buffer-snapshotter

Minor mode that keeps snapshots of changed versions buffers (visiting files or not) written to disk, and limits those snapshots by number or time

This was written because there are certain kinds of emacs editing situations where you have a buffer that you want essentially auto-saving, but because it's not visiting a file none of the internal mechanisms will cleanly do it.

It runs some timers and when one goes off it will check the (buffer-chars-modified-tick) to work out if it needs to save a snapshot or if the buffer remains unchanged, it only snapshots changed buffers with this mode enabled.

Notably this can be used to protect Emacs Atomic Chrome buffers.

It's pretty simple and should depend on nothing external to Emacs but it was written and tested against an Emacs 29.1 install so caution is recommended if you're using something else. It should be ok from some basic use and testing.

Built in Emacs features and other packages to consider first

If you want to have a saved copy of the file your buffer is visiting consider using Auto-Save and auto-save-mode.

If you want to automatically write back to the original file your buffer is visiting regularly then look at auto-save-visiting-mode.

If you want to save a backup of the original version of a file (before your changes) when you write to it look at the make-backup-files variable, you can also make Emacs save multiple versions of each file by changing the version-control variable, read more about Backup Files in Emacs.

If you want persistant undo/redo across sessions then look at undo-fu-session (optionally with something like vundo) or undo-tree.

buffer-snapshotter is for essentially doing a sort of mix of an auto-save with a versioned backup and works on buffers not linked to visiting files.

Activating buffer-snapshotter mode

Essentially all you need is this:

(use-package buffer-snapshotter-mode)

Then M-x buffer-snapshotter-mode in a buffer you feel needs this mode.

If you want it to clean up old snapshot files whenever you start Emacs then try something like this:

(use-package buffer-snapshotter-mode
    :config
    (buffer-snapshotter-cleanup-directory))

Where (buffer-snapshotter-cleanup-directory) goes through the snapshots directory and deletes every file older than buffer-snapshotter-maximum-age seconds, this defaults to 259200 so should cleanup all files older than 3 days.

If you want it to automatically start for certain modes use tricks like this:

(add-hook 'atomic-chrome-edit-mode-hook
           (lambda () (buffer-snapshotter-mode 1)))

If you want it to automatically run globally for every buffer then use something like this (by default it will refuse to run in minibuffer mode or any file accessed via EasyPG Assistant):

(add-hook 'after-change-major-mode-hook #'buffer-snapshotter-mode)

Configuring buffer-snapshotter-mode

Interesting variables to play around with and read the docstrings of include:

• buffer-snapshotter-directory the directory to store snapshots in, defaults to "buffer-snapshots" under the users user-emacs-directory
• buffer-snapshotter-cleanup-method-function allows you to pick cleanup method, it defaults to #'buffer-snapshotter-delete-excess-by-number which keeps N versions of each files snapshots at most, set to #'buffer-snapshotter-delete-excess-by-time if you want to keep snapshots for a specific amount of time, or create your own function.
• buffer-snapshotter-keep-versions numbers of versions to keep if keeping by number
• buffer-snapshotter-keep-time seconds to keep a snapshot, defaults to 21600 so 6 hours.
• buffer-snapshotter-frequency seconds of idle time before checking to see if a snapshot is worth taking, defaults to 30s.
• buffer-snapshotter-force-frequency seconds of time before checking to see if a snapshot is worth taking idle or not, defaults to 600s (10 minutes).
• buffer-snapshotter-namegen-function should be a #'function-symbol to a function used to generate consistent and filesystem safe names for snapshot files, the default replaces characters with their codepoint numbers for everything not matching [a-zA-Z0-9_-]
• buffer-snapshotter-include-hostname if snapshotting a buffer this will include the hostname into the name generation by default, this is to help distinguish between similarly named buffers on different machines if you synchronise your Emacs directory. If you want to turn off this behaviour so that different machines can see the same snapshots linked to the same named buffers then set this to nil.
• buffer-snapshotter-never-activate-mode-list list of modes (both major and minor) that if active in a buffer will cause it to not activate.
• buffer-snapshotter-never-activate-function function that if it returns t will cause it to not activate.

There are also two hooks to append to:

• buffer-snapshotter-mode-after-hook runs after the mode is activated/deactivated.
• buffer-snapshotter-save-copy-after-hook runs after a new snapshot file is created.