link to a similar question, however, I have not yet been able to make it work with a process string where the exact string is unkown -- the title of the thread is: Emacs - Disable Some Minibuffer Messages:

https://superuser.com/questions/669701/emacs-disable-some-minibuffer-messages

Answer 1

First Rough Draft (January 3, 2015):  Revised initial draft based on the helpful comment of @phils regarding using the function messages-buffer to locate or create the appropriate buffer (and put it into messages-buffer-mode); and, added a check for whether point-max is at the beginning of the line (if not, then inset a new line before inserting the message string).

EDIT (January 4, 2015):  There are situations where the inserted string may not necessarily end in a new line, and the function message does not have a check to ensure it is at the beginning of a new line, so we take care of that in this function. Thus, at any point when message inserts a new line, said line will begin at flush-left of the buffer.

(defun rsync-process-filter (proc string)
  (let ((inhibit-read-only t))
    (when (not (or
        (string-match "files...\r" string)
        (string-match "files to consider\n" string)))
      (with-current-buffer (messages-buffer)
        (goto-char (point-max))
        (when (not (bolp))
          (insert "\n"))
        (insert string)
        (when (not (bolp))
          (insert "\n"))))))

Answer 2

You can suppress display in the minibuffer by setting minibuffer-message-timeout to 0.

For example, I use something like this in a few places where I want to toggle a minor mode while in a minibuffer prompt (like ido find-file) without being interrupted by a 'mode enabled' message:

(let ((minibuffer-message-timeout 0))
    (toggle-some-mode))

Answer 3

Going through the docstring of message it seems it should be possible to achieve what you want by calling message with a nil argument immediately after calling message with your desired content. From the docstring of message

If the first argument is nil or the empty string, the function clears any existing message; this lets the minibuffer contents show.

So modifying your function to something like the following should work

(defun rsync-process-filter (proc string)
  (when (not (or
      (string-match "files...\r" string)
      (string-match "files to consider\n" string)))
    (message "%s" string)
    (message nil)))

I tested it by doing the following

(defun test ()
  (message "%s" "Test")
  (message nil))

(run-at-time 5 5 #'test)

And it seems to work