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master bf28b019a85: Fix problems resulting from modification of the undo
From: |
Po Lu |
Subject: |
master bf28b019a85: Fix problems resulting from modification of the undo list |
Date: |
Tue, 6 Jun 2023 09:01:21 -0400 (EDT) |
branch: master
commit bf28b019a85fcc4e16bc7ecad6304c30e48a3223
Author: Po Lu <luangruo@yahoo.com>
Commit: Po Lu <luangruo@yahoo.com>
Fix problems resulting from modification of the undo list
* doc/lispref/text.texi (Atomic Changes): Describe what not to
do inside an atomic change group.
* lisp/elec-pair.el (electric-pair-inhibit-if-helps-balance):
Don't call `delete-char'; that edits the undo list by removing
boundary markers.
* lisp/subr.el (atomic-change-group, prepare-change-group): Warn
against modifying the undo list inside.
---
doc/lispref/text.texi | 11 +++++++++++
lisp/elec-pair.el | 4 +++-
lisp/subr.el | 9 +++++++++
3 files changed, 23 insertions(+), 1 deletion(-)
diff --git a/doc/lispref/text.texi b/doc/lispref/text.texi
index f15b3c33e0c..dac8d0d8ce9 100644
--- a/doc/lispref/text.texi
+++ b/doc/lispref/text.texi
@@ -6181,6 +6181,17 @@ would expect. Non-nested use of change groups for the
same buffer
will get Emacs confused, so don't let it happen; the first change
group you start for any given buffer should be the last one finished.
+ Emacs keeps track of change groups by assuming that by following
+each cdr in @code{buffer-undo-list}, it will eventually arrive at the
+cons it was set to at the time @code{prepare-change-group} was called.
+
+ If @code{buffer-undo-list} no longer contains that cons, Emacs will
+lose track of any change groups, resulting in an error when the change
+group is cancelled. To avoid this, do not call any functions which
+may edit the undo list in such a manner, when a change group is
+active: notably, ``amalgamating'' commands such as @code{delete-char},
+which call @code{undo-auto-amalgamate}.
+
@node Change Hooks
@section Change Hooks
@cindex change hooks
diff --git a/lisp/elec-pair.el b/lisp/elec-pair.el
index b894965eae4..416c95e7a34 100644
--- a/lisp/elec-pair.el
+++ b/lisp/elec-pair.el
@@ -439,7 +439,9 @@ happened."
;; position some markers. The real fix would be to compute the
;; result without having to modify the buffer at all.
(atomic-change-group
- (delete-char -1)
+ ;; Don't use `delete-char'; that may modify the head of the
+ ;; undo list.
+ (delete-region (point) (1- (point)))
(throw
'done
(cond ((eq ?\( syntax)
diff --git a/lisp/subr.el b/lisp/subr.el
index cef631a69c3..1384215498b 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -3773,6 +3773,9 @@ This means that if BODY exits abnormally,
all of its changes to the current buffer are undone.
This works regardless of whether undo is enabled in the buffer.
+Do not call functions which edit the undo list within BODY; see
+`prepare-change-group'.
+
This mechanism is transparent to ordinary use of undo;
if undo is enabled in the buffer and BODY succeeds, the
user can undo the change normally."
@@ -3839,6 +3842,12 @@ Once you finish the group, don't use the handle
again--don't try to
finish the same group twice. For a simple example of correct use, see
the source code of `atomic-change-group'.
+As long as this handle is still in use, do not call functions
+which edit the undo list: if it no longer contains its current
+value, Emacs will not be able to cancel the change group. This
+includes any \"amalgamating\" commands, such as `delete-char',
+which call `undo-auto-amalgamate'.
+
The handle records only the specified buffer. To make a multibuffer
change group, call this function once for each buffer you want to
cover, then use `nconc' to combine the returned values, like this:
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