Obscure Emacs package appreciation: backup-walker
| emacs
The Emacs Carnival theme for September is obscure
packages, which made me think of how the
backup-walker package saved me from having to
write some code all over again. Something went
wrong when I was editing my config in Org Mode. I
probably accidentally deleted a subtree due to
over-enthusiastic speed commands. (… Maybe I
should make my k shortcut for
my-org-cut-subtree-or-list-item only work in my
Inbox.org and news.org files.) Chunks of my
literate Emacs configuration were gone, including
the code that defined my-org-insert-link-dwim.
Before I noticed, I'd already exported my (now
slightly shorter) Emacs configuration file with
org-babel-tangle and restarted Emacs. I couldn't
recover the definition from memory using
symbol-function. I couldn't use vundo to browse
the Emacs undo tree. As usual, I'd been neglecting
to commit my config changes to Git, so I couldn't
restore a previous version. Oops.
Well, not the first time I've needed to rewrite code from scratch because of a brain hiccup. I started to reimplement the function. Then I remembered that I had other backups. I have a 2 TB SSD in my laptop, and I had configured Emacs to neatly save numbered backups in a separate directory, keeping all the versions without deleting any of the old ones.
(setq backup-directory-alist '(("\\.env$" . nil)
("." . "~/.config/emacs/backups")))
(with-eval-after-load 'tramp
(setq tramp-backup-directory-alist nil))
(setq delete-old-versions -1)
(setq version-control t)
(setq auto-save-file-name-transforms '((".*" "~/.config/emacs/auto-save-list/" t)))
At the moment, there are about 12,633 files adding up to 3 GB. Totally worth it for peace of mind. I could probably use grep to search for the function, but it wasn't easy to see what changed between versions.
I had learned about backup-walker in the process
of writing about Thinking about time travel with
the Emacs text editor, Org Mode, and backups. So I
used backup-walker to flip through my file's
numbered backups in much the same way that
git-timemachine lets you flip through Git versions
of a file. After M-x backup-walker-start, I
tapped p to go through the previous backups. The
diff it showed me made it easy to check with C-s
(isearch-forward) if this was the version I was
looking for. When I found the change, I pressed
RET to load the version with the function in it.
Once I found it, it was easy to restore that
section. I also restored a couple of other
sections that I'd accidentally deleted too, like
the custom plain text publishing backend I use to
export Emacs News with less punctuation. It took
maybe 5 minutes to figure this out. Hooray for
backup-walker!
Note that the backup-walker diff was the other way
around from what I expected. It goes "diff new
old" instead of "diff old new", so the green
regions marked with + indicate stuff that was
removed by the newer version (compared to the
one a little older than it) and the red regions
marked with - indicate stuff that was added.
This could be useful if you think backwards in
time, kind of like the Emacs Antinews file, but my
mind doesn't quite work that way. I wanted it to
look like a regular diff, with the additions in
newer versions marked with +. Emacs being Emacs,
I changed it. Here's an example showing what it
looks like now:
The following code makes it behave the way I expect:
(defun my-backup-walker-refresh ()
(let* ((index (cdr (assq :index backup-walker-data-alist)))
(suffixes (cdr (assq :backup-suffix-list backup-walker-data-alist)))
(prefix (cdr (assq :backup-prefix backup-walker-data-alist)))
(right-file (concat prefix (nth index suffixes)))
(right-version (format "%i" (backup-walker-get-version right-file)))
diff-buff left-file left-version)
(if (eq index 0)
(setq left-file (cdr (assq :original-file backup-walker-data-alist))
left-version "orig")
(setq left-file (concat prefix (nth (1- index) suffixes))
left-version (format "%i" (backup-walker-get-version left-file))))
;; we change this to go the other way here
(setq diff-buf (diff-no-select right-file left-file nil 'noasync))
(setq buffer-read-only nil)
(delete-region (point-min) (point-max))
(insert-buffer diff-buf)
(set-buffer-modified-p nil)
(setq buffer-read-only t)
(force-mode-line-update)
(setq header-line-format
(concat (format "{{ ~%s~ → ~%s~ }} "
(propertize left-version 'face 'font-lock-variable-name-face)
(propertize right-version 'face 'font-lock-variable-name-face))
(if (nth (1+ index) suffixes)
(concat (propertize "<p>" 'face 'italic)
" ~"
(propertize (int-to-string
(backup-walker-get-version (nth (1+ index) suffixes)))
'face 'font-lock-keyword-face)
"~ ")
"")
(if (eq index 0)
""
(concat (propertize "<n>" 'face 'italic)
" ~"
(propertize (int-to-string (backup-walker-get-version (nth (1- index) suffixes)))
'face 'font-lock-keyword-face)
"~ "))
(propertize "<return>" 'face 'italic)
" open ~"
(propertize (propertize (int-to-string (backup-walker-get-version right-file))
'face 'font-lock-keyword-face))
"~"))
(kill-buffer diff-buf)))
(with-eval-after-load 'backup-walker
(advice-add 'backup-walker-refresh :override #'my-backup-walker-refresh))
backup-walker is not actually a real package in
the sense of M-x package-install, but
fortunately, recent Emacs makes it easier to
install from a repository. I needed to
install it from
https://github.com/lewang/backup-walker. It was
written so long ago that I needed to
defalias some functions that were removed in
Emacs 26.1. Here's the use-package snippet from my
configuration:
(use-package backup-walker
:vc (:url "https://github.com/lewang/backup-walker")
:commands backup-walker-start
:init
(defalias 'string-to-int 'string-to-number) ; removed in 26.1
(defalias 'display-buffer-other-window 'display-buffer))
So there's an obscure package recommendation: backup-walker. It hasn't been updated for more than a decade, and it's not even installable the regular way, but it's still handy.
I can imagine all sorts of ways this workflow
could be even better. It might be nice to dust off
backup-walker off, switch out the obsolete
functions, add an option for the diff direction,
and maybe sort things out so that you can reverse
the diff, split hunks, and apply hunks to your
original file. And maybe a way to walk the backup
history for changes in a specific region? I
suppose someone could make a spiffy
Transient-based user interface to modernize it.
But it's fine, it works. Maybe there's a more
modern equivalent, but I didn't see anything in a
quick search of M-x list-packages / N
(package-menu-filter-by-name-or-description) for
"backup~, except maybe vc-backup.1 Is there a general-purpose VC equivalent to
git-timemachine? That might be useful.
I should really be saving things in proper version
control, but this was a good backup. That reminds
me: I should backup my backup backups. I had
initially excluded my ~/.config directory from
borgbackup because of the extra bits and bobs that
I wouldn't need when restoring from backup (like
all the Emacs packages I'd just re-download). But
my file backups… Yeah, that's worth it. I
changed my --exclude-from to --patterns-from
and changing my borg-patterns file to look like
this:
+ /home/sacha/.config/emacs/backups - /home/sacha/.config/* # ... other rules
May backup-walker save you from a future oops!
Footnotes
vc-backup: The original repo is missing, but you can read it via ELPA's copy. Update: It's over on Codeberg now, and presumably the info on ELPA will be updated soon.
