master 8a149966dd5 2/5: Add command to view ERC-NEWS

[F. Jason Park]

branch: master
commit 8a149966dd50a801abe25e97d446ce363252cdf9
Author: F. Jason Park <jp@neverwas.me>
Commit: F. Jason Park <jp@neverwas.me>

    Add command to view ERC-NEWS
    
    * etc/ERC-NEWS: Mention `erc-news' command.
    * lisp/erc/erc.el (erc--news-url, erc--news-temp-file, erc-news): Add
    new command and supporting variable.
---
 etc/ERC-NEWS    |  7 ++++---
 lisp/erc/erc.el | 45 +++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 49 insertions(+), 3 deletions(-)

diff --git a/etc/ERC-NEWS b/etc/ERC-NEWS
index d257bdcbf51..5e5779cfcd5 100644
--- a/etc/ERC-NEWS
+++ b/etc/ERC-NEWS
@@ -136,9 +136,10 @@ Some minor quality-of-life niceties have finally made 
their way to
 ERC.  For example, the function 'erc-echo-timestamp' is now
 interactive and can be invoked on any message to view its timestamp in
 the echo area.  The command 'erc-button-previous' now moves to the
-beginning instead of the end of buttons.  And the 'irccontrols' module
-now supports additional colors and special handling for "spoilers"
-(hidden text).
+beginning instead of the end of buttons.  A new command, 'erc-news',
+can now be invoked to visit this very file.  And the 'irccontrols'
+module now supports additional colors and special handling for
+"spoilers" (hidden text).
 
 ** Changes in the library API.
 
diff --git a/lisp/erc/erc.el b/lisp/erc/erc.el
index 0be9eb69432..aad6f7d9d54 100644
--- a/lisp/erc/erc.el
+++ b/lisp/erc/erc.el
@@ -7428,6 +7428,51 @@ If BUFFER is nil, update the mode line in all ERC 
buffers."
     (goto-char (point-min))
     (insert "X-Debbugs-CC: emacs-erc@gnu.org\n")))
 
+(defconst erc--news-url
+  "https://git.savannah.gnu.org/cgit/emacs.git/plain/etc/ERC-NEWS";)
+
+(defvar erc--news-temp-file nil)
+
+(defun erc-news (arg)
+  "Show ERC news in a manner similar to `view-emacs-news'.
+With ARG, download and display the latest revision, which may
+contain more up-to-date information, even for older versions."
+  (interactive "P")
+  (find-file
+   (or (and arg erc--news-temp-file
+            (time-less-p (current-time) (car erc--news-temp-file))
+            (cdr erc--news-temp-file))
+       (and arg
+            (with-current-buffer (url-retrieve-synchronously erc--news-url)
+              (goto-char (point-min))
+              (search-forward "200 OK" (pos-eol))
+              (search-forward "\n\n")
+              (delete-region (point-min) (point))
+              (let ((tempfile (make-temp-file "erc-news.")))
+                (write-region (point-min) (point-max) tempfile)
+                (kill-buffer)
+                (cdr (setq erc--news-temp-file
+                           (cons (time-add (current-time) (* 60 60 12))
+                                 tempfile))))))
+       (and-let* ((file (or (eval-when-compile (macroexp-file-name))
+                            (locate-library "erc")))
+                  (dir (file-name-directory file))
+                  (adjacent (expand-file-name "ERC-NEWS" dir))
+                  ((file-exists-p adjacent)))
+         adjacent)
+       (expand-file-name "ERC-NEWS" data-directory)))
+  (when (fboundp 'emacs-news-view-mode)
+    (emacs-news-view-mode))
+  (let ((v (mapcar #'number-to-string
+                   (seq-take-while #'natnump (version-to-list erc-version)))))
+    (while (and v
+                (goto-char (point-min))
+                (not (search-forward (concat "\014\n* Changes in ERC "
+                                             (string-join v "."))
+                                     nil t)))
+      (setq v (butlast v))))
+  (beginning-of-line))
+
 (defun erc-port-to-string (p)
   "Convert port P to a string.
 P may be an integer or a service name."