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+#+begin_src emacs-lisp
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+
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+; Warn and ask user before saving buffers that have shurnk too much
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+;
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+; My .org files usually only grow. If one shrinks much,
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+; I want to know/confirming it before saving.
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+; The code below accomplishes it this.
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+;
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+; There is a hack. before-save-hook provides no way for a hook
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+; function to abort the write (e.g. if the user answers "no").
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+; The only solution I could come up with to throw an "error"
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+; is to set the file read-only and let the rest of the write
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+; process complain. This is not elegant. Suggestions for
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+; improvment solicited.
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+
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+; Maybe do this instead. Or in addition
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+; https://karl-voit.at/2014/08/20/org-losses-determining-post-commit/
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+
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+;; Copyright © 2023 by George Jones
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+
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+;; Author: George Jones (http://port111.com/george)
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+;; Version:
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+;; Package-Version:
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+;; Package-Commit:
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+;; Created: 2023-01-15
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+;; Package-Requires:
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+;; Keywords:
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+;; License: GPL v3
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+;; URL: http://ergoemacs.org/emacs/xah-lookup.html
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+
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+;; License: GPL v3
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+
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+(defvar check-buffer-shrinkage-delta (* 1 1000)
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+ "Warn the user before saving is the buffer shrinks more than this many bytes")
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+(setq check-buffer-shrinkage-delta 100)
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+
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+(defvar check-buffer-shrinkage-name "org$"
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+ "Warn user before saving files matching this pattern that have shurnk")
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+
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+(defun check-buffer-size-on-save ()
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+ "Warn user before writing if buffer has shrunk too much since last save.
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+ Check for shrinkage large than CHECK-BUFFER-SHRINKAGE-DELTA bytes.
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+ Match file names matching CHECK-BUFFER-SHRINKAGE-NAME"
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+
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+ (if (string-match check-buffer-shrinkage-name (buffer-file-name))
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+ (let ((delta (- buffer-saved-size (buffer-size))))
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+ (when (> delta check-buffer-shrinkage-delta)
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+ (unless (y-or-n-p (format "WARNING: Buffer has shurnk by %d bytes since last save?" delta))
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+ (progn
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+ ; This is a hack. There appears to be no way to have
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+ ; a before-save-hook abort. I've tried user-error, throw,
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+ ; and debug-on-error. It ignores them all, so here once
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+ ; I've determined that I DON'T want the write to succeed
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+ ; I will set mode to read-only.
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+ (message (format "Setting %s to mode 444 to keep write from succeeding" (buffer-file-name)))
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+ (set-file-modes (buffer-file-name) #o444 'nofollow)
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+ ; This should abort, but is being ignored when hooks run.
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+ (user-error "Error: %s" "Not saved. User aborted.")
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+ ) ; progn
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+ ) ; unless
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+ ) ; when
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+ ) ; let
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+) ; if
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+) ; defun
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+
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+;; This debug is NOT being honored. No way to abort save
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+;; (setq debug-on-error t)
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+(add-hook 'before-save-hook 'check-buffer-size-on-save)
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+#+end_src
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