signel is in the blog

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-#+title: signel, a barebones signal chat on top of signal-cli
-#+date: <2020-02-23 05:03>
-#+filetags: emacs norss
-#+PROPERTY: header-args :tangle yes :comments yes :results silent
-
-Unlike most chat systems in common use, [[https://signal.org][Signal]] lacks a decent emacs
-client.   All i could find was [[https://github.com/mrkrd/signal-msg][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
-[[https://github.com/AsamK/signal-cli][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 [[https://gitlab.com/jaor/elibs/-/blob/master/net/signel.org][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):
-
-#+begin_src emacs-lisp
-(require 'cl-lib)
-
-(defvar signel-cli-user "+44744xxxxxx")
-(defvar signel-contact-names '(("+447xxxxxxxx" . "john")
-                               ("+346xxxxxxxx" . "anna")))
-#+end_src
-
-and a simple function to get a contact name given its telephone
-number:
-
-#+begin_src emacs-lisp
-(defun signel--contact-name (src)
-  (or (alist-get src signel-contact-names nil nil #'string-equal) src))
-#+end_src
-
-We are also going to need the path for our signal-cli executable
-
-#+begin_src emacs-lisp
-(defvar signel-cli-exec "signal-cli")
-#+end_src
-
-Starting the signal-cli process is easy: ~make-process~ provides all the
-necessary bits. What we need is essentially calling
-
-#+begin_src shell
-signal-cli -u +44744xxxxxx daemon --json
-#+end_src
-
-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.
-
-#+begin_src emacs-lisp
-(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)))
-#+end_src
-
-#+begin_src emacs-lisp
-(defun signel-start ()
-  "Start the underlying signal-cli process if needed."
-  (interactive)
-  (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
-                               "-u"
-                               ,signel-cli-user
-                               "daemon" "--json")
-                    :filter #'signel--filter)
-      (message "Listening to signals!"))))
-#+end_src
-
-* 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:
-
-#+begin_src json :tangle no
-{
-  "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
-  }
-}
-#+end_src
-
-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.:
-
-#+begin_src json :tangle no
-{
-  "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": [
-        1582397111524
-      ]
-    }
-  }
-}
-#+end_src
-
-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~:
-
-#+begin_src json :tangle no
-{
-  "envelope": {
-    "source": "+346xxxxxxxx",
-    "sourceDevice": 1,
-    "relay": null,
-    "timestamp": 1582476539281,
-    "isReceipt": true,
-    "dataMessage": null,
-    "syncMessage": null,
-    "callMessage": null,
-    "receiptMessage": null
-  }
-}
-#+end_src
-
-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):
-
-#+begin_src emacs-lisp
-(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))))
-#+end_src
-
-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 [[https://lispcast.com/nil-punning/][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:
-
-#+begin_src emacs-lisp
-(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))))
-#+end_src
-
-* 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:
-
-#+begin_src emacs-lisp
-(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)))))
-#+end_src
-
-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:
-
-#+begin_src emacs-lisp
-(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))))
-        (save-excursion
-          ;; 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)))))))
-#+end_src
-
-* 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:
-
-#+begin_src emacs-lisp
-(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
-        (signel-chat-mode)
-        (setq-local signel-user source)
-        (insert signel-prompt)))
-    buffer))
-#+end_src
-
-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):
-
-#+begin_src emacs-lisp
-(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))
-#+end_src
-
-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:
-
-#+begin_src emacs-lisp
-(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)
-
-#+end_src
-
-
-* 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:
-
-#+begin_src emacs-lisp
-(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)))
-    (signel--delete-prompt)))
-
-#+end_src
-
-With that, we're finally ready to insert messages in our signel chat
-buffers:
-
-#+begin_src emacs-lisp
-(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 ()
-  (signel--insert-prompt)
-  (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
-        (signel--delete-last-prompt)
-        (if data
-            (let ((p (point)))
-              (insert (signel--timestamp "[" stamp "] ")
-                      (signel--contact (signel--contact-name source))
-                      signel-prompt
-                      data
-                      "\n")
-              (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)"))
-                    "\n")))
-        (signel--prompt-and-notify)
-        (end-of-line)))))
-#+end_src
-
-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 [[https://github.com/jorgenschaefer/circe/blob/master/tracking.el][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
-[[https://github.com/AsamK/signal-cli/wiki/DBus-service][DBUS interface]].
-In a nutshell, if you want to send ~MESSAGETEXT~ to a
-~RECIPIENT~ you'd invoke something like:
-
-#+begin_src shell :tangle no
-dbus-send --session --type=method_call \
-          --dest="org.asamk.Signal" \
-          /org/asamk/Signal \
-          org.asamk.Signal.sendMessage \
-          string:MESSAGETEXT array:string: string:RECIPIENT
-#+end_src
-
-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:
-
-#+begin_src emacs-lisp
-(defun signel--send-message (user msg)
-  (dbus-call-method :session "org.asamk.Signal" "/org/asamk/Signal"
-                    "org.asamk.Signal" "sendMessage"
-                    :string msg
-                    '(:array)
-                    :string user))
-#+end_src
-
-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:
-
-#+begin_src emacs-lisp
-(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."
-  (interactive)
-  (goto-char (point-max))
-  (beginning-of-line)
-  (let* ((p (point))
-         (plen (length signel-prompt))
-         (msg (buffer-substring (+ p plen) (point-max))))
-    (signel--delete-prompt)
-    (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")
-    (signel--insert-prompt)))
-#+end_src
-
-and we can bind it to the return key in signal chat buffers:
-
-#+begin_src emacs-lisp
-(define-key signel-chat-mode-map "\C-m" #'signel-send)
-#+end_src
-
-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:
-
-#+begin_src emacs-lisp
-(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))))
-#+end_src
-
-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!