[elpa] externals/undo-tree 4dd6905 028/195: Discard marker adjustment undo entries.

[Stefan Monnier]

branch: externals/undo-tree
commit 4dd690526560c56821073bd67e2e787fe11e0c2d
Author: tsc25 <tsc25@cantab.net>
Commit: tsc25 <tsc25@cantab.net>

    Discard marker adjustment undo entries.
    
    In buffer-undo-list, these entries are treated specially by Emacs'
    garbage-collection code in alloc.c (they don't count during the mark phase,
    and the sweep phase deletes entries for garbage-collected markers). But we
    can't emulate this behaviour from Elisp.
    
    Discarding these entries at least prevents dead markers being resurrected by
    undo-tree-mode, and allows them to be garbage-collected. Standard Emacs undo
    fails to restore markers correctly anyway (byg#4803), so the impact should
    hopefully be small.
---
 undo-tree.el | 33 +++++++++++++++++++++++++++++----
 1 file changed, 29 insertions(+), 4 deletions(-)

diff --git a/undo-tree.el b/undo-tree.el
index 1e5bf25..e34f712 100644
--- a/undo-tree.el
+++ b/undo-tree.el
@@ -406,6 +406,26 @@
 ;; and it can optionally display time-stamps for each buffer state. (There's
 ;; one other tiny difference: the visualizer puts the most recent branch on
 ;; the left rather than the right.)
+;;
+;;
+;;
+;; Drawbacks
+;; =========
+;;
+;; Emacs' undo system is deeply embedded in Emacs. In particular, garbage
+;; collection treats the `buffer-undo-list' specially: references to markers
+;; in `buffer-undo-list' don't count during the mark phase, and the sweep
+;; phase removes undo entries for markers that have been garbage-collected.
+;; This behaviour is implemented in C as part of the garbage collection code,
+;; and it is difficult or impossible to emulate in Elisp.
+;;
+;; To avoid dead markers being resurrected in `undo-tree-mode', and to allow
+;; them to be garbage-collected, `undo-tree-mode' doesn't record marker
+;; adjustments. Markers are rarely explicitly created by users, so the impact
+;; of this will primarily be through its effects on other features that make
+;; use of markers. However, undoing marker adjustments is anyway buggy in
+;; standard Emacs' undo (see Emacs bug#4803), so it seems plausible that
+;; relatively little code relies heavily on correct marker restoration.
 
 
 
@@ -913,16 +933,21 @@ which is defined in the `warnings' library.\n")
 
 (defun undo-list-pop-changeset ()
   ;; Pop changeset from `buffer-undo-list'.
-  ;; discard undo boundaries at head of list
-  (while (null (car buffer-undo-list))
+  ;; discard undo boundaries and marker adjustment entries at head of list
+  (while (or (null (car buffer-undo-list))
+            (and (consp (car buffer-undo-list))
+                 (markerp (caar buffer-undo-list))))
     (setq buffer-undo-list (cdr buffer-undo-list)))
+  ;; pop elements up to next undo boundary
   (unless (eq (car buffer-undo-list) 'undo-tree-canary)
-    ;; pop elements up to next undo boundary
     (let* ((changeset (cons (pop buffer-undo-list) nil))
            (p changeset))
       (while (car buffer-undo-list)
         (setcdr p (cons (pop buffer-undo-list) nil))
-        (setq p (cdr p)))
+       ;; discard marker adjustment entries (see Commentary, above)
+       (if (and (consp (cadr p)) (markerp (car (cadr p))))
+           (setcdr p nil)
+         (setq p (cdr p))))
       changeset)))