Bug#105889: dpkg-dev-el; Adding hooks to enter mode for Debian native logs

[Peter S Galbraith <psg@debian.org>]

Package: dpkg-dev-el
Version: 1.42-1
Severity: wishlist

Background:

 There is often a Local variables block in Debian changelogs
 which debian-changelog-mode has been partially deleting in
 recent versions.

 The "mode: debian-changelog" line is still useful for Debian
 native packages because the mode then gets invoked properly when
 you edit an installed log file,
 e.g. /usr/share/doc/debiandoc-sgml/changelog.gz (It's not named
 changelog.Debian.gz so we can't autoload based on the name)

Discussion:

 Following a discussion on debian-devel, I received two ideas
 that would let debian-changelog-mode offer to delete the block
 completely as it would no longer add any benefit:

 Alan Shutko wrote:

 > I'd put something in change-log-mode-hook which does the check, and
 > sets major-mode appropriately.  Unfortunately, interpreter-mode-alist
 > is specific to files which start with #!.

 Tollef Fog Heen wrote:

 > Something like:
 > 
 > (setq find-file-hooks
 >       (cons #'(lambda ()
 > 	       ;; Invoke proper modes when we don't know file extensions
 > 	       (cond ((looking-at "#!.*/perl") (perl-mode))
 > 		     ((looking-at "#!.*/tclsh") (tcl-mode))
 > 		     ((looking-at "#!.*/wish") (tcl-mode))))
 > 	    find-file-hooks))
 > 
 > And add the proper regexp for Debian changelogs.

Solution:

 If this function were added to /etc/emacs/site-start.d/50dpkg-dev-el.el :

 (defun debian-changelog-find-file-hook ()
   (if (looking-at
        "\\S-+ +(\\([0-9]:\\)?[0-9][0-9a-zA-Z.+:]+\\(-\\([0-9a-zA-Z.+]+\\)\\)?) +[^\n]*")
       (debian-changelog-mode)))

 We could then use either of these hooks in the file as well:

 (add-hook 'find-file-hooks 'debian-changelog-find-file-hook t)
 (add-hook 'change-log-mode-hook 'debian-changelog-find-file-hook t)

 Note that the name of the function assumes we'd pick the first
 one, otherwise I'd change it.

 The first hook catches the process early on.  For _every_ file
 loaded into Emacs, the first line is compared to the regexp and
 if it matches debian-changelog-mode is entered.

 The second hook assumes that change-log-mode will end up catching
 the log file.  After the mode is fully loaded, the hook checks
 the first line of the file for a match against the regexp and
 enters debian-changelog-mode if matched.  The advantage is that
 only log files will be checked, and not all files visited by
 Emacs.  The disadvantage is that change-log-mode might have setup
 unwanted variables that are not undone before entering
 debian-changelog-mode. 

 What do you think?
 
Peter