programming musings
23 Feb 2020

signel, a barebones signal chat on top of signal-cli

Unlike most chat systems in common use, Signal lacks a decent emacs client. All i could find was signal-msg, which is able only to send messages and has a readme that explicitly warns that its is not a chat application. Skimming over signal-msg's code i learnt about signal-cli, a java-based daemon that knows how to send and receive signal messages, and how to link to a nearby phone, or register new users. And playing with it i saw that it can output its activities formatted as JSON, and that offers (when run in daemon mode) a DBUS service that can be used to send messages.

Now, emacs knows how to run a process and capture its output handling it to a filter function, and comes equipped with a JSON parser and a set of built-in functions to talk to DBUS buses.

So how about writing a simple Signal chat app for emacs? Let's call it signel, and write it as a blog post in literate org-mode.

Starting a process

We are going to need a variable for our identity (telephone number), and a list of contact names (until i discover how to get them directly from signal-cli):

(require 'cl-lib)

(defvar signel-cli-user "+44744xxxxxx")
(defvar signel-contact-names '(("+447xxxxxxxx" . "john")
                               ("+346xxxxxxxx" . "anna")))

and a simple function to get a contact name given its telephone number:

(defun signel--contact-name (src)
  (or (alist-get src signel-contact-names nil nil #'string-equal) src))

We are also going to need the path for our signal-cli executable

(defvar signel-cli-exec "signal-cli")

Starting the signal-cli process is easy: make-process provides all the necessary bits. What we need is essentially calling

signal-cli -u +44744xxxxxx daemon --json

associating to the process a buffer selected by the function signel--proc-buffer . While we are at it, we'll write also little helpers for users of our API.

(defun signel--proc-buffer ()
  (get-buffer-create "*signal-cli*"))

(defun signel-signal-cli-buffer ()
  (get-buffer "*signal-cli*"))

(defun signel-signal-cli-process ()
  (when-let ((proc (get-buffer-process (signel-signal-cli-buffer))))
    (and (process-live-p proc) proc)))
(defun signel-start ()
  "Start the underlying signal-cli process if needed."
  (if (signel-signal-cli-process)
      (message "signal-cli is already running!")
    (let ((b (signel--proc-buffer)))
      (make-process :name "signal-cli"
                    :buffer b
                    :command `(,signel-cli-exec
                               "daemon" "--json")
                    :filter #'signel--filter)
      (message "Listening to signals!"))))

Parsing JSON

We've told emacs to handle any ouput of the process to the function signel--filter, which we're going to write next. This function will receive the process object and its latest output as a string representing a JSON object. Here's an example of the kind of outputs that signal-cli emits:

  "envelope": {
    "source": "+4473xxxxxxxx",
    "sourceDevice": 1,
    "relay": null,
    "timestamp": 1582396178696,
    "isReceipt": false,
    "dataMessage": {
      "timestamp": 1582396178696,
      "message": "Hello there!",
      "expiresInSeconds": 0,
      "attachments": [],
      "groupInfo": null
    "syncMessage": null,
    "callMessage": null,
    "receiptMessage": null

Everything seems to be always inside envelope, which contains objects for the possible messages received. In the example above, we're receiving a message from a source contact. We can also receive receipt messages, telling us whether our last message has been received or read; e.g.:

  "envelope": {
    "source": "+4473xxxxxxxx",
    "sourceDevice": 1,
    "relay": null,
    "timestamp": 1582397117584,
    "isReceipt": false,
    "dataMessage": null,
    "syncMessage": null,
    "callMessage": null,
    "receiptMessage": {
      "when": 1582397117584,
      "isDelivery": true,
      "isRead": false,
      "timestamps": [

A bit confusingly, that delivery notification has a receiptMessage, but its isReceipt flag is set to false. At other times, we get isReceipt but no receiptMessage:

  "envelope": {
    "source": "+346xxxxxxxx",
    "sourceDevice": 1,
    "relay": null,
    "timestamp": 1582476539281,
    "isReceipt": true,
    "dataMessage": null,
    "syncMessage": null,
    "callMessage": null,
    "receiptMessage": null

It is very easy to parse JSON in emacs and extract signal-cli's envelopes (and it's become faster in emacs 27, but the interface is a bit different):

(defun signel--parse-json (str)
  (if (> emacs-major-version 26)
      (json-parse-string str
                         :null-object nil
                         :false-object nil
                         :object-type 'alist
                         :array-type 'list)
    (json-read-from-string str)))

(defun signel--msg-contents (str)
  (alist-get 'envelope (ignore-errors (signel--parse-json str))))

Here i am being old-school and opting to receive JSON dicitionaries as alists (rather than hash maps, the default), and arrays as lists rather than vectors just because lisps are lisps for a reason. I'm also going to do some mild nil punning, hence the choice for null and false representations.

Once the contents of the envelope is extracted, it's trivial (and boring) to get into its components:

(defun signel--msg-source (msg) (alist-get 'source msg))

(defun signel--msg-data (msg)
  (alist-get 'message (alist-get 'dataMessage msg)))

(defun signel--msg-timestamp (msg)
  (if-let (msecs (alist-get 'timestamp msg))
      (format-time-string "%H:%M" (/ msecs 1000))

;; emacs 26 compat
(defun signel--not-false (x)
  (and (not (eq :json-false x)) x))

(defun signel--msg-receipt (msg)
  (alist-get 'receiptMessage msg))

(defun signel--msg-is-receipt (msg)
  (signel--not-false (alist-get 'isReceipt msg)))

(defun signel--msg-receipt-timestamp (msg)
  (when-let (msecs (alist-get 'when (signel--msg-receipt msg)))
    (format-time-string "%H:%M" (/ msecs 1000))))

(defun signel--msg-is-delivery (msg)
  (when-let ((receipt (signel--msg-receipt msg)))
    (signel--not-false (alist-get 'isDelivery msg))))

(defun signel--msg-is-read (msg)
  (when-let ((receipt (signel--msg-receipt msg)))
    (signel--not-false (alist-get 'isRead msg))))

A process output filter

We're almost ready to write our filter. It will:

  • For debugging purposes, insert the raw JSON string in the process buffer.
  • Parse the received JSON string and extract its envelope contents.
  • Check wether it has a source and either message data or a receipt timestamp.
  • Dispatch to a helper function that will insert the data or notification in a chat buffer.

Or, in elisp:

(defvar signel--line-buffer "")

(defun signel--filter (proc str)
  (signel--ordinary-insertion-filter proc str)
  (let ((str (concat signel--line-buffer str)))
    (if-let ((msg (signel--msg-contents str)))
        (let ((source (signel--msg-source msg))
              (stamp (signel--msg-timestamp msg))
              (data (signel--msg-data msg))
              (rec-stamp (signel--msg-receipt-timestamp msg)))
          (setq signel--line-buffer "")
          (when source
            (signel--update-chat-buffer source data stamp rec-stamp msg)))
      (setq signel--line-buffer
            (if (string-match-p ".*\n$" str) "" str)))))

We've had to take care of the case when the filter receives input that is not a complete JSON expression: in the case of signal-cli, that only happens when we haven't seen yet an end of line.

The function to insert the raw contents in the process buffer is surprisingly hard to get right, but the emacs manual spells out a reasonable implementation, which i just copied:

(defun signel--ordinary-insertion-filter (proc string)
  (when (and proc (buffer-live-p (process-buffer proc)))
    (with-current-buffer (process-buffer proc)
      (let ((moving (= (point) (process-mark proc))))
          ;; Insert the text, advancing the process marker.
          (goto-char (process-mark proc))
          (insert string)
          (set-marker (process-mark proc) (point)))
        (if moving (goto-char (process-mark proc)))))))

It's not an emacs app if it doesn't have a new mode

With that out of the way, we just have to insert our data in an appropriate buffer. We are going to associate a separate buffer to each source, using for that its name:

(defvar-local signel-user nil)

(defun signel--contact-buffer (source)
  (let* ((name (format "*%s" (signel--contact-name source)))
         (buffer (get-buffer name)))
    (unless buffer
      (setq buffer (get-buffer-create name))
      (with-current-buffer buffer
        (setq-local signel-user source)
        (insert signel-prompt)))

where, as is often the case in emacs, we are going to have a dedicated major mode for chat buffers, called signel-chat-mode. For now, let's keep it really simple (for the record, this is essentially a copy of what ERC does for its erc-mode):

(defvar signel-prompt ": ")

(define-derived-mode signel-chat-mode fundamental-mode "Signal"
  "Major mode for Signal chats."
  (when (boundp 'next-line-add-newlines)
    (set (make-local-variable 'next-line-add-newlines) nil))
  (setq line-move-ignore-invisible t)
  (set (make-local-variable 'paragraph-separate)
       (concat "\C-l\\|\\(^" (regexp-quote signel-prompt) "\\)"))
  (set (make-local-variable 'paragraph-start)
       (concat "\\(" (regexp-quote signel-prompt) "\\)"))
  (setq-local completion-ignore-case t))

Note how, in signel--contact-buffer, we're storing the user identity associated with the buffer (its source) in a buffer-local variable named signel-user that is set after enabling signel-chat-mode: order here matters because the major mode activation cleans up the values of any local variables previously set (i always forget that!).

And a customization group

We're going to need a couple of new faces for the different parts of inserted messages, so we'll take the chance to be tidy and introduce a customization group:

(defgroup signel nil "Signel")

(defface signel-contact '((t :weight bold))
  "Face for contact names."
  :group 'signel)

(defface signel-timestamp '((t :foreground "grey70"))
  "Face for timestamp names."
  :group 'signel)

(defface signel-notice '((t :inherit signel-timestamp))
  "Face for delivery notices."
  :group 'signel)

(defface signel-prompt '((t :weight bold))
  "Face for the input prompt marker."
  :group 'signel)

(defface signel-user '((t :foreground "orangered"))
  "Face for sent messages."
  :group 'signel)

(defface signel-notification '((t :foreground "burlywood"))
  "Face for notifications shown by tracking, when available."
  :group 'signel)

Displaying incoming messages

We have now almost all the ingredients to write signel--update-chat-buffer, the function that inserts the received message data into the chat buffer. Let's define a few little functions to format those parts:

(defun signel--contact (name)
  (propertize name 'face 'signel-contact))

(defun signel--timestamp (&rest p)
  (propertize (apply #'concat p) 'face 'signel-timestamp))

(defun signel--notice (notice)
  (propertize notice 'face 'signel-notice))

(defun signel--insert-prompt ()
  (let ((inhibit-read-only t)
        (p (point)))
    (insert signel-prompt)
    (set-text-properties p (- (point) 1)
                         '(face signel-prompt
                           read-only t front-sticky t rear-sticky t))))

(defun signel--delete-prompt ()
  (when (looking-at-p (regexp-quote signel-prompt))
    (let ((inhibit-read-only t))
      (delete-char (length signel-prompt)))))

(defun signel--delete-last-prompt ()
  (goto-char (point-max))
  (when (re-search-backward (concat "^" (regexp-quote signel-prompt)))

With that, we're finally ready to insert messages in our signel chat buffers:

(defcustom signel-report-deliveries nil
  "Whether to show message delivery notices."
  :group 'signel
  :type 'boolean)

(defcustom signel-report-read t
  "Whether to show message read notices."
  :group 'signel
  :type 'boolean)

(defun signel--prompt-and-notify ()
  (when (fboundp 'tracking-add-buffer)
    (tracking-add-buffer (current-buffer) '(signel-notification))))

(defun signel--needs-insert-p (data stamp rec-stamp msg)
  (or data
      (and (or rec-stamp stamp)
           (not (string= source signel-cli-user))
           (or signel-report-deliveries
               (and signel-report-read (signel--msg-is-read msg))))))

(defun signel--update-chat-buffer (source data stamp rec-stamp msg)
  (when (signel--needs-insert-p data stamp rec-stamp msg)
    (when-let ((b (signel--contact-buffer source)))
      (with-current-buffer b
        (if data
            (let ((p (point)))
              (insert (signel--timestamp "[" stamp "] ")
                      (signel--contact (signel--contact-name source))
              (fill-region p (point)))
          (let ((is-read (signel--msg-is-read msg)))
            (insert (signel--timestamp "*" (or rec-stamp stamp) "* ")
                    (signel--notice (if is-read "(read)" "(delivered)"))

There are some rough edges in the above implementation that must be polished should signel ever be released in the wild. For one, proper handling of timestamps and their formats. And of course notifications should be much more customizable (here i'm using Circe's tracking.el if available).

Sending messages: the DBUS interface

With that, we're going to receive and display messages and simple receipts, and i'm sure that we will feel the urge to answer some of them. As mentioned above, signal-cli let's us send messages via its DBUS interface. In a nutshell, if you want to send MESSAGETEXT to a RECIPIENT you'd invoke something like:

dbus-send --session --type=method_call \
          --dest="org.asamk.Signal" \
          /org/asamk/Signal \
          org.asamk.Signal.sendMessage \
          string:MESSAGETEXT array:string: string:RECIPIENT

That is, call the method sendMessage of the corresponding service interface with three arguments (the second one empty). Using emacs' dbus libray one can write the above as:

(defun signel--send-message (user msg)
  (dbus-call-method :session "org.asamk.Signal" "/org/asamk/Signal"
                    "org.asamk.Signal" "sendMessage"
                    :string msg
                    :string user))

The only complicated bit is being careful with the specification of the types of the method arguments: if one gets them wrong, DBUS will simply complain and say that the method is not defined, which was confusing me at first (but of course makes sense because DBUS allows overloading method names, so the full method spec must include its signature).

We want to read whatever our user writes after the last prompt and send it via the little helper above. Here's our interactive command for that:

(defun signel-send ()
  "Read text inserted in the current buffer after the last prompt and send it.

The recipient of the message is looked up in a local variable set
when the buffer was created."
  (goto-char (point-max))
  (let* ((p (point))
         (plen (length signel-prompt))
         (msg (buffer-substring (+ p plen) (point-max))))
    (signel--send-message signel-user msg)
    (insert (signel--timestamp (format-time-string "(%H:%M) ")))
    (fill-region p (point-max))
    (goto-char (point-max))
    (set-text-properties p (point) '(face signel-user))
    (insert "\n")

and we can bind it to the return key in signal chat buffers:

(define-key signel-chat-mode-map "\C-m" #'signel-send)

And we are going sometimes to want to talk to contacts that don't have yet said anything and have, therefore, no associated chat buffer:

(defun signel-query (contact)
  "Start a conversation with a signal contact."
  (interactive (list (completing-read "Signal to: "
                                      (mapcar #'cdr-safe signel-contact-names))))
  (let ((phone (alist-get contact
                          (cl-pairlis (mapcar #'cdr signel-contact-names)
                                      (mapcar #'car signel-contact-names))
                          nil nil #'string-equal)))
    (when (not phone)
      (error "Unknown contact %s" contact))
    (pop-to-buffer (signel--contact-buffer phone))))

There are of course lots of rough edges and missing functionality in this incipient signel, but it's already usable and a nice demonstration of how easy it is to get the ball rolling in this lisp machine of ours!

Tags: emacs
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