I was recently working on a porcelain for local database management in Emacs, tablemacs (name tbd). The general idea here is to give a magit style interface for interacting with a local database. This mode is built off SQLi (sql-interactive-mode) and uses a hidden comint buffer to execute commands. Everything was working great till I encountered a really weird issue. Let me preface everything with, I’m still very new to elisp and am still very much a beginner. Not only is it a radically different language than what I’m used to, the paradigms are also just very unique to emacs. If some of the code here looks wrong, it’s a mistake in translation as some of it was modified for ease of understanding.

The process & the Issue

Right now tablemacs creates a hidden comint buffer with sql-interactive-mode engaged. I then use comint-redirect-send-command-to-process which redirects the output of a comint command to an aribtrary buffer, which is my tablemacs-status buffer.

(comint-redirect-send-command-to-process COMMAND OUTPUT-BUFFER PROCESS ECHO &optional NO-DISPLAY)


Send COMMAND to PROCESS, with output to OUTPUT-BUFFER. With prefix arg, echo output in process buffer. If NO-DISPLAY is non-nil, do not show the output buffer.

This works as you’d expect however, there’s some artifacts in the output. Here’s what I get for my show-tables command which just runs show tables;:

show tables;^ M
+--------------------------+^ M
| Tables_in_tablemacs_test |
+--------------------------+^ M
| test_table               |^ M
+--------------------------+^ M

All those =^M=s means it’s displaying the carriage returns in the redirected buffer. Obviously, I wanted to remove those.

I searched around for something that could help and I had already known about comint filters. These allow you to run filter functions on the strings as they or after they’ve interacted with the comint buffer. Here’s a non-comprehensive list of a few of the available “filters” list variables you can add filter functions too:

  • comint-input-filter-functions
  • comint-output-filter-functions
  • comint-preoutput-filter-functions
  • comint-redirect-filter-functions
  • comint-redirect-original-filter-function

There’s a few more but those are the ones that were interesting to me in this situation. Looking at the documentation, comint-redirect-filter-functions seemed perfect.

List of functions to call before inserting redirected process output. Each function gets one argument, a string containing the text received from the subprocess. It should return the string to insert, perhaps the same string that was received, or perhaps a modified or transformed string.

The functions on the list are called sequentially, and each one is given the string returned by the previous one. The string returned by the last function is the text that is actually inserted in the redirection buffer.

You can use `add-hook’ to add functions to this list either globally or locally.

Seems ok so far! So I plugged it in with:

(add-hook 'tablemacs-minor-mode-hook (lambda () (push 'comint-strip-ctrl-m comint-redirect-filter-functions) ))

It did not work.


I then moved to setting the comint-redirect-filter-functions globally and still it did not work. I thought surely I was doing something wrong, but when I used describe-variable on comint-redirect-filter-functions it appeared to have comint-strip-ctrl-m as it should. I’m still a beginner when it comes to elisp so I thought I was doing something wrong. So I wrote my own filter just to see:

(defun tablemacs--comint-strip-ctrl-m-test (str)
  "test filter"
  (message "ran filter!")

Low and behold I got the message in my minibuffer. So what gives?

Well the next thing to do was to look at describe-function for comint-strip-ctrl-m which is as follows:

(defun comint-strip-ctrl-m (&optional _string interactive)
  "Strip trailing `^M' characters from the current output group.
This function could be on `comint-output-filter-functions' or bound to a key."
  (interactive (list nil t))
  (let ((process (get-buffer-process (current-buffer))))
    (if (not process)
        ;; This function may be used in
        ;; `comint-output-filter-functions', and in that case, if
        ;; there's no process, then we should do nothing.  If
        ;; interactive, report an error.
        (when interactive
          (error "No process in the current buffer"))
      (let ((pmark (process-mark process)))
          (condition-case nil
	       (if interactive
	           comint-last-input-end comint-last-output-start))
	    (error nil))
          (while (re-search-forward "\r+$" pmark t)
	    (replace-match "" t t)))))))

Herein lies the culprit. This filter takes in an &optional _string and usually, variables prefixed with _ means they aren’t used. So if it’s not using the passed in string, what’s it doing? Well it’s using (get-buffer-process (current-buffer)) and then marking where the process command output starts and then searching through with (research-forward "\r+$" pmark t) which is what actually replaces the carriage returns. The big red flag here is that it’s using the (current-buffer) which, in my use case, isn’t the buffer that the process is running in, instead its my porcelein buffer.

So the issue turned out to be the implementation of comint-strip-ctrl-m and not the way I was using it.

What to do next?

It’s pretty clear to me that the function of `comint-strip-ctrl-m` doesn’t match the documentation. Emacs documentation is exceptional compared to anything else I’ve used, I mean it’s known as the “self documenting text editor” for a reason. However, this is a very specific case where the documentation, or expected implicit behavior derrived from the documentation, doesn’t line up with reality. So what should I do?

My fix

In my code, I just wrote my own filter to do exactly comint-strip-ctrl-m should do. It looks like this:

(defun tablemacs--comint-strip-ctrl-m (str)
  "Filter function to remove carriage returns from comint output
   This is needed because one provided by comint rely's on `current-buffer`
   to get the process and it's always going to be wrong."
  (replace-regexp-in-string "\r" "" str))

So this now works with comint-redirect-filter-functions as expected.

show tables;
| Tables_in_tablemacs_test |
| test_table               |

Emacs bug report?

At this point I’m considering filing a report, or at least a request to update the documentation for this rather specific small bug. It’s not like this is a huge breaking bug for most users, and it’s a pretty specific use case. But this might open up a potential contribution opporunity or at least a way to get involved with the emacs maintainer community at least a little bit. Possible fixes could consist of one of the following:

  1. Updating the documentation for comint-strip-ctrl-m to explictely state it uses current-buffer instead of just the passed in string.
  2. Updating comint-strip-ctrl-m to actually use the string it’s passed and perform the same string editing functions.
  3. Creating a new comint-strip-ctrl-m-filter (name TBD?) which takes in a string, modifies it and returns a modified string.

I don’t know. Maybe someone will let me know if this is in fact an issue or if I’m just missing something else important.

Happy Hacking.