Subject: bug#58797: 29.0.50; Revise format of stored message tags in ERC

From: "J.P." <[email-masked]>
Date: Wed, 26 Oct 2022 06:20:32 -0700

Tags: patch

I'm proposing we change the format (type) of the "tags" field in the
`erc-response' struct from

  (STRING . LIST)

where LIST contains at most one (possibly empty) string, to

  (SYMBOL . OPT-STRING)

where OPT-STRING, when non-nil, is a nonempty string. For ERC 5.5 and
Emacs 29, this change would merely manifest as a warning when first
encountered. In subsequent releases, it would still be reserved only for
users who opt in by activating optional modules. Rationale below.

Thanks,
J.P.

Imagined FAQ:

  Why bother?

    In the existing implementation, all code accessing a processed tag
    value must check that it's both non-nil and nonempty. This stands to
    become a common occurrence with the arrival of IRCv3 in future
    releases. The majority of instances will only contain one item (a
    timestamp) and will persist beyond the parsing stage as text
    properties in ERC buffers. And since we need to redo the parsing
    anyway to abide by all the escaping rules and other spec minutiae,
    we might as well make the processed goods as easy to manage as
    possible, no?

  Isn't this a breaking change?

    Yes. But I believe preserving backward compatibility (by adding an
    additional field and keeping the old one around) is less important
    in this case because the field currently goes unused in ERC's client
    code and is likely underused in user code (based on a cursory survey
    of Melpa packages, popular shared configs, and wiki snippets). While
    message tags *are* currently parsed when present, the field is only
    ever written to and never again accessed (on account of ERC's lack
    of general IRCv3 support). That said, the library function as well
    as the `erc-response' structure are indeed public (exported).

    There is also the matter of authoritative intent to consider. The
    doc string for the function `erc-parse-tags' describes its return
    type as being an alist of "(tag . value)". For anyone willing to
    entertain "value" as meaning "logical tag value," this change
    becomes a belated bug fix.

  Why now? Why not wait?

    It's true that the main benefit of doing this won't be felt for a
    couple more ERC releases. As stated above, the immediate effect of
    this change will merely be as a warning for any user code that
    depends on this field. This approach seems somewhat in keeping with
    the overall Emacs policy for function and variable deprecation.

In GNU Emacs 29.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version
 3.24.34, cairo version 1.17.6) of 2022-10-26 built on localhost
Repository revision: 832bd2dfe51b2b9ea5e0e56e0e6ee60b2d8ae2ed
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12014000
System Description: Fedora Linux 36 (Workstation Edition)

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>From 44cb69befc28ae4e14499521fb2f8a09d7acaf5c Mon Sep 17 00:00:00 2001
From: "F. Jason Park" <[email-masked]>
Date: Mon, 24 Oct 2022 22:58:13 -0700
Subject: [PATCH] Warn of future breaking change to erc-response.tags

* lisp/erc/erc-backend.el (erc-parse-tags-format): New option to
determine type of the `erc-response' "tags" field.
(erc-parse-tags): Defer to internal generic function.
(erc--parse-tags): New function to hold original `erc-parse-tags'
implementation.
(erc--parse-message-tags): New generic function that conditionally
calls `erc--parse-tags', perhaps emitting a warning beforehand.
(erc-parse-server-response): Call `erc--parse-message-tags'.
---
 lisp/erc/erc-backend.el | 59 ++++++++++++++++++++++++++++++++++++++---
 1 file changed, 56 insertions(+), 3 deletions(-)

diff --git a/lisp/erc/erc-backend.el b/lisp/erc/erc-backend.el
index df9efe4b0c..9854e863c3 100644
--- a/lisp/erc/erc-backend.el
+++ b/lisp/erc/erc-backend.el
@@ -992,8 +992,39 @@ erc-send-ctcp-notice
 
 ;;;; Handling responses
 
+(defcustom erc-tags-format 'overridable
+  "Shape of the `tags' alist in `erc-response' objects.
+When set to `legacy', pre-5.5 parsing behavior takes effect for
+the tags portion of every message.  The resulting alist contains
+conses of the form (STRING . LIST), in which LIST is comprised of
+at most one (possibly empty) string.
+
+When nil, ERC only parses tags if an active module defines an
+implementation.  It otherwise ignores them.  In such cases, each
+alist element is a cons of a symbol and an optional, non-empty
+string.
+
+With the default value of `overridable', ERC behaves as it does
+with `legacy' except that it emits a warning whenever first
+encountering a message containing tags in a given Emacs session.
+But it only does so when a module implementing overriding,
+non-legacy behavior isn't already active in the current network
+context.
+
+Note that future bundled modules providing IRCv3 functionality
+may not be compatible with the legacy format.  User code should
+eventually transition to expecting this \"5.5+ variant\" and set
+this option to nil."
+  :package-version '(ERC . "5.4.1") ; FIXME increment on next release
+  :type '(choice (const nil)
+                 (const legacy)
+                 (const overridable)))
+
 (defun erc-parse-tags (string)
   "Parse IRCv3 tags list in STRING to a (tag . value) alist."
+  (erc--parse-message-tags string))
+
+(defun erc--parse-tags (string)
   (let ((tags)
         (tag-strings (split-string string ";")))
     (dolist (tag-string tag-strings tags)
@@ -1003,6 +1034,28 @@ erc-parse-tags
                 `(,pair))
               tags)))))
 
+;; One benefit of this function being internal is to avoid having to
+;; define a separate method just to ensure an `erc-tags-format' value
+;; of `legacy' always wins.  The downside is that module code must
+;; take care to preserve that promise manually.
+
+(cl-defgeneric erc--parse-message-tags (string)
+  "Parse STRING into an alist of (TAG . VALUE) conses.
+TAG is a symbol.  VALUE is nil or a non-empty string.  Composite
+raw input values containing commas are never split but always
+left as a single string."
+  (when erc-tags-format
+    (unless (or (eq erc-tags-format 'legacy)
+                (get 'erc-parse-tags 'erc-v3-warned-p))
+      (put 'erc-parse-tags 'erc-v3-warned-p t)
+      (display-warning
+       'ERC
+       (concat
+        "Legacy ERC tags behavior is currently in effect, but other modules,"
+        " including those bundled with ERC, may override this in future"
+        " releases.  See `erc-tags-format' for more info.")))
+    (erc--parse-tags string)))
+
 (defun erc-parse-server-response (proc string)
   "Parse and act upon a complete line from an IRC server.
 PROC is the process (connection) from which STRING was received.
@@ -1012,9 +1065,9 @@ erc-parse-server-response
       (let* ((tag-list (when (eq (aref string 0) ?@)
                          (substring string 1
                                     (string-search " " string))))
-             (msg (make-erc-response :unparsed string :tags (when tag-list
-                                                              (erc-parse-tags
-                                                               tag-list))))
+             (msg (make-erc-response :unparsed string :tags
+                                     (when tag-list
+                                       (erc--parse-message-tags tag-list))))
              (string (if tag-list
                          (substring string (+ 1 (string-search " " string)))
                        string))
-- 
2.37.3